diff --git a/Documentation/RCU/00-INDEX b/Documentation/RCU/00-INDEX
index 461481d..7dc0695 100644
--- a/Documentation/RCU/00-INDEX
+++ b/Documentation/RCU/00-INDEX
@@ -16,6 +16,8 @@
 	- List of RCU papers (bibliography) going back to 1980.
 torture.txt
 	- RCU Torture Test Operation (CONFIG_RCU_TORTURE_TEST)
+trace.txt
+	- CONFIG_RCU_TRACE debugfs files and formats
 UP.txt
 	- RCU on Uniprocessor Systems
 whatisRCU.txt
diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
new file mode 100644
index 0000000..0688482
--- /dev/null
+++ b/Documentation/RCU/trace.txt
@@ -0,0 +1,413 @@
+CONFIG_RCU_TRACE debugfs Files and Formats
+
+
+The rcupreempt and rcutree implementations of RCU provide debugfs trace
+output that summarizes counters and state.  This information is useful for
+debugging RCU itself, and can sometimes also help to debug abuses of RCU.
+Note that the rcuclassic implementation of RCU does not provide debugfs
+trace output.
+
+The following sections describe the debugfs files and formats for
+preemptable RCU (rcupreempt) and hierarchical RCU (rcutree).
+
+
+Preemptable RCU debugfs Files and Formats
+
+This implementation of RCU provides three debugfs files under the
+top-level directory RCU: rcu/rcuctrs (which displays the per-CPU
+counters used by preemptable RCU) rcu/rcugp (which displays grace-period
+counters), and rcu/rcustats (which internal counters for debugging RCU).
+
+The output of "cat rcu/rcuctrs" looks as follows:
+
+CPU last cur F M
+  0    5  -5 0 0
+  1   -1   0 0 0
+  2    0   1 0 0
+  3    0   1 0 0
+  4    0   1 0 0
+  5    0   1 0 0
+  6    0   2 0 0
+  7    0  -1 0 0
+  8    0   1 0 0
+ggp = 26226, state = waitzero
+
+The per-CPU fields are as follows:
+
+o	"CPU" gives the CPU number.  Offline CPUs are not displayed.
+
+o	"last" gives the value of the counter that is being decremented
+	for the current grace period phase.  In the example above,
+	the counters sum to 4, indicating that there are still four
+	RCU read-side critical sections still running that started
+	before the last counter flip.
+
+o	"cur" gives the value of the counter that is currently being
+	both incremented (by rcu_read_lock()) and decremented (by
+	rcu_read_unlock()).  In the example above, the counters sum to
+	1, indicating that there is only one RCU read-side critical section
+	still running that started after the last counter flip.
+
+o	"F" indicates whether RCU is waiting for this CPU to acknowledge
+	a counter flip.  In the above example, RCU is not waiting on any,
+	which is consistent with the state being "waitzero" rather than
+	"waitack".
+
+o	"M" indicates whether RCU is waiting for this CPU to execute a
+	memory barrier.  In the above example, RCU is not waiting on any,
+	which is consistent with the state being "waitzero" rather than
+	"waitmb".
+
+o	"ggp" is the global grace-period counter.
+
+o	"state" is the RCU state, which can be one of the following:
+
+	o	"idle": there is no grace period in progress.
+
+	o	"waitack": RCU just incremented the global grace-period
+		counter, which has the effect of reversing the roles of
+		the "last" and "cur" counters above, and is waiting for
+		all the CPUs to acknowledge the flip.  Once the flip has
+		been acknowledged, CPUs will no longer be incrementing
+		what are now the "last" counters, so that their sum will
+		decrease monotonically down to zero.
+
+	o	"waitzero": RCU is waiting for the sum of the "last" counters
+		to decrease to zero.
+
+	o	"waitmb": RCU is waiting for each CPU to execute a memory
+		barrier, which ensures that instructions from a given CPU's
+		last RCU read-side critical section cannot be reordered
+		with instructions following the memory-barrier instruction.
+
+The output of "cat rcu/rcugp" looks as follows:
+
+oldggp=48870  newggp=48873
+
+Note that reading from this file provokes a synchronize_rcu().  The
+"oldggp" value is that of "ggp" from rcu/rcuctrs above, taken before
+executing the synchronize_rcu(), and the "newggp" value is also the
+"ggp" value, but taken after the synchronize_rcu() command returns.
+
+
+The output of "cat rcu/rcugp" looks as follows:
+
+na=1337955 nl=40 wa=1337915 wl=44 da=1337871 dl=0 dr=1337871 di=1337871
+1=50989 e1=6138 i1=49722 ie1=82 g1=49640 a1=315203 ae1=265563 a2=49640
+z1=1401244 ze1=1351605 z2=49639 m1=5661253 me1=5611614 m2=49639
+
+These are counters tracking internal preemptable-RCU events, however,
+some of them may be useful for debugging algorithms using RCU.  In
+particular, the "nl", "wl", and "dl" values track the number of RCU
+callbacks in various states.  The fields are as follows:
+
+o	"na" is the total number of RCU callbacks that have been enqueued
+	since boot.
+
+o	"nl" is the number of RCU callbacks waiting for the previous
+	grace period to end so that they can start waiting on the next
+	grace period.
+
+o	"wa" is the total number of RCU callbacks that have started waiting
+	for a grace period since boot.  "na" should be roughly equal to
+	"nl" plus "wa".
+
+o	"wl" is the number of RCU callbacks currently waiting for their
+	grace period to end.
+
+o	"da" is the total number of RCU callbacks whose grace periods
+	have completed since boot.  "wa" should be roughly equal to
+	"wl" plus "da".
+
+o	"dr" is the total number of RCU callbacks that have been removed
+	from the list of callbacks ready to invoke.  "dr" should be roughly
+	equal to "da".
+
+o	"di" is the total number of RCU callbacks that have been invoked
+	since boot.  "di" should be roughly equal to "da", though some
+	early versions of preemptable RCU had a bug so that only the
+	last CPU's count of invocations was displayed, rather than the
+	sum of all CPU's counts.
+
+o	"1" is the number of calls to rcu_try_flip().  This should be
+	roughly equal to the sum of "e1", "i1", "a1", "z1", and "m1"
+	described below.  In other words, the number of times that
+	the state machine is visited should be equal to the sum of the
+	number of times that each state is visited plus the number of
+	times that the state-machine lock acquisition failed.
+
+o	"e1" is the number of times that rcu_try_flip() was unable to
+	acquire the fliplock.
+
+o	"i1" is the number of calls to rcu_try_flip_idle().
+
+o	"ie1" is the number of times rcu_try_flip_idle() exited early
+	due to the calling CPU having no work for RCU.
+
+o	"g1" is the number of times that rcu_try_flip_idle() decided
+	to start a new grace period.  "i1" should be roughly equal to
+	"ie1" plus "g1".
+
+o	"a1" is the number of calls to rcu_try_flip_waitack().
+
+o	"ae1" is the number of times that rcu_try_flip_waitack() found
+	that at least one CPU had not yet acknowledge the new grace period
+	(AKA "counter flip").
+
+o	"a2" is the number of time rcu_try_flip_waitack() found that
+	all CPUs had acknowledged.  "a1" should be roughly equal to
+	"ae1" plus "a2".  (This particular output was collected on
+	a 128-CPU machine, hence the smaller-than-usual fraction of
+	calls to rcu_try_flip_waitack() finding all CPUs having already
+	acknowledged.)
+
+o	"z1" is the number of calls to rcu_try_flip_waitzero().
+
+o	"ze1" is the number of times that rcu_try_flip_waitzero() found
+	that not all of the old RCU read-side critical sections had
+	completed.
+
+o	"z2" is the number of times that rcu_try_flip_waitzero() finds
+	the sum of the counters equal to zero, in other words, that
+	all of the old RCU read-side critical sections had completed.
+	The value of "z1" should be roughly equal to "ze1" plus
+	"z2".
+
+o	"m1" is the number of calls to rcu_try_flip_waitmb().
+
+o	"me1" is the number of times that rcu_try_flip_waitmb() finds
+	that at least one CPU has not yet executed a memory barrier.
+
+o	"m2" is the number of times that rcu_try_flip_waitmb() finds that
+	all CPUs have executed a memory barrier.
+
+
+Hierarchical RCU debugfs Files and Formats
+
+This implementation of RCU provides three debugfs files under the
+top-level directory RCU: rcu/rcudata (which displays fields in struct
+rcu_data), rcu/rcugp (which displays grace-period counters), and
+rcu/rcuhier (which displays the struct rcu_node hierarchy).
+
+The output of "cat rcu/rcudata" looks as follows:
+
+rcu:
+  0 c=4011 g=4012 pq=1 pqc=4011 qp=0 rpfq=1 rp=3c2a dt=23301/73 dn=2 df=1882 of=0 ri=2126 ql=2 b=10
+  1 c=4011 g=4012 pq=1 pqc=4011 qp=0 rpfq=3 rp=39a6 dt=78073/1 dn=2 df=1402 of=0 ri=1875 ql=46 b=10
+  2 c=4010 g=4010 pq=1 pqc=4010 qp=0 rpfq=-5 rp=1d12 dt=16646/0 dn=2 df=3140 of=0 ri=2080 ql=0 b=10
+  3 c=4012 g=4013 pq=1 pqc=4012 qp=1 rpfq=3 rp=2b50 dt=21159/1 dn=2 df=2230 of=0 ri=1923 ql=72 b=10
+  4 c=4012 g=4013 pq=1 pqc=4012 qp=1 rpfq=3 rp=1644 dt=5783/1 dn=2 df=3348 of=0 ri=2805 ql=7 b=10
+  5 c=4012 g=4013 pq=0 pqc=4011 qp=1 rpfq=3 rp=1aac dt=5879/1 dn=2 df=3140 of=0 ri=2066 ql=10 b=10
+  6 c=4012 g=4013 pq=1 pqc=4012 qp=1 rpfq=3 rp=ed8 dt=5847/1 dn=2 df=3797 of=0 ri=1266 ql=10 b=10
+  7 c=4012 g=4013 pq=1 pqc=4012 qp=1 rpfq=3 rp=1fa2 dt=6199/1 dn=2 df=2795 of=0 ri=2162 ql=28 b=10
+rcu_bh:
+  0 c=-268 g=-268 pq=1 pqc=-268 qp=0 rpfq=-145 rp=21d6 dt=23301/73 dn=2 df=0 of=0 ri=0 ql=0 b=10
+  1 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-170 rp=20ce dt=78073/1 dn=2 df=26 of=0 ri=5 ql=0 b=10
+  2 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-83 rp=fbd dt=16646/0 dn=2 df=28 of=0 ri=4 ql=0 b=10
+  3 c=-268 g=-268 pq=1 pqc=-268 qp=0 rpfq=-105 rp=178c dt=21159/1 dn=2 df=28 of=0 ri=2 ql=0 b=10
+  4 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-30 rp=b54 dt=5783/1 dn=2 df=32 of=0 ri=0 ql=0 b=10
+  5 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-29 rp=df5 dt=5879/1 dn=2 df=30 of=0 ri=3 ql=0 b=10
+  6 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-28 rp=788 dt=5847/1 dn=2 df=32 of=0 ri=0 ql=0 b=10
+  7 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-53 rp=1098 dt=6199/1 dn=2 df=30 of=0 ri=3 ql=0 b=10
+
+The first section lists the rcu_data structures for rcu, the second for
+rcu_bh.  Each section has one line per CPU, or eight for this 8-CPU system.
+The fields are as follows:
+
+o	The number at the beginning of each line is the CPU number.
+	CPUs numbers followed by an exclamation mark are offline,
+	but have been online at least once since boot.	There will be
+	no output for CPUs that have never been online, which can be
+	a good thing in the surprisingly common case where NR_CPUS is
+	substantially larger than the number of actual CPUs.
+
+o	"c" is the count of grace periods that this CPU believes have
+	completed.  CPUs in dynticks idle mode may lag quite a ways
+	behind, for example, CPU 4 under "rcu" above, which has slept
+	through the past 25 RCU grace periods.	It is not unusual to
+	see CPUs lagging by thousands of grace periods.
+
+o	"g" is the count of grace periods that this CPU believes have
+	started.  Again, CPUs in dynticks idle mode may lag behind.
+	If the "c" and "g" values are equal, this CPU has already
+	reported a quiescent state for the last RCU grace period that
+	it is aware of, otherwise, the CPU believes that it owes RCU a
+	quiescent state.
+
+o	"pq" indicates that this CPU has passed through a quiescent state
+	for the current grace period.  It is possible for "pq" to be
+	"1" and "c" different than "g", which indicates that although
+	the CPU has passed through a quiescent state, either (1) this
+	CPU has not yet reported that fact, (2) some other CPU has not
+	yet reported for this grace period, or (3) both.
+
+o	"pqc" indicates which grace period the last-observed quiescent
+	state for this CPU corresponds to.  This is important for handling
+	the race between CPU 0 reporting an extended dynticks-idle
+	quiescent state for CPU 1 and CPU 1 suddenly waking up and
+	reporting its own quiescent state.  If CPU 1 was the last CPU
+	for the current grace period, then the CPU that loses this race
+	will attempt to incorrectly mark CPU 1 as having checked in for
+	the next grace period!
+
+o	"qp" indicates that RCU still expects a quiescent state from
+	this CPU.
+
+o	"rpfq" is the number of rcu_pending() calls on this CPU required
+	to induce this CPU to invoke force_quiescent_state().
+
+o	"rp" is low-order four hex digits of the count of how many times
+	rcu_pending() has been invoked on this CPU.
+
+o	"dt" is the current value of the dyntick counter that is incremented
+	when entering or leaving dynticks idle state, either by the
+	scheduler or by irq.  The number after the "/" is the interrupt
+	nesting depth when in dyntick-idle state, or one greater than
+	the interrupt-nesting depth otherwise.
+
+	This field is displayed only for CONFIG_NO_HZ kernels.
+
+o	"dn" is the current value of the dyntick counter that is incremented
+	when entering or leaving dynticks idle state via NMI.  If both
+	the "dt" and "dn" values are even, then this CPU is in dynticks
+	idle mode and may be ignored by RCU.  If either of these two
+	counters is odd, then RCU must be alert to the possibility of
+	an RCU read-side critical section running on this CPU.
+
+	This field is displayed only for CONFIG_NO_HZ kernels.
+
+o	"df" is the number of times that some other CPU has forced a
+	quiescent state on behalf of this CPU due to this CPU being in
+	dynticks-idle state.
+
+	This field is displayed only for CONFIG_NO_HZ kernels.
+
+o	"of" is the number of times that some other CPU has forced a
+	quiescent state on behalf of this CPU due to this CPU being
+	offline.  In a perfect world, this might neve happen, but it
+	turns out that offlining and onlining a CPU can take several grace
+	periods, and so there is likely to be an extended period of time
+	when RCU believes that the CPU is online when it really is not.
+	Please note that erring in the other direction (RCU believing a
+	CPU is offline when it is really alive and kicking) is a fatal
+	error, so it makes sense to err conservatively.
+
+o	"ri" is the number of times that RCU has seen fit to send a
+	reschedule IPI to this CPU in order to get it to report a
+	quiescent state.
+
+o	"ql" is the number of RCU callbacks currently residing on
+	this CPU.  This is the total number of callbacks, regardless
+	of what state they are in (new, waiting for grace period to
+	start, waiting for grace period to end, ready to invoke).
+
+o	"b" is the batch limit for this CPU.  If more than this number
+	of RCU callbacks is ready to invoke, then the remainder will
+	be deferred.
+
+
+The output of "cat rcu/rcugp" looks as follows:
+
+rcu: completed=33062  gpnum=33063
+rcu_bh: completed=464  gpnum=464
+
+Again, this output is for both "rcu" and "rcu_bh".  The fields are
+taken from the rcu_state structure, and are as follows:
+
+o	"completed" is the number of grace periods that have completed.
+	It is comparable to the "c" field from rcu/rcudata in that a
+	CPU whose "c" field matches the value of "completed" is aware
+	that the corresponding RCU grace period has completed.
+
+o	"gpnum" is the number of grace periods that have started.  It is
+	comparable to the "g" field from rcu/rcudata in that a CPU
+	whose "g" field matches the value of "gpnum" is aware that the
+	corresponding RCU grace period has started.
+
+	If these two fields are equal (as they are for "rcu_bh" above),
+	then there is no grace period in progress, in other words, RCU
+	is idle.  On the other hand, if the two fields differ (as they
+	do for "rcu" above), then an RCU grace period is in progress.
+
+
+The output of "cat rcu/rcuhier" looks as follows, with very long lines:
+
+c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6
+1/1 0:127 ^0    
+3/3 0:35 ^0    0/0 36:71 ^1    0/0 72:107 ^2    0/0 108:127 ^3    
+3/3f 0:5 ^0    2/3 6:11 ^1    0/0 12:17 ^2    0/0 18:23 ^3    0/0 24:29 ^4    0/0 30:35 ^5    0/0 36:41 ^0    0/0 42:47 ^1    0/0 48:53 ^2    0/0 54:59 ^3    0/0 60:65 ^4    0/0 66:71 ^5    0/0 72:77 ^0    0/0 78:83 ^1    0/0 84:89 ^2    0/0 90:95 ^3    0/0 96:101 ^4    0/0 102:107 ^5    0/0 108:113 ^0    0/0 114:119 ^1    0/0 120:125 ^2    0/0 126:127 ^3    
+rcu_bh:
+c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0
+0/1 0:127 ^0    
+0/3 0:35 ^0    0/0 36:71 ^1    0/0 72:107 ^2    0/0 108:127 ^3    
+0/3f 0:5 ^0    0/3 6:11 ^1    0/0 12:17 ^2    0/0 18:23 ^3    0/0 24:29 ^4    0/0 30:35 ^5    0/0 36:41 ^0    0/0 42:47 ^1    0/0 48:53 ^2    0/0 54:59 ^3    0/0 60:65 ^4    0/0 66:71 ^5    0/0 72:77 ^0    0/0 78:83 ^1    0/0 84:89 ^2    0/0 90:95 ^3    0/0 96:101 ^4    0/0 102:107 ^5    0/0 108:113 ^0    0/0 114:119 ^1    0/0 120:125 ^2    0/0 126:127 ^3
+
+This is once again split into "rcu" and "rcu_bh" portions.  The fields are
+as follows:
+
+o	"c" is exactly the same as "completed" under rcu/rcugp.
+
+o	"g" is exactly the same as "gpnum" under rcu/rcugp.
+
+o	"s" is the "signaled" state that drives force_quiescent_state()'s
+	state machine.
+
+o	"jfq" is the number of jiffies remaining for this grace period
+	before force_quiescent_state() is invoked to help push things
+	along.  Note that CPUs in dyntick-idle mode thoughout the grace
+	period will not report on their own, but rather must be check by
+	some other CPU via force_quiescent_state().
+
+o	"j" is the low-order four hex digits of the jiffies counter.
+	Yes, Paul did run into a number of problems that turned out to
+	be due to the jiffies counter no longer counting.  Why do you ask?
+
+o	"nfqs" is the number of calls to force_quiescent_state() since
+	boot.
+
+o	"nfqsng" is the number of useless calls to force_quiescent_state(),
+	where there wasn't actually a grace period active.  This can
+	happen due to races.  The number in parentheses is the difference
+	between "nfqs" and "nfqsng", or the number of times that
+	force_quiescent_state() actually did some real work.
+
+o	"fqlh" is the number of calls to force_quiescent_state() that
+	exited immediately (without even being counted in nfqs above)
+	due to contention on ->fqslock.
+
+o	Each element of the form "1/1 0:127 ^0" represents one struct
+	rcu_node.  Each line represents one level of the hierarchy, from
+	root to leaves.  It is best to think of the rcu_data structures
+	as forming yet another level after the leaves.  Note that there
+	might be either one, two, or three levels of rcu_node structures,
+	depending on the relationship between CONFIG_RCU_FANOUT and
+	CONFIG_NR_CPUS.
+	
+	o	The numbers separated by the "/" are the qsmask followed
+		by the qsmaskinit.  The qsmask will have one bit
+		set for each entity in the next lower level that
+		has not yet checked in for the current grace period.
+		The qsmaskinit will have one bit for each entity that is
+		currently expected to check in during each grace period.
+		The value of qsmaskinit is assigned to that of qsmask
+		at the beginning of each grace period.
+
+		For example, for "rcu", the qsmask of the first entry
+		of the lowest level is 0x14, meaning that we are still
+		waiting for CPUs 2 and 4 to check in for the current
+		grace period.
+
+	o	The numbers separated by the ":" are the range of CPUs
+		served by this struct rcu_node.  This can be helpful
+		in working out how the hierarchy is wired together.
+
+		For example, the first entry at the lowest level shows
+		"0:5", indicating that it covers CPUs 0 through 5.
+
+	o	The number after the "^" indicates the bit in the
+		next higher level rcu_node structure that this
+		rcu_node structure corresponds to.
+
+		For example, the first entry at the lowest level shows
+		"^0", indicating that it corresponds to bit zero in
+		the first entry at the middle level.
diff --git a/arch/powerpc/platforms/pseries/rtasd.c b/arch/powerpc/platforms/pseries/rtasd.c
index f4e55be..afad9f5 100644
--- a/arch/powerpc/platforms/pseries/rtasd.c
+++ b/arch/powerpc/platforms/pseries/rtasd.c
@@ -208,6 +208,7 @@
 		break;
 	case ERR_TYPE_KERNEL_PANIC:
 	default:
+		WARN_ON_ONCE(!irqs_disabled()); /* @@@ DEBUG @@@ */
 		spin_unlock_irqrestore(&rtasd_log_lock, s);
 		return;
 	}
@@ -227,6 +228,7 @@
 	/* Check to see if we need to or have stopped logging */
 	if (fatal || !logging_enabled) {
 		logging_enabled = 0;
+		WARN_ON_ONCE(!irqs_disabled()); /* @@@ DEBUG @@@ */
 		spin_unlock_irqrestore(&rtasd_log_lock, s);
 		return;
 	}
@@ -249,11 +251,13 @@
 		else
 			rtas_log_start += 1;
 
+		WARN_ON_ONCE(!irqs_disabled()); /* @@@ DEBUG @@@ */
 		spin_unlock_irqrestore(&rtasd_log_lock, s);
 		wake_up_interruptible(&rtas_log_wait);
 		break;
 	case ERR_TYPE_KERNEL_PANIC:
 	default:
+		WARN_ON_ONCE(!irqs_disabled()); /* @@@ DEBUG @@@ */
 		spin_unlock_irqrestore(&rtasd_log_lock, s);
 		return;
 	}
diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index 181006c..9b70b92 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -118,13 +118,17 @@
 }
 #endif
 
-#if defined(CONFIG_PREEMPT_RCU) && defined(CONFIG_NO_HZ)
+#if defined(CONFIG_NO_HZ) && !defined(CONFIG_CLASSIC_RCU)
 extern void rcu_irq_enter(void);
 extern void rcu_irq_exit(void);
+extern void rcu_nmi_enter(void);
+extern void rcu_nmi_exit(void);
 #else
 # define rcu_irq_enter() do { } while (0)
 # define rcu_irq_exit() do { } while (0)
-#endif /* CONFIG_PREEMPT_RCU */
+# define rcu_nmi_enter() do { } while (0)
+# define rcu_nmi_exit() do { } while (0)
+#endif /* #if defined(CONFIG_NO_HZ) && !defined(CONFIG_CLASSIC_RCU) */
 
 /*
  * It is safe to do non-atomic ops on ->hardirq_context,
@@ -134,7 +138,6 @@
  */
 #define __irq_enter()					\
 	do {						\
-		rcu_irq_enter();			\
 		account_system_vtime(current);		\
 		add_preempt_count(HARDIRQ_OFFSET);	\
 		trace_hardirq_enter();			\
@@ -153,7 +156,6 @@
 		trace_hardirq_exit();			\
 		account_system_vtime(current);		\
 		sub_preempt_count(HARDIRQ_OFFSET);	\
-		rcu_irq_exit();				\
 	} while (0)
 
 /*
@@ -161,7 +163,7 @@
  */
 extern void irq_exit(void);
 
-#define nmi_enter()		do { lockdep_off(); __irq_enter(); } while (0)
-#define nmi_exit()		do { __irq_exit(); lockdep_on(); } while (0)
+#define nmi_enter()		do { lockdep_off(); rcu_nmi_enter(); __irq_enter(); } while (0)
+#define nmi_exit()		do { __irq_exit(); rcu_nmi_exit(); lockdep_on(); } while (0)
 
 #endif /* LINUX_HARDIRQ_H */
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 86f1f5e..bfd289a 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -52,11 +52,15 @@
 	void (*func)(struct rcu_head *head);
 };
 
-#ifdef CONFIG_CLASSIC_RCU
+#if defined(CONFIG_CLASSIC_RCU)
 #include <linux/rcuclassic.h>
-#else /* #ifdef CONFIG_CLASSIC_RCU */
+#elif defined(CONFIG_TREE_RCU)
+#include <linux/rcutree.h>
+#elif defined(CONFIG_PREEMPT_RCU)
 #include <linux/rcupreempt.h>
-#endif /* #else #ifdef CONFIG_CLASSIC_RCU */
+#else
+#error "Unknown RCU implementation specified to kernel configuration"
+#endif /* #else #if defined(CONFIG_CLASSIC_RCU) */
 
 #define RCU_HEAD_INIT 	{ .next = NULL, .func = NULL }
 #define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
new file mode 100644
index 0000000..d4368b7
--- /dev/null
+++ b/include/linux/rcutree.h
@@ -0,0 +1,329 @@
+/*
+ * Read-Copy Update mechanism for mutual exclusion (tree-based version)
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * Copyright IBM Corporation, 2008
+ *
+ * Author: Dipankar Sarma <dipankar@in.ibm.com>
+ *	   Paul E. McKenney <paulmck@linux.vnet.ibm.com> Hierarchical algorithm
+ *
+ * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
+ * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
+ *
+ * For detailed explanation of Read-Copy Update mechanism see -
+ * 	Documentation/RCU
+ */
+
+#ifndef __LINUX_RCUTREE_H
+#define __LINUX_RCUTREE_H
+
+#include <linux/cache.h>
+#include <linux/spinlock.h>
+#include <linux/threads.h>
+#include <linux/percpu.h>
+#include <linux/cpumask.h>
+#include <linux/seqlock.h>
+
+/*
+ * Define shape of hierarchy based on NR_CPUS and CONFIG_RCU_FANOUT.
+ * In theory, it should be possible to add more levels straightforwardly.
+ * In practice, this has not been tested, so there is probably some
+ * bug somewhere.
+ */
+#define MAX_RCU_LVLS 3
+#define RCU_FANOUT	      (CONFIG_RCU_FANOUT)
+#define RCU_FANOUT_SQ	      (RCU_FANOUT * RCU_FANOUT)
+#define RCU_FANOUT_CUBE	      (RCU_FANOUT_SQ * RCU_FANOUT)
+
+#if NR_CPUS <= RCU_FANOUT
+#  define NUM_RCU_LVLS	      1
+#  define NUM_RCU_LVL_0	      1
+#  define NUM_RCU_LVL_1	      (NR_CPUS)
+#  define NUM_RCU_LVL_2	      0
+#  define NUM_RCU_LVL_3	      0
+#elif NR_CPUS <= RCU_FANOUT_SQ
+#  define NUM_RCU_LVLS	      2
+#  define NUM_RCU_LVL_0	      1
+#  define NUM_RCU_LVL_1	      (((NR_CPUS) + RCU_FANOUT - 1) / RCU_FANOUT)
+#  define NUM_RCU_LVL_2	      (NR_CPUS)
+#  define NUM_RCU_LVL_3	      0
+#elif NR_CPUS <= RCU_FANOUT_CUBE
+#  define NUM_RCU_LVLS	      3
+#  define NUM_RCU_LVL_0	      1
+#  define NUM_RCU_LVL_1	      (((NR_CPUS) + RCU_FANOUT_SQ - 1) / RCU_FANOUT_SQ)
+#  define NUM_RCU_LVL_2	      (((NR_CPUS) + (RCU_FANOUT) - 1) / (RCU_FANOUT))
+#  define NUM_RCU_LVL_3	      NR_CPUS
+#else
+# error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
+#endif /* #if (NR_CPUS) <= RCU_FANOUT */
+
+#define RCU_SUM (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
+#define NUM_RCU_NODES (RCU_SUM - NR_CPUS)
+
+/*
+ * Dynticks per-CPU state.
+ */
+struct rcu_dynticks {
+	int dynticks_nesting;	/* Track nesting level, sort of. */
+	int dynticks;		/* Even value for dynticks-idle, else odd. */
+	int dynticks_nmi;	/* Even value for either dynticks-idle or */
+				/*  not in nmi handler, else odd.  So this */
+				/*  remains even for nmi from irq handler. */
+};
+
+/*
+ * Definition for node within the RCU grace-period-detection hierarchy.
+ */
+struct rcu_node {
+	spinlock_t lock;
+	unsigned long qsmask;	/* CPUs or groups that need to switch in */
+				/*  order for current grace period to proceed.*/
+	unsigned long qsmaskinit;
+				/* Per-GP initialization for qsmask. */
+	unsigned long grpmask;	/* Mask to apply to parent qsmask. */
+	int	grplo;		/* lowest-numbered CPU or group here. */
+	int	grphi;		/* highest-numbered CPU or group here. */
+	u8	grpnum;		/* CPU/group number for next level up. */
+	u8	level;		/* root is at level 0. */
+	struct rcu_node *parent;
+} ____cacheline_internodealigned_in_smp;
+
+/* Index values for nxttail array in struct rcu_data. */
+#define RCU_DONE_TAIL		0	/* Also RCU_WAIT head. */
+#define RCU_WAIT_TAIL		1	/* Also RCU_NEXT_READY head. */
+#define RCU_NEXT_READY_TAIL	2	/* Also RCU_NEXT head. */
+#define RCU_NEXT_TAIL		3
+#define RCU_NEXT_SIZE		4
+
+/* Per-CPU data for read-copy update. */
+struct rcu_data {
+	/* 1) quiescent-state and grace-period handling : */
+	long		completed;	/* Track rsp->completed gp number */
+					/*  in order to detect GP end. */
+	long		gpnum;		/* Highest gp number that this CPU */
+					/*  is aware of having started. */
+	long		passed_quiesc_completed;
+					/* Value of completed at time of qs. */
+	bool		passed_quiesc;	/* User-mode/idle loop etc. */
+	bool		qs_pending;	/* Core waits for quiesc state. */
+	bool		beenonline;	/* CPU online at least once. */
+	struct rcu_node *mynode;	/* This CPU's leaf of hierarchy */
+	unsigned long grpmask;		/* Mask to apply to leaf qsmask. */
+
+	/* 2) batch handling */
+	/*
+	 * If nxtlist is not NULL, it is partitioned as follows.
+	 * Any of the partitions might be empty, in which case the
+	 * pointer to that partition will be equal to the pointer for
+	 * the following partition.  When the list is empty, all of
+	 * the nxttail elements point to nxtlist, which is NULL.
+	 *
+	 * [*nxttail[RCU_NEXT_READY_TAIL], NULL = *nxttail[RCU_NEXT_TAIL]):
+	 *	Entries that might have arrived after current GP ended
+	 * [*nxttail[RCU_WAIT_TAIL], *nxttail[RCU_NEXT_READY_TAIL]):
+	 *	Entries known to have arrived before current GP ended
+	 * [*nxttail[RCU_DONE_TAIL], *nxttail[RCU_WAIT_TAIL]):
+	 *	Entries that batch # <= ->completed - 1: waiting for current GP
+	 * [nxtlist, *nxttail[RCU_DONE_TAIL]):
+	 *	Entries that batch # <= ->completed
+	 *	The grace period for these entries has completed, and
+	 *	the other grace-period-completed entries may be moved
+	 *	here temporarily in rcu_process_callbacks().
+	 */
+	struct rcu_head *nxtlist;
+	struct rcu_head **nxttail[RCU_NEXT_SIZE];
+	long		qlen; 	 	/* # of queued callbacks */
+	long		blimit;		/* Upper limit on a processed batch */
+
+#ifdef CONFIG_NO_HZ
+	/* 3) dynticks interface. */
+	struct rcu_dynticks *dynticks;	/* Shared per-CPU dynticks state. */
+	int dynticks_snap;		/* Per-GP tracking for dynticks. */
+	int dynticks_nmi_snap;		/* Per-GP tracking for dynticks_nmi. */
+#endif /* #ifdef CONFIG_NO_HZ */
+
+	/* 4) reasons this CPU needed to be kicked by force_quiescent_state */
+#ifdef CONFIG_NO_HZ
+	unsigned long dynticks_fqs;	/* Kicked due to dynticks idle. */
+#endif /* #ifdef CONFIG_NO_HZ */
+	unsigned long offline_fqs;	/* Kicked due to being offline. */
+	unsigned long resched_ipi;	/* Sent a resched IPI. */
+
+	/* 5) state to allow this CPU to force_quiescent_state on others */
+	long n_rcu_pending;		/* rcu_pending() calls since boot. */
+	long n_rcu_pending_force_qs;	/* when to force quiescent states. */
+
+	int cpu;
+};
+
+/* Values for signaled field in struct rcu_state. */
+#define RCU_GP_INIT		0	/* Grace period being initialized. */
+#define RCU_SAVE_DYNTICK	1	/* Need to scan dyntick state. */
+#define RCU_FORCE_QS		2	/* Need to force quiescent state. */
+#ifdef CONFIG_NO_HZ
+#define RCU_SIGNAL_INIT		RCU_SAVE_DYNTICK
+#else /* #ifdef CONFIG_NO_HZ */
+#define RCU_SIGNAL_INIT		RCU_FORCE_QS
+#endif /* #else #ifdef CONFIG_NO_HZ */
+
+#define RCU_JIFFIES_TILL_FORCE_QS	 3	/* for rsp->jiffies_force_qs */
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+#define RCU_SECONDS_TILL_STALL_CHECK   (10 * HZ)  /* for rsp->jiffies_stall */
+#define RCU_SECONDS_TILL_STALL_RECHECK (30 * HZ)  /* for rsp->jiffies_stall */
+#define RCU_STALL_RAT_DELAY		2	  /* Allow other CPUs time */
+						  /*  to take at least one */
+						  /*  scheduling clock irq */
+						  /*  before ratting on them. */
+
+#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
+
+/*
+ * RCU global state, including node hierarchy.  This hierarchy is
+ * represented in "heap" form in a dense array.  The root (first level)
+ * of the hierarchy is in ->node[0] (referenced by ->level[0]), the second
+ * level in ->node[1] through ->node[m] (->node[1] referenced by ->level[1]),
+ * and the third level in ->node[m+1] and following (->node[m+1] referenced
+ * by ->level[2]).  The number of levels is determined by the number of
+ * CPUs and by CONFIG_RCU_FANOUT.  Small systems will have a "hierarchy"
+ * consisting of a single rcu_node.
+ */
+struct rcu_state {
+	struct rcu_node node[NUM_RCU_NODES];	/* Hierarchy. */
+	struct rcu_node *level[NUM_RCU_LVLS];	/* Hierarchy levels. */
+	u32 levelcnt[MAX_RCU_LVLS + 1];		/* # nodes in each level. */
+	u8 levelspread[NUM_RCU_LVLS];		/* kids/node in each level. */
+	struct rcu_data *rda[NR_CPUS];		/* array of rdp pointers. */
+
+	/* The following fields are guarded by the root rcu_node's lock. */
+
+	u8	signaled ____cacheline_internodealigned_in_smp;
+						/* Force QS state. */
+	long	gpnum;				/* Current gp number. */
+	long	completed;			/* # of last completed gp. */
+	spinlock_t onofflock;			/* exclude on/offline and */
+						/*  starting new GP. */
+	spinlock_t fqslock;			/* Only one task forcing */
+						/*  quiescent states. */
+	unsigned long jiffies_force_qs;		/* Time at which to invoke */
+						/*  force_quiescent_state(). */
+	unsigned long n_force_qs;		/* Number of calls to */
+						/*  force_quiescent_state(). */
+	unsigned long n_force_qs_lh;		/* ~Number of calls leaving */
+						/*  due to lock unavailable. */
+	unsigned long n_force_qs_ngp;		/* Number of calls leaving */
+						/*  due to no GP active. */
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+	unsigned long gp_start;			/* Time at which GP started, */
+						/*  but in jiffies. */
+	unsigned long jiffies_stall;		/* Time at which to check */
+						/*  for CPU stalls. */
+#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
+#ifdef CONFIG_NO_HZ
+	long dynticks_completed;		/* Value of completed @ snap. */
+#endif /* #ifdef CONFIG_NO_HZ */
+};
+
+extern struct rcu_state rcu_state;
+DECLARE_PER_CPU(struct rcu_data, rcu_data);
+
+extern struct rcu_state rcu_bh_state;
+DECLARE_PER_CPU(struct rcu_data, rcu_bh_data);
+
+/*
+ * Increment the quiescent state counter.
+ * The counter is a bit degenerated: We do not need to know
+ * how many quiescent states passed, just if there was at least
+ * one since the start of the grace period. Thus just a flag.
+ */
+static inline void rcu_qsctr_inc(int cpu)
+{
+	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
+	rdp->passed_quiesc = 1;
+	rdp->passed_quiesc_completed = rdp->completed;
+}
+static inline void rcu_bh_qsctr_inc(int cpu)
+{
+	struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu);
+	rdp->passed_quiesc = 1;
+	rdp->passed_quiesc_completed = rdp->completed;
+}
+
+extern int rcu_pending(int cpu);
+extern int rcu_needs_cpu(int cpu);
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+extern struct lockdep_map rcu_lock_map;
+# define rcu_read_acquire()	\
+			lock_acquire(&rcu_lock_map, 0, 0, 2, 1, NULL, _THIS_IP_)
+# define rcu_read_release()	lock_release(&rcu_lock_map, 1, _THIS_IP_)
+#else
+# define rcu_read_acquire()	do { } while (0)
+# define rcu_read_release()	do { } while (0)
+#endif
+
+static inline void __rcu_read_lock(void)
+{
+	preempt_disable();
+	__acquire(RCU);
+	rcu_read_acquire();
+}
+static inline void __rcu_read_unlock(void)
+{
+	rcu_read_release();
+	__release(RCU);
+	preempt_enable();
+}
+static inline void __rcu_read_lock_bh(void)
+{
+	local_bh_disable();
+	__acquire(RCU_BH);
+	rcu_read_acquire();
+}
+static inline void __rcu_read_unlock_bh(void)
+{
+	rcu_read_release();
+	__release(RCU_BH);
+	local_bh_enable();
+}
+
+#define __synchronize_sched() synchronize_rcu()
+
+#define call_rcu_sched(head, func) call_rcu(head, func)
+
+static inline void rcu_init_sched(void)
+{
+}
+
+extern void __rcu_init(void);
+extern void rcu_check_callbacks(int cpu, int user);
+extern void rcu_restart_cpu(int cpu);
+
+extern long rcu_batches_completed(void);
+extern long rcu_batches_completed_bh(void);
+
+#ifdef CONFIG_NO_HZ
+void rcu_enter_nohz(void);
+void rcu_exit_nohz(void);
+#else /* CONFIG_NO_HZ */
+static inline void rcu_enter_nohz(void)
+{
+}
+static inline void rcu_exit_nohz(void)
+{
+}
+#endif /* CONFIG_NO_HZ */
+
+#endif /* __LINUX_RCUTREE_H */
diff --git a/init/Kconfig b/init/Kconfig
index f763762..9dd7958 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -928,10 +928,16 @@
 config PREEMPT_NOTIFIERS
 	bool
 
-config CLASSIC_RCU
-	def_bool !PREEMPT_RCU
+config TREE_RCU_TRACE
+	def_bool RCU_TRACE && TREE_RCU
+	select DEBUG_FS
 	help
-	  This option selects the classic RCU implementation that is
-	  designed for best read-side performance on non-realtime
-	  systems.  Classic RCU is the default.  Note that the
-	  PREEMPT_RCU symbol is used to select/deselect this option.
+	  This option provides tracing for the TREE_RCU implementation,
+	  permitting Makefile to trivially select kernel/rcutree_trace.c.
+
+config PREEMPT_RCU_TRACE
+	def_bool RCU_TRACE && PREEMPT_RCU
+	select DEBUG_FS
+	help
+	  This option provides tracing for the PREEMPT_RCU implementation,
+	  permitting Makefile to trivially select kernel/rcupreempt_trace.c.
diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index 9fdba03..463f297 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -52,10 +52,29 @@
 
 endchoice
 
+choice
+	prompt "RCU Implementation"
+	default CLASSIC_RCU
+
+config CLASSIC_RCU
+	bool "Classic RCU"
+	help
+	  This option selects the classic RCU implementation that is
+	  designed for best read-side performance on non-realtime
+	  systems.
+	  
+	  Select this option if you are unsure.
+
+config TREE_RCU
+	bool "Tree-based hierarchical RCU"
+	help
+	  This option selects the RCU implementation that is
+	  designed for very large SMP system with hundreds or
+	  thousands of CPUs.
+
 config PREEMPT_RCU
 	bool "Preemptible RCU"
 	depends on PREEMPT
-	default n
 	help
 	  This option reduces the latency of the kernel by making certain
 	  RCU sections preemptible. Normally RCU code is non-preemptible, if
@@ -64,16 +83,47 @@
 	  now-naive assumptions about each RCU read-side critical section
 	  remaining on a given CPU through its execution.
 
-	  Say N if you are unsure.
+endchoice
 
 config RCU_TRACE
-	bool "Enable tracing for RCU - currently stats in debugfs"
-	depends on PREEMPT_RCU
-	select DEBUG_FS
-	default y
+	bool "Enable tracing for RCU"
+	depends on TREE_RCU || PREEMPT_RCU
 	help
 	  This option provides tracing in RCU which presents stats
 	  in debugfs for debugging RCU implementation.
 
 	  Say Y here if you want to enable RCU tracing
 	  Say N if you are unsure.
+
+config RCU_FANOUT
+	int "Tree-based hierarchical RCU fanout value"
+	range 2 64 if 64BIT
+	range 2 32 if !64BIT
+	depends on TREE_RCU
+	default 64 if 64BIT
+	default 32 if !64BIT
+	help
+	  This option controls the fanout of hierarchical implementations
+	  of RCU, allowing RCU to work efficiently on machines with
+	  large numbers of CPUs.  This value must be at least the cube
+	  root of NR_CPUS, which allows NR_CPUS up to 32,768 for 32-bit
+	  systems and up to 262,144 for 64-bit systems.
+
+	  Select a specific number if testing RCU itself.
+	  Take the default if unsure.
+
+config RCU_FANOUT_EXACT
+	bool "Disable tree-based hierarchical RCU auto-balancing"
+	depends on TREE_RCU
+	default n
+	help
+	  This option forces use of the exact RCU_FANOUT value specified,
+	  regardless of imbalances in the hierarchy.  This is useful for
+	  testing RCU itself, and might one day be useful on systems with
+	  strong NUMA behavior.
+
+	  Without RCU_FANOUT_EXACT, the code will balance the hierarchy.
+
+	  Say n if unsure.
+
+	
diff --git a/kernel/Makefile b/kernel/Makefile
index 19fad00..b4fdbbf 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -74,10 +74,10 @@
 obj-$(CONFIG_SECCOMP) += seccomp.o
 obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
 obj-$(CONFIG_CLASSIC_RCU) += rcuclassic.o
+obj-$(CONFIG_TREE_RCU) += rcutree.o
 obj-$(CONFIG_PREEMPT_RCU) += rcupreempt.o
-ifeq ($(CONFIG_PREEMPT_RCU),y)
-obj-$(CONFIG_RCU_TRACE) += rcupreempt_trace.o
-endif
+obj-$(CONFIG_TREE_RCU_TRACE) += rcutree_trace.o
+obj-$(CONFIG_PREEMPT_RCU_TRACE) += rcupreempt_trace.o
 obj-$(CONFIG_RELAY) += relay.o
 obj-$(CONFIG_SYSCTL) += utsname_sysctl.o
 obj-$(CONFIG_TASK_DELAY_ACCT) += delayacct.o
diff --git a/kernel/rcupreempt.c b/kernel/rcupreempt.c
index 59236e8..0498265 100644
--- a/kernel/rcupreempt.c
+++ b/kernel/rcupreempt.c
@@ -551,6 +551,16 @@
 	}
 }
 
+void rcu_nmi_enter(void)
+{
+	rcu_irq_enter();
+}
+
+void rcu_nmi_exit(void)
+{
+	rcu_irq_exit();
+}
+
 static void dyntick_save_progress_counter(int cpu)
 {
 	struct rcu_dyntick_sched *rdssp = &per_cpu(rcu_dyntick_sched, cpu);
diff --git a/kernel/rcupreempt_trace.c b/kernel/rcupreempt_trace.c
index 35c2d33..7c2665c 100644
--- a/kernel/rcupreempt_trace.c
+++ b/kernel/rcupreempt_trace.c
@@ -149,12 +149,12 @@
 		sp->done_length += cp->done_length;
 		sp->done_add += cp->done_add;
 		sp->done_remove += cp->done_remove;
-		atomic_set(&sp->done_invoked, atomic_read(&cp->done_invoked));
+		atomic_add(atomic_read(&cp->done_invoked), &sp->done_invoked);
 		sp->rcu_check_callbacks += cp->rcu_check_callbacks;
-		atomic_set(&sp->rcu_try_flip_1,
-			   atomic_read(&cp->rcu_try_flip_1));
-		atomic_set(&sp->rcu_try_flip_e1,
-			   atomic_read(&cp->rcu_try_flip_e1));
+		atomic_add(atomic_read(&cp->rcu_try_flip_1),
+			   &sp->rcu_try_flip_1);
+		atomic_add(atomic_read(&cp->rcu_try_flip_e1),
+			   &sp->rcu_try_flip_e1);
 		sp->rcu_try_flip_i1 += cp->rcu_try_flip_i1;
 		sp->rcu_try_flip_ie1 += cp->rcu_try_flip_ie1;
 		sp->rcu_try_flip_g1 += cp->rcu_try_flip_g1;
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
new file mode 100644
index 0000000..a342b03
--- /dev/null
+++ b/kernel/rcutree.c
@@ -0,0 +1,1535 @@
+/*
+ * Read-Copy Update mechanism for mutual exclusion
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * Copyright IBM Corporation, 2008
+ *
+ * Authors: Dipankar Sarma <dipankar@in.ibm.com>
+ *	    Manfred Spraul <manfred@colorfullife.com>
+ *	    Paul E. McKenney <paulmck@linux.vnet.ibm.com> Hierarchical version
+ *
+ * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
+ * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
+ *
+ * For detailed explanation of Read-Copy Update mechanism see -
+ * 	Documentation/RCU
+ */
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/spinlock.h>
+#include <linux/smp.h>
+#include <linux/rcupdate.h>
+#include <linux/interrupt.h>
+#include <linux/sched.h>
+#include <asm/atomic.h>
+#include <linux/bitops.h>
+#include <linux/module.h>
+#include <linux/completion.h>
+#include <linux/moduleparam.h>
+#include <linux/percpu.h>
+#include <linux/notifier.h>
+#include <linux/cpu.h>
+#include <linux/mutex.h>
+#include <linux/time.h>
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+static struct lock_class_key rcu_lock_key;
+struct lockdep_map rcu_lock_map =
+	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
+EXPORT_SYMBOL_GPL(rcu_lock_map);
+#endif
+
+/* Data structures. */
+
+#define RCU_STATE_INITIALIZER(name) { \
+	.level = { &name.node[0] }, \
+	.levelcnt = { \
+		NUM_RCU_LVL_0,  /* root of hierarchy. */ \
+		NUM_RCU_LVL_1, \
+		NUM_RCU_LVL_2, \
+		NUM_RCU_LVL_3, /* == MAX_RCU_LVLS */ \
+	}, \
+	.signaled = RCU_SIGNAL_INIT, \
+	.gpnum = -300, \
+	.completed = -300, \
+	.onofflock = __SPIN_LOCK_UNLOCKED(&name.onofflock), \
+	.fqslock = __SPIN_LOCK_UNLOCKED(&name.fqslock), \
+	.n_force_qs = 0, \
+	.n_force_qs_ngp = 0, \
+}
+
+struct rcu_state rcu_state = RCU_STATE_INITIALIZER(rcu_state);
+DEFINE_PER_CPU(struct rcu_data, rcu_data);
+
+struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh_state);
+DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);
+
+#ifdef CONFIG_NO_HZ
+DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks);
+#endif /* #ifdef CONFIG_NO_HZ */
+
+static int blimit = 10;		/* Maximum callbacks per softirq. */
+static int qhimark = 10000;	/* If this many pending, ignore blimit. */
+static int qlowmark = 100;	/* Once only this many pending, use blimit. */
+
+static void force_quiescent_state(struct rcu_state *rsp, int relaxed);
+
+/*
+ * Return the number of RCU batches processed thus far for debug & stats.
+ */
+long rcu_batches_completed(void)
+{
+	return rcu_state.completed;
+}
+EXPORT_SYMBOL_GPL(rcu_batches_completed);
+
+/*
+ * Return the number of RCU BH batches processed thus far for debug & stats.
+ */
+long rcu_batches_completed_bh(void)
+{
+	return rcu_bh_state.completed;
+}
+EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
+
+/*
+ * Does the CPU have callbacks ready to be invoked?
+ */
+static int
+cpu_has_callbacks_ready_to_invoke(struct rcu_data *rdp)
+{
+	return &rdp->nxtlist != rdp->nxttail[RCU_DONE_TAIL];
+}
+
+/*
+ * Does the current CPU require a yet-as-unscheduled grace period?
+ */
+static int
+cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
+{
+	/* ACCESS_ONCE() because we are accessing outside of lock. */
+	return *rdp->nxttail[RCU_DONE_TAIL] &&
+	       ACCESS_ONCE(rsp->completed) == ACCESS_ONCE(rsp->gpnum);
+}
+
+/*
+ * Return the root node of the specified rcu_state structure.
+ */
+static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
+{
+	return &rsp->node[0];
+}
+
+#ifdef CONFIG_SMP
+
+/*
+ * If the specified CPU is offline, tell the caller that it is in
+ * a quiescent state.  Otherwise, whack it with a reschedule IPI.
+ * Grace periods can end up waiting on an offline CPU when that
+ * CPU is in the process of coming online -- it will be added to the
+ * rcu_node bitmasks before it actually makes it online.  The same thing
+ * can happen while a CPU is in the process of coming online.  Because this
+ * race is quite rare, we check for it after detecting that the grace
+ * period has been delayed rather than checking each and every CPU
+ * each and every time we start a new grace period.
+ */
+static int rcu_implicit_offline_qs(struct rcu_data *rdp)
+{
+	/*
+	 * If the CPU is offline, it is in a quiescent state.  We can
+	 * trust its state not to change because interrupts are disabled.
+	 */
+	if (cpu_is_offline(rdp->cpu)) {
+		rdp->offline_fqs++;
+		return 1;
+	}
+
+	/* The CPU is online, so send it a reschedule IPI. */
+	if (rdp->cpu != smp_processor_id())
+		smp_send_reschedule(rdp->cpu);
+	else
+		set_need_resched();
+	rdp->resched_ipi++;
+	return 0;
+}
+
+#endif /* #ifdef CONFIG_SMP */
+
+#ifdef CONFIG_NO_HZ
+static DEFINE_RATELIMIT_STATE(rcu_rs, 10 * HZ, 5);
+
+/**
+ * rcu_enter_nohz - inform RCU that current CPU is entering nohz
+ *
+ * Enter nohz mode, in other words, -leave- the mode in which RCU
+ * read-side critical sections can occur.  (Though RCU read-side
+ * critical sections can occur in irq handlers in nohz mode, a possibility
+ * handled by rcu_irq_enter() and rcu_irq_exit()).
+ */
+void rcu_enter_nohz(void)
+{
+	unsigned long flags;
+	struct rcu_dynticks *rdtp;
+
+	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
+	local_irq_save(flags);
+	rdtp = &__get_cpu_var(rcu_dynticks);
+	rdtp->dynticks++;
+	rdtp->dynticks_nesting--;
+	WARN_ON_RATELIMIT(rdtp->dynticks & 0x1, &rcu_rs);
+	local_irq_restore(flags);
+}
+
+/*
+ * rcu_exit_nohz - inform RCU that current CPU is leaving nohz
+ *
+ * Exit nohz mode, in other words, -enter- the mode in which RCU
+ * read-side critical sections normally occur.
+ */
+void rcu_exit_nohz(void)
+{
+	unsigned long flags;
+	struct rcu_dynticks *rdtp;
+
+	local_irq_save(flags);
+	rdtp = &__get_cpu_var(rcu_dynticks);
+	rdtp->dynticks++;
+	rdtp->dynticks_nesting++;
+	WARN_ON_RATELIMIT(!(rdtp->dynticks & 0x1), &rcu_rs);
+	local_irq_restore(flags);
+	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
+}
+
+/**
+ * rcu_nmi_enter - inform RCU of entry to NMI context
+ *
+ * If the CPU was idle with dynamic ticks active, and there is no
+ * irq handler running, this updates rdtp->dynticks_nmi to let the
+ * RCU grace-period handling know that the CPU is active.
+ */
+void rcu_nmi_enter(void)
+{
+	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
+
+	if (rdtp->dynticks & 0x1)
+		return;
+	rdtp->dynticks_nmi++;
+	WARN_ON_RATELIMIT(!(rdtp->dynticks_nmi & 0x1), &rcu_rs);
+	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
+}
+
+/**
+ * rcu_nmi_exit - inform RCU of exit from NMI context
+ *
+ * If the CPU was idle with dynamic ticks active, and there is no
+ * irq handler running, this updates rdtp->dynticks_nmi to let the
+ * RCU grace-period handling know that the CPU is no longer active.
+ */
+void rcu_nmi_exit(void)
+{
+	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
+
+	if (rdtp->dynticks & 0x1)
+		return;
+	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
+	rdtp->dynticks_nmi++;
+	WARN_ON_RATELIMIT(rdtp->dynticks_nmi & 0x1, &rcu_rs);
+}
+
+/**
+ * rcu_irq_enter - inform RCU of entry to hard irq context
+ *
+ * If the CPU was idle with dynamic ticks active, this updates the
+ * rdtp->dynticks to let the RCU handling know that the CPU is active.
+ */
+void rcu_irq_enter(void)
+{
+	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
+
+	if (rdtp->dynticks_nesting++)
+		return;
+	rdtp->dynticks++;
+	WARN_ON_RATELIMIT(!(rdtp->dynticks & 0x1), &rcu_rs);
+	smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
+}
+
+/**
+ * rcu_irq_exit - inform RCU of exit from hard irq context
+ *
+ * If the CPU was idle with dynamic ticks active, update the rdp->dynticks
+ * to put let the RCU handling be aware that the CPU is going back to idle
+ * with no ticks.
+ */
+void rcu_irq_exit(void)
+{
+	struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
+
+	if (--rdtp->dynticks_nesting)
+		return;
+	smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
+	rdtp->dynticks++;
+	WARN_ON_RATELIMIT(rdtp->dynticks & 0x1, &rcu_rs);
+
+	/* If the interrupt queued a callback, get out of dyntick mode. */
+	if (__get_cpu_var(rcu_data).nxtlist ||
+	    __get_cpu_var(rcu_bh_data).nxtlist)
+		set_need_resched();
+}
+
+/*
+ * Record the specified "completed" value, which is later used to validate
+ * dynticks counter manipulations.  Specify "rsp->completed - 1" to
+ * unconditionally invalidate any future dynticks manipulations (which is
+ * useful at the beginning of a grace period).
+ */
+static void dyntick_record_completed(struct rcu_state *rsp, long comp)
+{
+	rsp->dynticks_completed = comp;
+}
+
+#ifdef CONFIG_SMP
+
+/*
+ * Recall the previously recorded value of the completion for dynticks.
+ */
+static long dyntick_recall_completed(struct rcu_state *rsp)
+{
+	return rsp->dynticks_completed;
+}
+
+/*
+ * Snapshot the specified CPU's dynticks counter so that we can later
+ * credit them with an implicit quiescent state.  Return 1 if this CPU
+ * is already in a quiescent state courtesy of dynticks idle mode.
+ */
+static int dyntick_save_progress_counter(struct rcu_data *rdp)
+{
+	int ret;
+	int snap;
+	int snap_nmi;
+
+	snap = rdp->dynticks->dynticks;
+	snap_nmi = rdp->dynticks->dynticks_nmi;
+	smp_mb();	/* Order sampling of snap with end of grace period. */
+	rdp->dynticks_snap = snap;
+	rdp->dynticks_nmi_snap = snap_nmi;
+	ret = ((snap & 0x1) == 0) && ((snap_nmi & 0x1) == 0);
+	if (ret)
+		rdp->dynticks_fqs++;
+	return ret;
+}
+
+/*
+ * Return true if the specified CPU has passed through a quiescent
+ * state by virtue of being in or having passed through an dynticks
+ * idle state since the last call to dyntick_save_progress_counter()
+ * for this same CPU.
+ */
+static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
+{
+	long curr;
+	long curr_nmi;
+	long snap;
+	long snap_nmi;
+
+	curr = rdp->dynticks->dynticks;
+	snap = rdp->dynticks_snap;
+	curr_nmi = rdp->dynticks->dynticks_nmi;
+	snap_nmi = rdp->dynticks_nmi_snap;
+	smp_mb(); /* force ordering with cpu entering/leaving dynticks. */
+
+	/*
+	 * If the CPU passed through or entered a dynticks idle phase with
+	 * no active irq/NMI handlers, then we can safely pretend that the CPU
+	 * already acknowledged the request to pass through a quiescent
+	 * state.  Either way, that CPU cannot possibly be in an RCU
+	 * read-side critical section that started before the beginning
+	 * of the current RCU grace period.
+	 */
+	if ((curr != snap || (curr & 0x1) == 0) &&
+	    (curr_nmi != snap_nmi || (curr_nmi & 0x1) == 0)) {
+		rdp->dynticks_fqs++;
+		return 1;
+	}
+
+	/* Go check for the CPU being offline. */
+	return rcu_implicit_offline_qs(rdp);
+}
+
+#endif /* #ifdef CONFIG_SMP */
+
+#else /* #ifdef CONFIG_NO_HZ */
+
+static void dyntick_record_completed(struct rcu_state *rsp, long comp)
+{
+}
+
+#ifdef CONFIG_SMP
+
+/*
+ * If there are no dynticks, then the only way that a CPU can passively
+ * be in a quiescent state is to be offline.  Unlike dynticks idle, which
+ * is a point in time during the prior (already finished) grace period,
+ * an offline CPU is always in a quiescent state, and thus can be
+ * unconditionally applied.  So just return the current value of completed.
+ */
+static long dyntick_recall_completed(struct rcu_state *rsp)
+{
+	return rsp->completed;
+}
+
+static int dyntick_save_progress_counter(struct rcu_data *rdp)
+{
+	return 0;
+}
+
+static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
+{
+	return rcu_implicit_offline_qs(rdp);
+}
+
+#endif /* #ifdef CONFIG_SMP */
+
+#endif /* #else #ifdef CONFIG_NO_HZ */
+
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+
+static void record_gp_stall_check_time(struct rcu_state *rsp)
+{
+	rsp->gp_start = jiffies;
+	rsp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_CHECK;
+}
+
+static void print_other_cpu_stall(struct rcu_state *rsp)
+{
+	int cpu;
+	long delta;
+	unsigned long flags;
+	struct rcu_node *rnp = rcu_get_root(rsp);
+	struct rcu_node *rnp_cur = rsp->level[NUM_RCU_LVLS - 1];
+	struct rcu_node *rnp_end = &rsp->node[NUM_RCU_NODES];
+
+	/* Only let one CPU complain about others per time interval. */
+
+	spin_lock_irqsave(&rnp->lock, flags);
+	delta = jiffies - rsp->jiffies_stall;
+	if (delta < RCU_STALL_RAT_DELAY || rsp->gpnum == rsp->completed) {
+		spin_unlock_irqrestore(&rnp->lock, flags);
+		return;
+	}
+	rsp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
+	spin_unlock_irqrestore(&rnp->lock, flags);
+
+	/* OK, time to rat on our buddy... */
+
+	printk(KERN_ERR "INFO: RCU detected CPU stalls:");
+	for (; rnp_cur < rnp_end; rnp_cur++) {
+		if (rnp_cur->qsmask == 0)
+			continue;
+		for (cpu = 0; cpu <= rnp_cur->grphi - rnp_cur->grplo; cpu++)
+			if (rnp_cur->qsmask & (1UL << cpu))
+				printk(" %d", rnp_cur->grplo + cpu);
+	}
+	printk(" (detected by %d, t=%ld jiffies)\n",
+	       smp_processor_id(), (long)(jiffies - rsp->gp_start));
+	force_quiescent_state(rsp, 0);  /* Kick them all. */
+}
+
+static void print_cpu_stall(struct rcu_state *rsp)
+{
+	unsigned long flags;
+	struct rcu_node *rnp = rcu_get_root(rsp);
+
+	printk(KERN_ERR "INFO: RCU detected CPU %d stall (t=%lu jiffies)\n",
+			smp_processor_id(), jiffies - rsp->gp_start);
+	dump_stack();
+	spin_lock_irqsave(&rnp->lock, flags);
+	if ((long)(jiffies - rsp->jiffies_stall) >= 0)
+		rsp->jiffies_stall =
+			jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
+	spin_unlock_irqrestore(&rnp->lock, flags);
+	set_need_resched();  /* kick ourselves to get things going. */
+}
+
+static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
+{
+	long delta;
+	struct rcu_node *rnp;
+
+	delta = jiffies - rsp->jiffies_stall;
+	rnp = rdp->mynode;
+	if ((rnp->qsmask & rdp->grpmask) && delta >= 0) {
+
+		/* We haven't checked in, so go dump stack. */
+		print_cpu_stall(rsp);
+
+	} else if (rsp->gpnum != rsp->completed &&
+		   delta >= RCU_STALL_RAT_DELAY) {
+
+		/* They had two time units to dump stack, so complain. */
+		print_other_cpu_stall(rsp);
+	}
+}
+
+#else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
+
+static void record_gp_stall_check_time(struct rcu_state *rsp)
+{
+}
+
+static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
+{
+}
+
+#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
+
+/*
+ * Update CPU-local rcu_data state to record the newly noticed grace period.
+ * This is used both when we started the grace period and when we notice
+ * that someone else started the grace period.
+ */
+static void note_new_gpnum(struct rcu_state *rsp, struct rcu_data *rdp)
+{
+	rdp->qs_pending = 1;
+	rdp->passed_quiesc = 0;
+	rdp->gpnum = rsp->gpnum;
+	rdp->n_rcu_pending_force_qs = rdp->n_rcu_pending +
+				      RCU_JIFFIES_TILL_FORCE_QS;
+}
+
+/*
+ * Did someone else start a new RCU grace period start since we last
+ * checked?  Update local state appropriately if so.  Must be called
+ * on the CPU corresponding to rdp.
+ */
+static int
+check_for_new_grace_period(struct rcu_state *rsp, struct rcu_data *rdp)
+{
+	unsigned long flags;
+	int ret = 0;
+
+	local_irq_save(flags);
+	if (rdp->gpnum != rsp->gpnum) {
+		note_new_gpnum(rsp, rdp);
+		ret = 1;
+	}
+	local_irq_restore(flags);
+	return ret;
+}
+
+/*
+ * Start a new RCU grace period if warranted, re-initializing the hierarchy
+ * in preparation for detecting the next grace period.  The caller must hold
+ * the root node's ->lock, which is released before return.  Hard irqs must
+ * be disabled.
+ */
+static void
+rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
+	__releases(rcu_get_root(rsp)->lock)
+{
+	struct rcu_data *rdp = rsp->rda[smp_processor_id()];
+	struct rcu_node *rnp = rcu_get_root(rsp);
+	struct rcu_node *rnp_cur;
+	struct rcu_node *rnp_end;
+
+	if (!cpu_needs_another_gp(rsp, rdp)) {
+		spin_unlock_irqrestore(&rnp->lock, flags);
+		return;
+	}
+
+	/* Advance to a new grace period and initialize state. */
+	rsp->gpnum++;
+	rsp->signaled = RCU_GP_INIT; /* Hold off force_quiescent_state. */
+	rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
+	rdp->n_rcu_pending_force_qs = rdp->n_rcu_pending +
+				      RCU_JIFFIES_TILL_FORCE_QS;
+	record_gp_stall_check_time(rsp);
+	dyntick_record_completed(rsp, rsp->completed - 1);
+	note_new_gpnum(rsp, rdp);
+
+	/*
+	 * Because we are first, we know that all our callbacks will
+	 * be covered by this upcoming grace period, even the ones
+	 * that were registered arbitrarily recently.
+	 */
+	rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
+	rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
+
+	/* Special-case the common single-level case. */
+	if (NUM_RCU_NODES == 1) {
+		rnp->qsmask = rnp->qsmaskinit;
+		spin_unlock_irqrestore(&rnp->lock, flags);
+		return;
+	}
+
+	spin_unlock(&rnp->lock);  /* leave irqs disabled. */
+
+
+	/* Exclude any concurrent CPU-hotplug operations. */
+	spin_lock(&rsp->onofflock);  /* irqs already disabled. */
+
+	/*
+	 * Set the quiescent-state-needed bits in all the non-leaf RCU
+	 * nodes for all currently online CPUs.  This operation relies
+	 * on the layout of the hierarchy within the rsp->node[] array.
+	 * Note that other CPUs will access only the leaves of the
+	 * hierarchy, which still indicate that no grace period is in
+	 * progress.  In addition, we have excluded CPU-hotplug operations.
+	 *
+	 * We therefore do not need to hold any locks.  Any required
+	 * memory barriers will be supplied by the locks guarding the
+	 * leaf rcu_nodes in the hierarchy.
+	 */
+
+	rnp_end = rsp->level[NUM_RCU_LVLS - 1];
+	for (rnp_cur = &rsp->node[0]; rnp_cur < rnp_end; rnp_cur++)
+		rnp_cur->qsmask = rnp_cur->qsmaskinit;
+
+	/*
+	 * Now set up the leaf nodes.  Here we must be careful.  First,
+	 * we need to hold the lock in order to exclude other CPUs, which
+	 * might be contending for the leaf nodes' locks.  Second, as
+	 * soon as we initialize a given leaf node, its CPUs might run
+	 * up the rest of the hierarchy.  We must therefore acquire locks
+	 * for each node that we touch during this stage.  (But we still
+	 * are excluding CPU-hotplug operations.)
+	 *
+	 * Note that the grace period cannot complete until we finish
+	 * the initialization process, as there will be at least one
+	 * qsmask bit set in the root node until that time, namely the
+	 * one corresponding to this CPU.
+	 */
+	rnp_end = &rsp->node[NUM_RCU_NODES];
+	rnp_cur = rsp->level[NUM_RCU_LVLS - 1];
+	for (; rnp_cur < rnp_end; rnp_cur++) {
+		spin_lock(&rnp_cur->lock);	/* irqs already disabled. */
+		rnp_cur->qsmask = rnp_cur->qsmaskinit;
+		spin_unlock(&rnp_cur->lock);	/* irqs already disabled. */
+	}
+
+	rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state now OK. */
+	spin_unlock_irqrestore(&rsp->onofflock, flags);
+}
+
+/*
+ * Advance this CPU's callbacks, but only if the current grace period
+ * has ended.  This may be called only from the CPU to whom the rdp
+ * belongs.
+ */
+static void
+rcu_process_gp_end(struct rcu_state *rsp, struct rcu_data *rdp)
+{
+	long completed_snap;
+	unsigned long flags;
+
+	local_irq_save(flags);
+	completed_snap = ACCESS_ONCE(rsp->completed);  /* outside of lock. */
+
+	/* Did another grace period end? */
+	if (rdp->completed != completed_snap) {
+
+		/* Advance callbacks.  No harm if list empty. */
+		rdp->nxttail[RCU_DONE_TAIL] = rdp->nxttail[RCU_WAIT_TAIL];
+		rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_READY_TAIL];
+		rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
+
+		/* Remember that we saw this grace-period completion. */
+		rdp->completed = completed_snap;
+	}
+	local_irq_restore(flags);
+}
+
+/*
+ * Similar to cpu_quiet(), for which it is a helper function.  Allows
+ * a group of CPUs to be quieted at one go, though all the CPUs in the
+ * group must be represented by the same leaf rcu_node structure.
+ * That structure's lock must be held upon entry, and it is released
+ * before return.
+ */
+static void
+cpu_quiet_msk(unsigned long mask, struct rcu_state *rsp, struct rcu_node *rnp,
+	      unsigned long flags)
+	__releases(rnp->lock)
+{
+	/* Walk up the rcu_node hierarchy. */
+	for (;;) {
+		if (!(rnp->qsmask & mask)) {
+
+			/* Our bit has already been cleared, so done. */
+			spin_unlock_irqrestore(&rnp->lock, flags);
+			return;
+		}
+		rnp->qsmask &= ~mask;
+		if (rnp->qsmask != 0) {
+
+			/* Other bits still set at this level, so done. */
+			spin_unlock_irqrestore(&rnp->lock, flags);
+			return;
+		}
+		mask = rnp->grpmask;
+		if (rnp->parent == NULL) {
+
+			/* No more levels.  Exit loop holding root lock. */
+
+			break;
+		}
+		spin_unlock_irqrestore(&rnp->lock, flags);
+		rnp = rnp->parent;
+		spin_lock_irqsave(&rnp->lock, flags);
+	}
+
+	/*
+	 * Get here if we are the last CPU to pass through a quiescent
+	 * state for this grace period.  Clean up and let rcu_start_gp()
+	 * start up the next grace period if one is needed.  Note that
+	 * we still hold rnp->lock, as required by rcu_start_gp(), which
+	 * will release it.
+	 */
+	rsp->completed = rsp->gpnum;
+	rcu_process_gp_end(rsp, rsp->rda[smp_processor_id()]);
+	rcu_start_gp(rsp, flags);  /* releases rnp->lock. */
+}
+
+/*
+ * Record a quiescent state for the specified CPU, which must either be
+ * the current CPU or an offline CPU.  The lastcomp argument is used to
+ * make sure we are still in the grace period of interest.  We don't want
+ * to end the current grace period based on quiescent states detected in
+ * an earlier grace period!
+ */
+static void
+cpu_quiet(int cpu, struct rcu_state *rsp, struct rcu_data *rdp, long lastcomp)
+{
+	unsigned long flags;
+	unsigned long mask;
+	struct rcu_node *rnp;
+
+	rnp = rdp->mynode;
+	spin_lock_irqsave(&rnp->lock, flags);
+	if (lastcomp != ACCESS_ONCE(rsp->completed)) {
+
+		/*
+		 * Someone beat us to it for this grace period, so leave.
+		 * The race with GP start is resolved by the fact that we
+		 * hold the leaf rcu_node lock, so that the per-CPU bits
+		 * cannot yet be initialized -- so we would simply find our
+		 * CPU's bit already cleared in cpu_quiet_msk() if this race
+		 * occurred.
+		 */
+		rdp->passed_quiesc = 0;	/* try again later! */
+		spin_unlock_irqrestore(&rnp->lock, flags);
+		return;
+	}
+	mask = rdp->grpmask;
+	if ((rnp->qsmask & mask) == 0) {
+		spin_unlock_irqrestore(&rnp->lock, flags);
+	} else {
+		rdp->qs_pending = 0;
+
+		/*
+		 * This GP can't end until cpu checks in, so all of our
+		 * callbacks can be processed during the next GP.
+		 */
+		rdp = rsp->rda[smp_processor_id()];
+		rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
+
+		cpu_quiet_msk(mask, rsp, rnp, flags); /* releases rnp->lock */
+	}
+}
+
+/*
+ * Check to see if there is a new grace period of which this CPU
+ * is not yet aware, and if so, set up local rcu_data state for it.
+ * Otherwise, see if this CPU has just passed through its first
+ * quiescent state for this grace period, and record that fact if so.
+ */
+static void
+rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
+{
+	/* If there is now a new grace period, record and return. */
+	if (check_for_new_grace_period(rsp, rdp))
+		return;
+
+	/*
+	 * Does this CPU still need to do its part for current grace period?
+	 * If no, return and let the other CPUs do their part as well.
+	 */
+	if (!rdp->qs_pending)
+		return;
+
+	/*
+	 * Was there a quiescent state since the beginning of the grace
+	 * period? If no, then exit and wait for the next call.
+	 */
+	if (!rdp->passed_quiesc)
+		return;
+
+	/* Tell RCU we are done (but cpu_quiet() will be the judge of that). */
+	cpu_quiet(rdp->cpu, rsp, rdp, rdp->passed_quiesc_completed);
+}
+
+#ifdef CONFIG_HOTPLUG_CPU
+
+/*
+ * Remove the outgoing CPU from the bitmasks in the rcu_node hierarchy
+ * and move all callbacks from the outgoing CPU to the current one.
+ */
+static void __rcu_offline_cpu(int cpu, struct rcu_state *rsp)
+{
+	int i;
+	unsigned long flags;
+	long lastcomp;
+	unsigned long mask;
+	struct rcu_data *rdp = rsp->rda[cpu];
+	struct rcu_data *rdp_me;
+	struct rcu_node *rnp;
+
+	/* Exclude any attempts to start a new grace period. */
+	spin_lock_irqsave(&rsp->onofflock, flags);
+
+	/* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
+	rnp = rdp->mynode;
+	mask = rdp->grpmask;	/* rnp->grplo is constant. */
+	do {
+		spin_lock(&rnp->lock);		/* irqs already disabled. */
+		rnp->qsmaskinit &= ~mask;
+		if (rnp->qsmaskinit != 0) {
+			spin_unlock(&rnp->lock); /* irqs already disabled. */
+			break;
+		}
+		mask = rnp->grpmask;
+		spin_unlock(&rnp->lock);	/* irqs already disabled. */
+		rnp = rnp->parent;
+	} while (rnp != NULL);
+	lastcomp = rsp->completed;
+
+	spin_unlock(&rsp->onofflock);		/* irqs remain disabled. */
+
+	/* Being offline is a quiescent state, so go record it. */
+	cpu_quiet(cpu, rsp, rdp, lastcomp);
+
+	/*
+	 * Move callbacks from the outgoing CPU to the running CPU.
+	 * Note that the outgoing CPU is now quiscent, so it is now
+	 * (uncharacteristically) safe to access it rcu_data structure.
+	 * Note also that we must carefully retain the order of the
+	 * outgoing CPU's callbacks in order for rcu_barrier() to work
+	 * correctly.  Finally, note that we start all the callbacks
+	 * afresh, even those that have passed through a grace period
+	 * and are therefore ready to invoke.  The theory is that hotplug
+	 * events are rare, and that if they are frequent enough to
+	 * indefinitely delay callbacks, you have far worse things to
+	 * be worrying about.
+	 */
+	rdp_me = rsp->rda[smp_processor_id()];
+	if (rdp->nxtlist != NULL) {
+		*rdp_me->nxttail[RCU_NEXT_TAIL] = rdp->nxtlist;
+		rdp_me->nxttail[RCU_NEXT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
+		rdp->nxtlist = NULL;
+		for (i = 0; i < RCU_NEXT_SIZE; i++)
+			rdp->nxttail[i] = &rdp->nxtlist;
+		rdp_me->qlen += rdp->qlen;
+		rdp->qlen = 0;
+	}
+	local_irq_restore(flags);
+}
+
+/*
+ * Remove the specified CPU from the RCU hierarchy and move any pending
+ * callbacks that it might have to the current CPU.  This code assumes
+ * that at least one CPU in the system will remain running at all times.
+ * Any attempt to offline -all- CPUs is likely to strand RCU callbacks.
+ */
+static void rcu_offline_cpu(int cpu)
+{
+	__rcu_offline_cpu(cpu, &rcu_state);
+	__rcu_offline_cpu(cpu, &rcu_bh_state);
+}
+
+#else /* #ifdef CONFIG_HOTPLUG_CPU */
+
+static void rcu_offline_cpu(int cpu)
+{
+}
+
+#endif /* #else #ifdef CONFIG_HOTPLUG_CPU */
+
+/*
+ * Invoke any RCU callbacks that have made it to the end of their grace
+ * period.  Thottle as specified by rdp->blimit.
+ */
+static void rcu_do_batch(struct rcu_data *rdp)
+{
+	unsigned long flags;
+	struct rcu_head *next, *list, **tail;
+	int count;
+
+	/* If no callbacks are ready, just return.*/
+	if (!cpu_has_callbacks_ready_to_invoke(rdp))
+		return;
+
+	/*
+	 * Extract the list of ready callbacks, disabling to prevent
+	 * races with call_rcu() from interrupt handlers.
+	 */
+	local_irq_save(flags);
+	list = rdp->nxtlist;
+	rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
+	*rdp->nxttail[RCU_DONE_TAIL] = NULL;
+	tail = rdp->nxttail[RCU_DONE_TAIL];
+	for (count = RCU_NEXT_SIZE - 1; count >= 0; count--)
+		if (rdp->nxttail[count] == rdp->nxttail[RCU_DONE_TAIL])
+			rdp->nxttail[count] = &rdp->nxtlist;
+	local_irq_restore(flags);
+
+	/* Invoke callbacks. */
+	count = 0;
+	while (list) {
+		next = list->next;
+		prefetch(next);
+		list->func(list);
+		list = next;
+		if (++count >= rdp->blimit)
+			break;
+	}
+
+	local_irq_save(flags);
+
+	/* Update count, and requeue any remaining callbacks. */
+	rdp->qlen -= count;
+	if (list != NULL) {
+		*tail = rdp->nxtlist;
+		rdp->nxtlist = list;
+		for (count = 0; count < RCU_NEXT_SIZE; count++)
+			if (&rdp->nxtlist == rdp->nxttail[count])
+				rdp->nxttail[count] = tail;
+			else
+				break;
+	}
+
+	/* Reinstate batch limit if we have worked down the excess. */
+	if (rdp->blimit == LONG_MAX && rdp->qlen <= qlowmark)
+		rdp->blimit = blimit;
+
+	local_irq_restore(flags);
+
+	/* Re-raise the RCU softirq if there are callbacks remaining. */
+	if (cpu_has_callbacks_ready_to_invoke(rdp))
+		raise_softirq(RCU_SOFTIRQ);
+}
+
+/*
+ * Check to see if this CPU is in a non-context-switch quiescent state
+ * (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
+ * Also schedule the RCU softirq handler.
+ *
+ * This function must be called with hardirqs disabled.  It is normally
+ * invoked from the scheduling-clock interrupt.  If rcu_pending returns
+ * false, there is no point in invoking rcu_check_callbacks().
+ */
+void rcu_check_callbacks(int cpu, int user)
+{
+	if (user ||
+	    (idle_cpu(cpu) && !in_softirq() &&
+				hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
+
+		/*
+		 * Get here if this CPU took its interrupt from user
+		 * mode or from the idle loop, and if this is not a
+		 * nested interrupt.  In this case, the CPU is in
+		 * a quiescent state, so count it.
+		 *
+		 * No memory barrier is required here because both
+		 * rcu_qsctr_inc() and rcu_bh_qsctr_inc() reference
+		 * only CPU-local variables that other CPUs neither
+		 * access nor modify, at least not while the corresponding
+		 * CPU is online.
+		 */
+
+		rcu_qsctr_inc(cpu);
+		rcu_bh_qsctr_inc(cpu);
+
+	} else if (!in_softirq()) {
+
+		/*
+		 * Get here if this CPU did not take its interrupt from
+		 * softirq, in other words, if it is not interrupting
+		 * a rcu_bh read-side critical section.  This is an _bh
+		 * critical section, so count it.
+		 */
+
+		rcu_bh_qsctr_inc(cpu);
+	}
+	raise_softirq(RCU_SOFTIRQ);
+}
+
+#ifdef CONFIG_SMP
+
+/*
+ * Scan the leaf rcu_node structures, processing dyntick state for any that
+ * have not yet encountered a quiescent state, using the function specified.
+ * Returns 1 if the current grace period ends while scanning (possibly
+ * because we made it end).
+ */
+static int rcu_process_dyntick(struct rcu_state *rsp, long lastcomp,
+			       int (*f)(struct rcu_data *))
+{
+	unsigned long bit;
+	int cpu;
+	unsigned long flags;
+	unsigned long mask;
+	struct rcu_node *rnp_cur = rsp->level[NUM_RCU_LVLS - 1];
+	struct rcu_node *rnp_end = &rsp->node[NUM_RCU_NODES];
+
+	for (; rnp_cur < rnp_end; rnp_cur++) {
+		mask = 0;
+		spin_lock_irqsave(&rnp_cur->lock, flags);
+		if (rsp->completed != lastcomp) {
+			spin_unlock_irqrestore(&rnp_cur->lock, flags);
+			return 1;
+		}
+		if (rnp_cur->qsmask == 0) {
+			spin_unlock_irqrestore(&rnp_cur->lock, flags);
+			continue;
+		}
+		cpu = rnp_cur->grplo;
+		bit = 1;
+		for (; cpu <= rnp_cur->grphi; cpu++, bit <<= 1) {
+			if ((rnp_cur->qsmask & bit) != 0 && f(rsp->rda[cpu]))
+				mask |= bit;
+		}
+		if (mask != 0 && rsp->completed == lastcomp) {
+
+			/* cpu_quiet_msk() releases rnp_cur->lock. */
+			cpu_quiet_msk(mask, rsp, rnp_cur, flags);
+			continue;
+		}
+		spin_unlock_irqrestore(&rnp_cur->lock, flags);
+	}
+	return 0;
+}
+
+/*
+ * Force quiescent states on reluctant CPUs, and also detect which
+ * CPUs are in dyntick-idle mode.
+ */
+static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
+{
+	unsigned long flags;
+	long lastcomp;
+	struct rcu_data *rdp = rsp->rda[smp_processor_id()];
+	struct rcu_node *rnp = rcu_get_root(rsp);
+	u8 signaled;
+
+	if (ACCESS_ONCE(rsp->completed) == ACCESS_ONCE(rsp->gpnum))
+		return;  /* No grace period in progress, nothing to force. */
+	if (!spin_trylock_irqsave(&rsp->fqslock, flags)) {
+		rsp->n_force_qs_lh++; /* Inexact, can lose counts.  Tough! */
+		return;	/* Someone else is already on the job. */
+	}
+	if (relaxed &&
+	    (long)(rsp->jiffies_force_qs - jiffies) >= 0 &&
+	    (rdp->n_rcu_pending_force_qs - rdp->n_rcu_pending) >= 0)
+		goto unlock_ret; /* no emergency and done recently. */
+	rsp->n_force_qs++;
+	spin_lock(&rnp->lock);
+	lastcomp = rsp->completed;
+	signaled = rsp->signaled;
+	rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
+	rdp->n_rcu_pending_force_qs = rdp->n_rcu_pending +
+				      RCU_JIFFIES_TILL_FORCE_QS;
+	if (lastcomp == rsp->gpnum) {
+		rsp->n_force_qs_ngp++;
+		spin_unlock(&rnp->lock);
+		goto unlock_ret;  /* no GP in progress, time updated. */
+	}
+	spin_unlock(&rnp->lock);
+	switch (signaled) {
+	case RCU_GP_INIT:
+
+		break; /* grace period still initializing, ignore. */
+
+	case RCU_SAVE_DYNTICK:
+
+		if (RCU_SIGNAL_INIT != RCU_SAVE_DYNTICK)
+			break; /* So gcc recognizes the dead code. */
+
+		/* Record dyntick-idle state. */
+		if (rcu_process_dyntick(rsp, lastcomp,
+					dyntick_save_progress_counter))
+			goto unlock_ret;
+
+		/* Update state, record completion counter. */
+		spin_lock(&rnp->lock);
+		if (lastcomp == rsp->completed) {
+			rsp->signaled = RCU_FORCE_QS;
+			dyntick_record_completed(rsp, lastcomp);
+		}
+		spin_unlock(&rnp->lock);
+		break;
+
+	case RCU_FORCE_QS:
+
+		/* Check dyntick-idle state, send IPI to laggarts. */
+		if (rcu_process_dyntick(rsp, dyntick_recall_completed(rsp),
+					rcu_implicit_dynticks_qs))
+			goto unlock_ret;
+
+		/* Leave state in case more forcing is required. */
+
+		break;
+	}
+unlock_ret:
+	spin_unlock_irqrestore(&rsp->fqslock, flags);
+}
+
+#else /* #ifdef CONFIG_SMP */
+
+static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
+{
+	set_need_resched();
+}
+
+#endif /* #else #ifdef CONFIG_SMP */
+
+/*
+ * This does the RCU processing work from softirq context for the
+ * specified rcu_state and rcu_data structures.  This may be called
+ * only from the CPU to whom the rdp belongs.
+ */
+static void
+__rcu_process_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)
+{
+	unsigned long flags;
+
+	/*
+	 * If an RCU GP has gone long enough, go check for dyntick
+	 * idle CPUs and, if needed, send resched IPIs.
+	 */
+	if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0 ||
+	    (rdp->n_rcu_pending_force_qs - rdp->n_rcu_pending) < 0)
+		force_quiescent_state(rsp, 1);
+
+	/*
+	 * Advance callbacks in response to end of earlier grace
+	 * period that some other CPU ended.
+	 */
+	rcu_process_gp_end(rsp, rdp);
+
+	/* Update RCU state based on any recent quiescent states. */
+	rcu_check_quiescent_state(rsp, rdp);
+
+	/* Does this CPU require a not-yet-started grace period? */
+	if (cpu_needs_another_gp(rsp, rdp)) {
+		spin_lock_irqsave(&rcu_get_root(rsp)->lock, flags);
+		rcu_start_gp(rsp, flags);  /* releases above lock */
+	}
+
+	/* If there are callbacks ready, invoke them. */
+	rcu_do_batch(rdp);
+}
+
+/*
+ * Do softirq processing for the current CPU.
+ */
+static void rcu_process_callbacks(struct softirq_action *unused)
+{
+	/*
+	 * Memory references from any prior RCU read-side critical sections
+	 * executed by the interrupted code must be seen before any RCU
+	 * grace-period manipulations below.
+	 */
+	smp_mb(); /* See above block comment. */
+
+	__rcu_process_callbacks(&rcu_state, &__get_cpu_var(rcu_data));
+	__rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
+
+	/*
+	 * Memory references from any later RCU read-side critical sections
+	 * executed by the interrupted code must be seen after any RCU
+	 * grace-period manipulations above.
+	 */
+	smp_mb(); /* See above block comment. */
+}
+
+static void
+__call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
+	   struct rcu_state *rsp)
+{
+	unsigned long flags;
+	struct rcu_data *rdp;
+
+	head->func = func;
+	head->next = NULL;
+
+	smp_mb(); /* Ensure RCU update seen before callback registry. */
+
+	/*
+	 * Opportunistically note grace-period endings and beginnings.
+	 * Note that we might see a beginning right after we see an
+	 * end, but never vice versa, since this CPU has to pass through
+	 * a quiescent state betweentimes.
+	 */
+	local_irq_save(flags);
+	rdp = rsp->rda[smp_processor_id()];
+	rcu_process_gp_end(rsp, rdp);
+	check_for_new_grace_period(rsp, rdp);
+
+	/* Add the callback to our list. */
+	*rdp->nxttail[RCU_NEXT_TAIL] = head;
+	rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
+
+	/* Start a new grace period if one not already started. */
+	if (ACCESS_ONCE(rsp->completed) == ACCESS_ONCE(rsp->gpnum)) {
+		unsigned long nestflag;
+		struct rcu_node *rnp_root = rcu_get_root(rsp);
+
+		spin_lock_irqsave(&rnp_root->lock, nestflag);
+		rcu_start_gp(rsp, nestflag);  /* releases rnp_root->lock. */
+	}
+
+	/* Force the grace period if too many callbacks or too long waiting. */
+	if (unlikely(++rdp->qlen > qhimark)) {
+		rdp->blimit = LONG_MAX;
+		force_quiescent_state(rsp, 0);
+	} else if ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0 ||
+		   (rdp->n_rcu_pending_force_qs - rdp->n_rcu_pending) < 0)
+		force_quiescent_state(rsp, 1);
+	local_irq_restore(flags);
+}
+
+/*
+ * Queue an RCU callback for invocation after a grace period.
+ */
+void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
+{
+	__call_rcu(head, func, &rcu_state);
+}
+EXPORT_SYMBOL_GPL(call_rcu);
+
+/*
+ * Queue an RCU for invocation after a quicker grace period.
+ */
+void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
+{
+	__call_rcu(head, func, &rcu_bh_state);
+}
+EXPORT_SYMBOL_GPL(call_rcu_bh);
+
+/*
+ * Check to see if there is any immediate RCU-related work to be done
+ * by the current CPU, for the specified type of RCU, returning 1 if so.
+ * The checks are in order of increasing expense: checks that can be
+ * carried out against CPU-local state are performed first.  However,
+ * we must check for CPU stalls first, else we might not get a chance.
+ */
+static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
+{
+	rdp->n_rcu_pending++;
+
+	/* Check for CPU stalls, if enabled. */
+	check_cpu_stall(rsp, rdp);
+
+	/* Is the RCU core waiting for a quiescent state from this CPU? */
+	if (rdp->qs_pending)
+		return 1;
+
+	/* Does this CPU have callbacks ready to invoke? */
+	if (cpu_has_callbacks_ready_to_invoke(rdp))
+		return 1;
+
+	/* Has RCU gone idle with this CPU needing another grace period? */
+	if (cpu_needs_another_gp(rsp, rdp))
+		return 1;
+
+	/* Has another RCU grace period completed?  */
+	if (ACCESS_ONCE(rsp->completed) != rdp->completed) /* outside of lock */
+		return 1;
+
+	/* Has a new RCU grace period started? */
+	if (ACCESS_ONCE(rsp->gpnum) != rdp->gpnum) /* outside of lock */
+		return 1;
+
+	/* Has an RCU GP gone long enough to send resched IPIs &c? */
+	if (ACCESS_ONCE(rsp->completed) != ACCESS_ONCE(rsp->gpnum) &&
+	    ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0 ||
+	     (rdp->n_rcu_pending_force_qs - rdp->n_rcu_pending) < 0))
+		return 1;
+
+	/* nothing to do */
+	return 0;
+}
+
+/*
+ * Check to see if there is any immediate RCU-related work to be done
+ * by the current CPU, returning 1 if so.  This function is part of the
+ * RCU implementation; it is -not- an exported member of the RCU API.
+ */
+int rcu_pending(int cpu)
+{
+	return __rcu_pending(&rcu_state, &per_cpu(rcu_data, cpu)) ||
+	       __rcu_pending(&rcu_bh_state, &per_cpu(rcu_bh_data, cpu));
+}
+
+/*
+ * Check to see if any future RCU-related work will need to be done
+ * by the current CPU, even if none need be done immediately, returning
+ * 1 if so.  This function is part of the RCU implementation; it is -not-
+ * an exported member of the RCU API.
+ */
+int rcu_needs_cpu(int cpu)
+{
+	/* RCU callbacks either ready or pending? */
+	return per_cpu(rcu_data, cpu).nxtlist ||
+	       per_cpu(rcu_bh_data, cpu).nxtlist;
+}
+
+/*
+ * Initialize a CPU's per-CPU RCU data.  We take this "scorched earth"
+ * approach so that we don't have to worry about how long the CPU has
+ * been gone, or whether it ever was online previously.  We do trust the
+ * ->mynode field, as it is constant for a given struct rcu_data and
+ * initialized during early boot.
+ *
+ * Note that only one online or offline event can be happening at a given
+ * time.  Note also that we can accept some slop in the rsp->completed
+ * access due to the fact that this CPU cannot possibly have any RCU
+ * callbacks in flight yet.
+ */
+static void
+rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
+{
+	unsigned long flags;
+	int i;
+	long lastcomp;
+	unsigned long mask;
+	struct rcu_data *rdp = rsp->rda[cpu];
+	struct rcu_node *rnp = rcu_get_root(rsp);
+
+	/* Set up local state, ensuring consistent view of global state. */
+	spin_lock_irqsave(&rnp->lock, flags);
+	lastcomp = rsp->completed;
+	rdp->completed = lastcomp;
+	rdp->gpnum = lastcomp;
+	rdp->passed_quiesc = 0;  /* We could be racing with new GP, */
+	rdp->qs_pending = 1;	 /*  so set up to respond to current GP. */
+	rdp->beenonline = 1;	 /* We have now been online. */
+	rdp->passed_quiesc_completed = lastcomp - 1;
+	rdp->grpmask = 1UL << (cpu - rdp->mynode->grplo);
+	rdp->nxtlist = NULL;
+	for (i = 0; i < RCU_NEXT_SIZE; i++)
+		rdp->nxttail[i] = &rdp->nxtlist;
+	rdp->qlen = 0;
+	rdp->blimit = blimit;
+#ifdef CONFIG_NO_HZ
+	rdp->dynticks = &per_cpu(rcu_dynticks, cpu);
+#endif /* #ifdef CONFIG_NO_HZ */
+	rdp->cpu = cpu;
+	spin_unlock(&rnp->lock);		/* irqs remain disabled. */
+
+	/*
+	 * A new grace period might start here.  If so, we won't be part
+	 * of it, but that is OK, as we are currently in a quiescent state.
+	 */
+
+	/* Exclude any attempts to start a new GP on large systems. */
+	spin_lock(&rsp->onofflock);		/* irqs already disabled. */
+
+	/* Add CPU to rcu_node bitmasks. */
+	rnp = rdp->mynode;
+	mask = rdp->grpmask;
+	do {
+		/* Exclude any attempts to start a new GP on small systems. */
+		spin_lock(&rnp->lock);	/* irqs already disabled. */
+		rnp->qsmaskinit |= mask;
+		mask = rnp->grpmask;
+		spin_unlock(&rnp->lock); /* irqs already disabled. */
+		rnp = rnp->parent;
+	} while (rnp != NULL && !(rnp->qsmaskinit & mask));
+
+	spin_unlock(&rsp->onofflock);		/* irqs remain disabled. */
+
+	/*
+	 * A new grace period might start here.  If so, we will be part of
+	 * it, and its gpnum will be greater than ours, so we will
+	 * participate.  It is also possible for the gpnum to have been
+	 * incremented before this function was called, and the bitmasks
+	 * to not be filled out until now, in which case we will also
+	 * participate due to our gpnum being behind.
+	 */
+
+	/* Since it is coming online, the CPU is in a quiescent state. */
+	cpu_quiet(cpu, rsp, rdp, lastcomp);
+	local_irq_restore(flags);
+}
+
+static void __cpuinit rcu_online_cpu(int cpu)
+{
+#ifdef CONFIG_NO_HZ
+	struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
+
+	rdtp->dynticks_nesting = 1;
+	rdtp->dynticks |= 1; 	/* need consecutive #s even for hotplug. */
+	rdtp->dynticks_nmi = (rdtp->dynticks_nmi + 1) & ~0x1;
+#endif /* #ifdef CONFIG_NO_HZ */
+	rcu_init_percpu_data(cpu, &rcu_state);
+	rcu_init_percpu_data(cpu, &rcu_bh_state);
+	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
+}
+
+/*
+ * Handle CPU online/offline notifcation events.
+ */
+static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
+				unsigned long action, void *hcpu)
+{
+	long cpu = (long)hcpu;
+
+	switch (action) {
+	case CPU_UP_PREPARE:
+	case CPU_UP_PREPARE_FROZEN:
+		rcu_online_cpu(cpu);
+		break;
+	case CPU_DEAD:
+	case CPU_DEAD_FROZEN:
+	case CPU_UP_CANCELED:
+	case CPU_UP_CANCELED_FROZEN:
+		rcu_offline_cpu(cpu);
+		break;
+	default:
+		break;
+	}
+	return NOTIFY_OK;
+}
+
+/*
+ * Compute the per-level fanout, either using the exact fanout specified
+ * or balancing the tree, depending on CONFIG_RCU_FANOUT_EXACT.
+ */
+#ifdef CONFIG_RCU_FANOUT_EXACT
+static void __init rcu_init_levelspread(struct rcu_state *rsp)
+{
+	int i;
+
+	for (i = NUM_RCU_LVLS - 1; i >= 0; i--)
+		rsp->levelspread[i] = CONFIG_RCU_FANOUT;
+}
+#else /* #ifdef CONFIG_RCU_FANOUT_EXACT */
+static void __init rcu_init_levelspread(struct rcu_state *rsp)
+{
+	int ccur;
+	int cprv;
+	int i;
+
+	cprv = NR_CPUS;
+	for (i = NUM_RCU_LVLS - 1; i >= 0; i--) {
+		ccur = rsp->levelcnt[i];
+		rsp->levelspread[i] = (cprv + ccur - 1) / ccur;
+		cprv = ccur;
+	}
+}
+#endif /* #else #ifdef CONFIG_RCU_FANOUT_EXACT */
+
+/*
+ * Helper function for rcu_init() that initializes one rcu_state structure.
+ */
+static void __init rcu_init_one(struct rcu_state *rsp)
+{
+	int cpustride = 1;
+	int i;
+	int j;
+	struct rcu_node *rnp;
+
+	/* Initialize the level-tracking arrays. */
+
+	for (i = 1; i < NUM_RCU_LVLS; i++)
+		rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
+	rcu_init_levelspread(rsp);
+
+	/* Initialize the elements themselves, starting from the leaves. */
+
+	for (i = NUM_RCU_LVLS - 1; i >= 0; i--) {
+		cpustride *= rsp->levelspread[i];
+		rnp = rsp->level[i];
+		for (j = 0; j < rsp->levelcnt[i]; j++, rnp++) {
+			spin_lock_init(&rnp->lock);
+			rnp->qsmask = 0;
+			rnp->qsmaskinit = 0;
+			rnp->grplo = j * cpustride;
+			rnp->grphi = (j + 1) * cpustride - 1;
+			if (rnp->grphi >= NR_CPUS)
+				rnp->grphi = NR_CPUS - 1;
+			if (i == 0) {
+				rnp->grpnum = 0;
+				rnp->grpmask = 0;
+				rnp->parent = NULL;
+			} else {
+				rnp->grpnum = j % rsp->levelspread[i - 1];
+				rnp->grpmask = 1UL << rnp->grpnum;
+				rnp->parent = rsp->level[i - 1] +
+					      j / rsp->levelspread[i - 1];
+			}
+			rnp->level = i;
+		}
+	}
+}
+
+/*
+ * Helper macro for __rcu_init().  To be used nowhere else!
+ * Assigns leaf node pointers into each CPU's rcu_data structure.
+ */
+#define RCU_DATA_PTR_INIT(rsp, rcu_data) \
+do { \
+	rnp = (rsp)->level[NUM_RCU_LVLS - 1]; \
+	j = 0; \
+	for_each_possible_cpu(i) { \
+		if (i > rnp[j].grphi) \
+			j++; \
+		per_cpu(rcu_data, i).mynode = &rnp[j]; \
+		(rsp)->rda[i] = &per_cpu(rcu_data, i); \
+	} \
+} while (0)
+
+static struct notifier_block __cpuinitdata rcu_nb = {
+	.notifier_call	= rcu_cpu_notify,
+};
+
+void __init __rcu_init(void)
+{
+	int i;			/* All used by RCU_DATA_PTR_INIT(). */
+	int j;
+	struct rcu_node *rnp;
+
+	printk(KERN_WARNING "Experimental hierarchical RCU implementation.\n");
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+	printk(KERN_INFO "RCU-based detection of stalled CPUs is enabled.\n");
+#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
+	rcu_init_one(&rcu_state);
+	RCU_DATA_PTR_INIT(&rcu_state, rcu_data);
+	rcu_init_one(&rcu_bh_state);
+	RCU_DATA_PTR_INIT(&rcu_bh_state, rcu_bh_data);
+
+	for_each_online_cpu(i)
+		rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE, (void *)(long)i);
+	/* Register notifier for non-boot CPUs */
+	register_cpu_notifier(&rcu_nb);
+	printk(KERN_WARNING "Experimental hierarchical RCU init done.\n");
+}
+
+module_param(blimit, int, 0);
+module_param(qhimark, int, 0);
+module_param(qlowmark, int, 0);
diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c
new file mode 100644
index 0000000..d6db3e83
--- /dev/null
+++ b/kernel/rcutree_trace.c
@@ -0,0 +1,271 @@
+/*
+ * Read-Copy Update tracing for classic implementation
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * Copyright IBM Corporation, 2008
+ *
+ * Papers:  http://www.rdrop.com/users/paulmck/RCU
+ *
+ * For detailed explanation of Read-Copy Update mechanism see -
+ * 		Documentation/RCU
+ *
+ */
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/spinlock.h>
+#include <linux/smp.h>
+#include <linux/rcupdate.h>
+#include <linux/interrupt.h>
+#include <linux/sched.h>
+#include <asm/atomic.h>
+#include <linux/bitops.h>
+#include <linux/module.h>
+#include <linux/completion.h>
+#include <linux/moduleparam.h>
+#include <linux/percpu.h>
+#include <linux/notifier.h>
+#include <linux/cpu.h>
+#include <linux/mutex.h>
+#include <linux/debugfs.h>
+#include <linux/seq_file.h>
+
+static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
+{
+	if (!rdp->beenonline)
+		return;
+	seq_printf(m, "%3d%cc=%ld g=%ld pq=%d pqc=%ld qp=%d rpfq=%ld rp=%x",
+		   rdp->cpu,
+		   cpu_is_offline(rdp->cpu) ? '!' : ' ',
+		   rdp->completed, rdp->gpnum,
+		   rdp->passed_quiesc, rdp->passed_quiesc_completed,
+		   rdp->qs_pending,
+		   rdp->n_rcu_pending_force_qs - rdp->n_rcu_pending,
+		   (int)(rdp->n_rcu_pending & 0xffff));
+#ifdef CONFIG_NO_HZ
+	seq_printf(m, " dt=%d/%d dn=%d df=%lu",
+		   rdp->dynticks->dynticks,
+		   rdp->dynticks->dynticks_nesting,
+		   rdp->dynticks->dynticks_nmi,
+		   rdp->dynticks_fqs);
+#endif /* #ifdef CONFIG_NO_HZ */
+	seq_printf(m, " of=%lu ri=%lu", rdp->offline_fqs, rdp->resched_ipi);
+	seq_printf(m, " ql=%ld b=%ld\n", rdp->qlen, rdp->blimit);
+}
+
+#define PRINT_RCU_DATA(name, func, m) \
+	do { \
+		int _p_r_d_i; \
+		\
+		for_each_possible_cpu(_p_r_d_i) \
+			func(m, &per_cpu(name, _p_r_d_i)); \
+	} while (0)
+
+static int show_rcudata(struct seq_file *m, void *unused)
+{
+	seq_puts(m, "rcu:\n");
+	PRINT_RCU_DATA(rcu_data, print_one_rcu_data, m);
+	seq_puts(m, "rcu_bh:\n");
+	PRINT_RCU_DATA(rcu_bh_data, print_one_rcu_data, m);
+	return 0;
+}
+
+static int rcudata_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, show_rcudata, NULL);
+}
+
+static struct file_operations rcudata_fops = {
+	.owner = THIS_MODULE,
+	.open = rcudata_open,
+	.read = seq_read,
+	.llseek = seq_lseek,
+	.release = single_release,
+};
+
+static void print_one_rcu_data_csv(struct seq_file *m, struct rcu_data *rdp)
+{
+	if (!rdp->beenonline)
+		return;
+	seq_printf(m, "%d,%s,%ld,%ld,%d,%ld,%d,%ld,%ld",
+		   rdp->cpu,
+		   cpu_is_offline(rdp->cpu) ? "\"Y\"" : "\"N\"",
+		   rdp->completed, rdp->gpnum,
+		   rdp->passed_quiesc, rdp->passed_quiesc_completed,
+		   rdp->qs_pending,
+		   rdp->n_rcu_pending_force_qs - rdp->n_rcu_pending,
+		   rdp->n_rcu_pending);
+#ifdef CONFIG_NO_HZ
+	seq_printf(m, ",%d,%d,%d,%lu",
+		   rdp->dynticks->dynticks,
+		   rdp->dynticks->dynticks_nesting,
+		   rdp->dynticks->dynticks_nmi,
+		   rdp->dynticks_fqs);
+#endif /* #ifdef CONFIG_NO_HZ */
+	seq_printf(m, ",%lu,%lu", rdp->offline_fqs, rdp->resched_ipi);
+	seq_printf(m, ",%ld,%ld\n", rdp->qlen, rdp->blimit);
+}
+
+static int show_rcudata_csv(struct seq_file *m, void *unused)
+{
+	seq_puts(m, "\"CPU\",\"Online?\",\"c\",\"g\",\"pq\",\"pqc\",\"pq\",\"rpfq\",\"rp\",");
+#ifdef CONFIG_NO_HZ
+	seq_puts(m, "\"dt\",\"dt nesting\",\"dn\",\"df\",");
+#endif /* #ifdef CONFIG_NO_HZ */
+	seq_puts(m, "\"of\",\"ri\",\"ql\",\"b\"\n");
+	seq_puts(m, "\"rcu:\"\n");
+	PRINT_RCU_DATA(rcu_data, print_one_rcu_data_csv, m);
+	seq_puts(m, "\"rcu_bh:\"\n");
+	PRINT_RCU_DATA(rcu_bh_data, print_one_rcu_data_csv, m);
+	return 0;
+}
+
+static int rcudata_csv_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, show_rcudata_csv, NULL);
+}
+
+static struct file_operations rcudata_csv_fops = {
+	.owner = THIS_MODULE,
+	.open = rcudata_csv_open,
+	.read = seq_read,
+	.llseek = seq_lseek,
+	.release = single_release,
+};
+
+static void print_one_rcu_state(struct seq_file *m, struct rcu_state *rsp)
+{
+	int level = 0;
+	struct rcu_node *rnp;
+
+	seq_printf(m, "c=%ld g=%ld s=%d jfq=%ld j=%x "
+	              "nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu\n",
+		   rsp->completed, rsp->gpnum, rsp->signaled,
+		   (long)(rsp->jiffies_force_qs - jiffies),
+		   (int)(jiffies & 0xffff),
+		   rsp->n_force_qs, rsp->n_force_qs_ngp,
+		   rsp->n_force_qs - rsp->n_force_qs_ngp,
+		   rsp->n_force_qs_lh);
+	for (rnp = &rsp->node[0]; rnp - &rsp->node[0] < NUM_RCU_NODES; rnp++) {
+		if (rnp->level != level) {
+			seq_puts(m, "\n");
+			level = rnp->level;
+		}
+		seq_printf(m, "%lx/%lx %d:%d ^%d    ",
+			   rnp->qsmask, rnp->qsmaskinit,
+			   rnp->grplo, rnp->grphi, rnp->grpnum);
+	}
+	seq_puts(m, "\n");
+}
+
+static int show_rcuhier(struct seq_file *m, void *unused)
+{
+	seq_puts(m, "rcu:\n");
+	print_one_rcu_state(m, &rcu_state);
+	seq_puts(m, "rcu_bh:\n");
+	print_one_rcu_state(m, &rcu_bh_state);
+	return 0;
+}
+
+static int rcuhier_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, show_rcuhier, NULL);
+}
+
+static struct file_operations rcuhier_fops = {
+	.owner = THIS_MODULE,
+	.open = rcuhier_open,
+	.read = seq_read,
+	.llseek = seq_lseek,
+	.release = single_release,
+};
+
+static int show_rcugp(struct seq_file *m, void *unused)
+{
+	seq_printf(m, "rcu: completed=%ld  gpnum=%ld\n",
+		   rcu_state.completed, rcu_state.gpnum);
+	seq_printf(m, "rcu_bh: completed=%ld  gpnum=%ld\n",
+		   rcu_bh_state.completed, rcu_bh_state.gpnum);
+	return 0;
+}
+
+static int rcugp_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, show_rcugp, NULL);
+}
+
+static struct file_operations rcugp_fops = {
+	.owner = THIS_MODULE,
+	.open = rcugp_open,
+	.read = seq_read,
+	.llseek = seq_lseek,
+	.release = single_release,
+};
+
+static struct dentry *rcudir, *datadir, *datadir_csv, *hierdir, *gpdir;
+static int __init rcuclassic_trace_init(void)
+{
+	rcudir = debugfs_create_dir("rcu", NULL);
+	if (!rcudir)
+		goto out;
+
+	datadir = debugfs_create_file("rcudata", 0444, rcudir,
+						NULL, &rcudata_fops);
+	if (!datadir)
+		goto free_out;
+
+	datadir_csv = debugfs_create_file("rcudata.csv", 0444, rcudir,
+						NULL, &rcudata_csv_fops);
+	if (!datadir_csv)
+		goto free_out;
+
+	gpdir = debugfs_create_file("rcugp", 0444, rcudir, NULL, &rcugp_fops);
+	if (!gpdir)
+		goto free_out;
+
+	hierdir = debugfs_create_file("rcuhier", 0444, rcudir,
+						NULL, &rcuhier_fops);
+	if (!hierdir)
+		goto free_out;
+	return 0;
+free_out:
+	if (datadir)
+		debugfs_remove(datadir);
+	if (datadir_csv)
+		debugfs_remove(datadir_csv);
+	if (gpdir)
+		debugfs_remove(gpdir);
+	debugfs_remove(rcudir);
+out:
+	return 1;
+}
+
+static void __exit rcuclassic_trace_cleanup(void)
+{
+	debugfs_remove(datadir);
+	debugfs_remove(datadir_csv);
+	debugfs_remove(gpdir);
+	debugfs_remove(hierdir);
+	debugfs_remove(rcudir);
+}
+
+
+module_init(rcuclassic_trace_init);
+module_exit(rcuclassic_trace_cleanup);
+
+MODULE_AUTHOR("Paul E. McKenney");
+MODULE_DESCRIPTION("Read-Copy Update tracing for hierarchical implementation");
+MODULE_LICENSE("GPL");
diff --git a/kernel/softirq.c b/kernel/softirq.c
index e7c69a7..80d323e 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -269,6 +269,7 @@
 {
 	int cpu = smp_processor_id();
 
+	rcu_irq_enter();
 	if (idle_cpu(cpu) && !in_interrupt()) {
 		__irq_enter();
 		tick_check_idle(cpu);
@@ -295,9 +296,9 @@
 
 #ifdef CONFIG_NO_HZ
 	/* Make sure that timer wheel updates are propagated */
-	if (!in_interrupt() && idle_cpu(smp_processor_id()) && !need_resched())
-		tick_nohz_stop_sched_tick(0);
 	rcu_irq_exit();
+	if (idle_cpu(smp_processor_id()) && !in_interrupt() && !need_resched())
+		tick_nohz_stop_sched_tick(0);
 #endif
 	preempt_enable_no_resched();
 }
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index b0f239e..465d822 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -619,6 +619,19 @@
 
 	  Say N if you are unsure.
 
+config RCU_CPU_STALL_DETECTOR
+	bool "Check for stalled CPUs delaying RCU grace periods"
+	depends on CLASSIC_RCU || TREE_RCU
+	default n
+	help
+	  This option causes RCU to printk information on which
+	  CPUs are delaying the current grace period, but only when
+	  the grace period extends for excessive time periods.
+
+	  Say Y if you want RCU to perform such checks.
+
+	  Say N if you are unsure.
+
 config KPROBES_SANITY_TEST
 	bool "Kprobes sanity tests"
 	depends on DEBUG_KERNEL