1 RCU Concepts
2
3
4 The basic idea behind RCU (read-copy update) is to split destructive
5 operations into two parts, one that prevents anyone from seeing the data
6 item being destroyed, and one that actually carries out the destruction.
7 A "grace period" must elapse between the two parts, and this grace period
8 must be long enough that any readers accessing the item being deleted have
9 since dropped their references. For example, an RCU-protected deletion
10 from a linked list would first remove the item from the list, wait for
11 a grace period to elapse, then free the element. See the listRCU.txt
12 file for more information on using RCU with linked lists.
13
14
15 Frequently Asked Questions
16
17 o 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
27 o 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
39 o If I am running on a uniprocessor kernel, which can only do one
40 thing at a time, why should I wait for a grace period?
41
42 See the UP.txt file in this directory.
43
44 o How can I see where RCU is currently used in the Linux kernel?
45
46 Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
47 "rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
48 "synchronize_rcu", and "synchronize_net".
49
50 o What guidelines should I follow when writing code that uses RCU?
51
52 See the checklist.txt file in this directory.
53
54 o Why the name "RCU"?
55
56 "RCU" stands for "read-copy update". The file listRCU.txt has
57 more information on where this name came from, search for
58 "read-copy update" to find it.
59
60 o I hear that RCU is patented? What is with that?
61
62 Yes, it is. There are several known patents related to RCU,
63 search for the string "Patent" in RTFP.txt to find them.
64 Of these, one was allowed to lapse by the assignee, and the
65 others have been contributed to the Linux kernel under GPL.
66
67 o I hear that RCU needs work in order to support realtime kernels?
68
69 Yes, work in progress.
70
71 o Where can I find more information on RCU?
72
73 See the RTFP.txt file in this directory.
74 Or point your browser at http://www.rdrop.com/users/paulmck/RCU/.
75
76 o What are all these files in this directory?
77
78
79 NMI-RCU.txt
80
81 Describes how to use RCU to implement dynamic
82 NMI handlers, which can be revectored on the fly,
83 without rebooting.
84
85 RTFP.txt
86
87 List of RCU-related publications and web sites.
88
89 UP.txt
90
91 Discussion of RCU usage in UP kernels.
92
93 arrayRCU.txt
94
95 Describes how to use RCU to protect arrays, with
96 resizeable arrays whose elements reference other
97 data structures being of the most interest.
98
99 checklist.txt
100
101 Lists things to check for when inspecting code that
102 uses RCU.
103
104 listRCU.txt
105
106 Describes how to use RCU to protect linked lists.
107 This is the simplest and most common use of RCU
108 in the Linux kernel.
109
110 rcu.txt
111
112 You are reading it!
113
114 rcuref.txt
115
116 Describes how to combine use of reference counts
117 with RCU.
118
119 whatisRCU.txt
120
121 Overview of how the RCU implementation works. Along
122 the way, presents a conceptual view of RCU.
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