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Linus Torvalds1da177e2005-04-16 15:20:36 -07001RCU Concepts
2
3
4The basic idea behind RCU (read-copy update) is to split destructive
5operations into two parts, one that prevents anyone from seeing the data
6item being destroyed, and one that actually carries out the destruction.
7A "grace period" must elapse between the two parts, and this grace period
8must be long enough that any readers accessing the item being deleted have
9since dropped their references. For example, an RCU-protected deletion
10from a linked list would first remove the item from the list, wait for
11a grace period to elapse, then free the element. See the listRCU.txt
12file for more information on using RCU with linked lists.
13
14
15Frequently Asked Questions
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17o Why would anyone want to use RCU?
18
19 The advantage of RCU's two-part approach is that RCU readers need
20 not acquire any locks, perform any atomic instructions, write to
21 shared memory, or (on CPUs other than Alpha) execute any memory
22 barriers. The fact that these operations are quite expensive
23 on modern CPUs is what gives RCU its performance advantages
24 in read-mostly situations. The fact that RCU readers need not
25 acquire locks can also greatly simplify deadlock-avoidance code.
26
27o How can the updater tell when a grace period has completed
28 if the RCU readers give no indication when they are done?
29
30 Just as with spinlocks, RCU readers are not permitted to
31 block, switch to user-mode execution, or enter the idle loop.
32 Therefore, as soon as a CPU is seen passing through any of these
33 three states, we know that that CPU has exited any previous RCU
34 read-side critical sections. So, if we remove an item from a
35 linked list, and then wait until all CPUs have switched context,
36 executed in user mode, or executed in the idle loop, we can
37 safely free up that item.
38
Paul E. McKenney6b3ef482009-08-22 13:56:53 -070039 Preemptible variants of RCU (CONFIG_TREE_PREEMPT_RCU) get the
Paul E. McKenneyf85d6c72008-01-25 21:08:25 +010040 same effect, but require that the readers manipulate CPU-local
41 counters. These counters allow limited types of blocking
42 within RCU read-side critical sections. SRCU also uses
43 CPU-local counters, and permits general blocking within
44 RCU read-side critical sections. These two variants of
45 RCU detect grace periods by sampling these counters.
46
Linus Torvalds1da177e2005-04-16 15:20:36 -070047o If I am running on a uniprocessor kernel, which can only do one
48 thing at a time, why should I wait for a grace period?
49
50 See the UP.txt file in this directory.
51
52o How can I see where RCU is currently used in the Linux kernel?
53
Paul E. McKenneya83f1fe2005-05-01 08:59:05 -070054 Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
55 "rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
Paul E. McKenney621934e2006-10-04 02:17:02 -070056 "srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
Paul E. McKenneyf85d6c72008-01-25 21:08:25 +010057 "synchronize_net", "synchronize_srcu", and the other RCU
58 primitives. Or grab one of the cscope databases from:
59
60 http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html
Linus Torvalds1da177e2005-04-16 15:20:36 -070061
62o What guidelines should I follow when writing code that uses RCU?
63
64 See the checklist.txt file in this directory.
65
66o Why the name "RCU"?
67
68 "RCU" stands for "read-copy update". The file listRCU.txt has
69 more information on where this name came from, search for
70 "read-copy update" to find it.
71
72o I hear that RCU is patented? What is with that?
73
74 Yes, it is. There are several known patents related to RCU,
75 search for the string "Patent" in RTFP.txt to find them.
76 Of these, one was allowed to lapse by the assignee, and the
77 others have been contributed to the Linux kernel under GPL.
78
Paul E. McKenneydd81eca2005-09-10 00:26:24 -070079o I hear that RCU needs work in order to support realtime kernels?
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Paul E. McKenneyf85d6c72008-01-25 21:08:25 +010081 This work is largely completed. Realtime-friendly RCU can be
Paul E. McKenney6b3ef482009-08-22 13:56:53 -070082 enabled via the CONFIG_TREE_PREEMPT_RCU kernel configuration
83 parameter. However, work is in progress for enabling priority
84 boosting of preempted RCU read-side critical sections. This is
85 needed if you have CPU-bound realtime threads.
Paul E. McKenneydd81eca2005-09-10 00:26:24 -070086
Linus Torvalds1da177e2005-04-16 15:20:36 -070087o Where can I find more information on RCU?
88
89 See the RTFP.txt file in this directory.
Paul E. McKenneydd81eca2005-09-10 00:26:24 -070090 Or point your browser at http://www.rdrop.com/users/paulmck/RCU/.
91
92o What are all these files in this directory?
93
94
95 NMI-RCU.txt
96
97 Describes how to use RCU to implement dynamic
98 NMI handlers, which can be revectored on the fly,
99 without rebooting.
100
101 RTFP.txt
102
103 List of RCU-related publications and web sites.
104
105 UP.txt
106
107 Discussion of RCU usage in UP kernels.
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109 arrayRCU.txt
110
111 Describes how to use RCU to protect arrays, with
112 resizeable arrays whose elements reference other
113 data structures being of the most interest.
114
115 checklist.txt
116
117 Lists things to check for when inspecting code that
118 uses RCU.
119
120 listRCU.txt
121
122 Describes how to use RCU to protect linked lists.
123 This is the simplest and most common use of RCU
124 in the Linux kernel.
125
126 rcu.txt
127
128 You are reading it!
129
Paul E. McKenneyd19720a2006-02-01 03:06:42 -0800130 rcuref.txt
131
132 Describes how to combine use of reference counts
133 with RCU.
134
Paul E. McKenneydd81eca2005-09-10 00:26:24 -0700135 whatisRCU.txt
136
137 Overview of how the RCU implementation works. Along
138 the way, presents a conceptual view of RCU.