Review Checklist for RCU Patches


This document contains a checklist for producing and reviewing patches
that make use of RCU.  Violating any of the rules listed below will
result in the same sorts of problems that leaving out a locking primitive
would cause.  This list is based on experiences reviewing such patches
over a rather long period of time, but improvements are always welcome!

0.	Is RCU being applied to a read-mostly situation?  If the data
	structure is updated more than about 10% of the time, then you
	should strongly consider some other approach, unless detailed
	performance measurements show that RCU is nonetheless the right
	tool for the job.  Yes, RCU does reduce read-side overhead by
	increasing write-side overhead, which is exactly why normal uses
	of RCU will do much more reading than updating.

	Another exception is where performance is not an issue, and RCU
	provides a simpler implementation.  An example of this situation
	is the dynamic NMI code in the Linux 2.6 kernel, at least on
	architectures where NMIs are rare.

	Yet another exception is where the low real-time latency of RCU's
	read-side primitives is critically important.

1.	Does the update code have proper mutual exclusion?

	RCU does allow -readers- to run (almost) naked, but -writers- must
	still use some sort of mutual exclusion, such as:

	a.	locking,
	b.	atomic operations, or
	c.	restricting updates to a single task.

	If you choose #b, be prepared to describe how you have handled
	memory barriers on weakly ordered machines (pretty much all of
	them -- even x86 allows later loads to be reordered to precede
	earlier stores), and be prepared to explain why this added
	complexity is worthwhile.  If you choose #c, be prepared to
	explain how this single task does not become a major bottleneck on
	big multiprocessor machines (for example, if the task is updating
	information relating to itself that other tasks can read, there
	by definition can be no bottleneck).

2.	Do the RCU read-side critical sections make proper use of
	rcu_read_lock() and friends?  These primitives are needed
	to prevent grace periods from ending prematurely, which
	could result in data being unceremoniously freed out from
	under your read-side code, which can greatly increase the
	actuarial risk of your kernel.

	As a rough rule of thumb, any dereference of an RCU-protected
	pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(),
	rcu_read_lock_sched(), or by the appropriate update-side lock.
	Disabling of preemption can serve as rcu_read_lock_sched(), but
	is less readable.

3.	Does the update code tolerate concurrent accesses?

	The whole point of RCU is to permit readers to run without
	any locks or atomic operations.  This means that readers will
	be running while updates are in progress.  There are a number
	of ways to handle this concurrency, depending on the situation:

	a.	Use the RCU variants of the list and hlist update
		primitives to add, remove, and replace elements on
		an RCU-protected list.	Alternatively, use the other
		RCU-protected data structures that have been added to
		the Linux kernel.

		This is almost always the best approach.

	b.	Proceed as in (a) above, but also maintain per-element
		locks (that are acquired by both readers and writers)
		that guard per-element state.  Of course, fields that
		the readers refrain from accessing can be guarded by
		some other lock acquired only by updaters, if desired.

		This works quite well, also.

	c.	Make updates appear atomic to readers.  For example,
		pointer updates to properly aligned fields will
		appear atomic, as will individual atomic primitives.
		Sequences of perations performed under a lock will -not-
		appear to be atomic to RCU readers, nor will sequences
		of multiple atomic primitives.

		This can work, but is starting to get a bit tricky.

	d.	Carefully order the updates and the reads so that
		readers see valid data at all phases of the update.
		This is often more difficult than it sounds, especially
		given modern CPUs' tendency to reorder memory references.
		One must usually liberally sprinkle memory barriers
		(smp_wmb(), smp_rmb(), smp_mb()) through the code,
		making it difficult to understand and to test.

		It is usually better to group the changing data into
		a separate structure, so that the change may be made
		to appear atomic by updating a pointer to reference
		a new structure containing updated values.

4.	Weakly ordered CPUs pose special challenges.  Almost all CPUs
	are weakly ordered -- even x86 CPUs allow later loads to be
	reordered to precede earlier stores.  RCU code must take all of
	the following measures to prevent memory-corruption problems:

	a.	Readers must maintain proper ordering of their memory
		accesses.  The rcu_dereference() primitive ensures that
		the CPU picks up the pointer before it picks up the data
		that the pointer points to.  This really is necessary
		on Alpha CPUs.	If you don't believe me, see:

			http://www.openvms.compaq.com/wizard/wiz_2637.html

		The rcu_dereference() primitive is also an excellent
		documentation aid, letting the person reading the code
		know exactly which pointers are protected by RCU.
		Please note that compilers can also reorder code, and
		they are becoming increasingly aggressive about doing
		just that.  The rcu_dereference() primitive therefore
		also prevents destructive compiler optimizations.

		The rcu_dereference() primitive is used by the
		various "_rcu()" list-traversal primitives, such
		as the list_for_each_entry_rcu().  Note that it is
		perfectly legal (if redundant) for update-side code to
		use rcu_dereference() and the "_rcu()" list-traversal
		primitives.  This is particularly useful in code that
		is common to readers and updaters.  However, neither
		rcu_dereference() nor the "_rcu()" list-traversal
		primitives can substitute for a good concurrency design
		coordinating among multiple updaters.

	b.	If the list macros are being used, the list_add_tail_rcu()
		and list_add_rcu() primitives must be used in order
		to prevent weakly ordered machines from misordering
		structure initialization and pointer planting.
		Similarly, if the hlist macros are being used, the
		hlist_add_head_rcu() primitive is required.

	c.	If the list macros are being used, the list_del_rcu()
		primitive must be used to keep list_del()'s pointer
		poisoning from inflicting toxic effects on concurrent
		readers.  Similarly, if the hlist macros are being used,
		the hlist_del_rcu() primitive is required.

		The list_replace_rcu() and hlist_replace_rcu() primitives
		may be used to replace an old structure with a new one
		in their respective types of RCU-protected lists.

	d.	Rules similar to (4b) and (4c) apply to the "hlist_nulls"
		type of RCU-protected linked lists.

	e.	Updates must ensure that initialization of a given
		structure happens before pointers to that structure are
		publicized.  Use the rcu_assign_pointer() primitive
		when publicizing a pointer to a structure that can
		be traversed by an RCU read-side critical section.

5.	If call_rcu(), or a related primitive such as call_rcu_bh() or
	call_rcu_sched(), is used, the callback function must be
	written to be called from softirq context.  In particular,
	it cannot block.

6.	Since synchronize_rcu() can block, it cannot be called from
	any sort of irq context.  The same rule applies for
	synchronize_rcu_bh(), synchronize_sched(), synchronize_srcu(),
	synchronize_rcu_expedited(), synchronize_rcu_bh_expedited(),
	synchronize_sched_expedite(), and synchronize_srcu_expedited().

	The expedited forms of these primitives have the same semantics
	as the non-expedited forms, but expediting is both expensive
	and unfriendly to real-time workloads.	Use of the expedited
	primitives should be restricted to rare configuration-change
	operations that would not normally be undertaken while a real-time
	workload is running.

7.	If the updater uses call_rcu() or synchronize_rcu(), then the
	corresponding readers must use rcu_read_lock() and
	rcu_read_unlock().  If the updater uses call_rcu_bh() or
	synchronize_rcu_bh(), then the corresponding readers must
	use rcu_read_lock_bh() and rcu_read_unlock_bh().  If the
	updater uses call_rcu_sched() or synchronize_sched(), then
	the corresponding readers must disable preemption, possibly
	by calling rcu_read_lock_sched() and rcu_read_unlock_sched().
	If the updater uses synchronize_srcu(), the the corresponding
	readers must use srcu_read_lock() and srcu_read_unlock(),
	and with the same srcu_struct.	The rules for the expedited
	primitives are the same as for their non-expedited counterparts.
	Mixing things up will result in confusion and broken kernels.

	One exception to this rule: rcu_read_lock() and rcu_read_unlock()
	may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
	in cases where local bottom halves are already known to be
	disabled, for example, in irq or softirq context.  Commenting
	such cases is a must, of course!  And the jury is still out on
	whether the increased speed is worth it.

8.	Although synchronize_rcu() is slower than is call_rcu(), it
	usually results in simpler code.  So, unless update performance
	is critically important or the updaters cannot block,
	synchronize_rcu() should be used in preference to call_rcu().

	An especially important property of the synchronize_rcu()
	primitive is that it automatically self-limits: if grace periods
	are delayed for whatever reason, then the synchronize_rcu()
	primitive will correspondingly delay updates.  In contrast,
	code using call_rcu() should explicitly limit update rate in
	cases where grace periods are delayed, as failing to do so can
	result in excessive realtime latencies or even OOM conditions.

	Ways of gaining this self-limiting property when using call_rcu()
	include:

	a.	Keeping a count of the number of data-structure elements
		used by the RCU-protected data structure, including those
		waiting for a grace period to elapse.  Enforce a limit
		on this number, stalling updates as needed to allow
		previously deferred frees to complete.

		Alternatively, limit only the number awaiting deferred
		free rather than the total number of elements.

	b.	Limiting update rate.  For example, if updates occur only
		once per hour, then no explicit rate limiting is required,
		unless your system is already badly broken.  The dcache
		subsystem takes this approach -- updates are guarded
		by a global lock, limiting their rate.

	c.	Trusted update -- if updates can only be done manually by
		superuser or some other trusted user, then it might not
		be necessary to automatically limit them.  The theory
		here is that superuser already has lots of ways to crash
		the machine.

	d.	Use call_rcu_bh() rather than call_rcu(), in order to take
		advantage of call_rcu_bh()'s faster grace periods.

	e.	Periodically invoke synchronize_rcu(), permitting a limited
		number of updates per grace period.

	The same cautions apply to call_rcu_bh() and call_rcu_sched().

9.	All RCU list-traversal primitives, which include
	rcu_dereference(), list_for_each_entry_rcu(),
	list_for_each_continue_rcu(), and list_for_each_safe_rcu(),
	must be either within an RCU read-side critical section or
	must be protected by appropriate update-side locks.  RCU
	read-side critical sections are delimited by rcu_read_lock()
	and rcu_read_unlock(), or by similar primitives such as
	rcu_read_lock_bh() and rcu_read_unlock_bh().

	The reason that it is permissible to use RCU list-traversal
	primitives when the update-side lock is held is that doing so
	can be quite helpful in reducing code bloat when common code is
	shared between readers and updaters.

10.	Conversely, if you are in an RCU read-side critical section,
	and you don't hold the appropriate update-side lock, you -must-
	use the "_rcu()" variants of the list macros.  Failing to do so
	will break Alpha, cause aggressive compilers to generate bad code,
	and confuse people trying to read your code.

11.	Note that synchronize_rcu() -only- guarantees to wait until
	all currently executing rcu_read_lock()-protected RCU read-side
	critical sections complete.  It does -not- necessarily guarantee
	that all currently running interrupts, NMIs, preempt_disable()
	code, or idle loops will complete.  Therefore, if you do not have
	rcu_read_lock()-protected read-side critical sections, do -not-
	use synchronize_rcu().

	Similarly, disabling preemption is not an acceptable substitute
	for rcu_read_lock().  Code that attempts to use preemption
	disabling where it should be using rcu_read_lock() will break
	in real-time kernel builds.

	If you want to wait for interrupt handlers, NMI handlers, and
	code under the influence of preempt_disable(), you instead
	need to use synchronize_irq() or synchronize_sched().

12.	Any lock acquired by an RCU callback must be acquired elsewhere
	with softirq disabled, e.g., via spin_lock_irqsave(),
	spin_lock_bh(), etc.  Failing to disable irq on a given
	acquisition of that lock will result in deadlock as soon as
	the RCU softirq handler happens to run your RCU callback while
	interrupting that acquisition's critical section.

13.	RCU callbacks can be and are executed in parallel.  In many cases,
	the callback code simply wrappers around kfree(), so that this
	is not an issue (or, more accurately, to the extent that it is
	an issue, the memory-allocator locking handles it).  However,
	if the callbacks do manipulate a shared data structure, they
	must use whatever locking or other synchronization is required
	to safely access and/or modify that data structure.

	RCU callbacks are -usually- executed on the same CPU that executed
	the corresponding call_rcu(), call_rcu_bh(), or call_rcu_sched(),
	but are by -no- means guaranteed to be.  For example, if a given
	CPU goes offline while having an RCU callback pending, then that
	RCU callback will execute on some surviving CPU.  (If this was
	not the case, a self-spawning RCU callback would prevent the
	victim CPU from ever going offline.)

14.	SRCU (srcu_read_lock(), srcu_read_unlock(), synchronize_srcu(),
	and synchronize_srcu_expedited()) may only be invoked from
	process context.  Unlike other forms of RCU, it -is- permissible
	to block in an SRCU read-side critical section (demarked by
	srcu_read_lock() and srcu_read_unlock()), hence the "SRCU":
	"sleepable RCU".  Please note that if you don't need to sleep
	in read-side critical sections, you should be using RCU rather
	than SRCU, because RCU is almost always faster and easier to
	use than is SRCU.

	Also unlike other forms of RCU, explicit initialization
	and cleanup is required via init_srcu_struct() and
	cleanup_srcu_struct().	These are passed a "struct srcu_struct"
	that defines the scope of a given SRCU domain.	Once initialized,
	the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
	synchronize_srcu(), and synchronize_srcu_expedited().  A given
	synchronize_srcu() waits only for SRCU read-side critical
	sections governed by srcu_read_lock() and srcu_read_unlock()
	calls that have been passed the same srcu_struct.  This property
	is what makes sleeping read-side critical sections tolerable --
	a given subsystem delays only its own updates, not those of other
	subsystems using SRCU.	Therefore, SRCU is less prone to OOM the
	system than RCU would be if RCU's read-side critical sections
	were permitted to sleep.

	The ability to sleep in read-side critical sections does not
	come for free.	First, corresponding srcu_read_lock() and
	srcu_read_unlock() calls must be passed the same srcu_struct.
	Second, grace-period-detection overhead is amortized only
	over those updates sharing a given srcu_struct, rather than
	being globally amortized as they are for other forms of RCU.
	Therefore, SRCU should be used in preference to rw_semaphore
	only in extremely read-intensive situations, or in situations
	requiring SRCU's read-side deadlock immunity or low read-side
	realtime latency.

	Note that, rcu_assign_pointer() and rcu_dereference() relate to
	SRCU just as they do to other forms of RCU.

15.	The whole point of call_rcu(), synchronize_rcu(), and friends
	is to wait until all pre-existing readers have finished before
	carrying out some otherwise-destructive operation.  It is
	therefore critically important to -first- remove any path
	that readers can follow that could be affected by the
	destructive operation, and -only- -then- invoke call_rcu(),
	synchronize_rcu(), or friends.

	Because these primitives only wait for pre-existing readers, it
	is the caller's responsibility to guarantee that any subsequent
	readers will execute safely.

16.	The various RCU read-side primitives do -not- necessarily contain
	memory barriers.  You should therefore plan for the CPU
	and the compiler to freely reorder code into and out of RCU
	read-side critical sections.  It is the responsibility of the
	RCU update-side primitives to deal with this.