1.. _rcu_doc:
2
3RCU Concepts
4============
5
6The basic idea behind RCU (read-copy update) is to split destructive
7operations into two parts, one that prevents anyone from seeing the data
8item being destroyed, and one that actually carries out the destruction.
9A "grace period" must elapse between the two parts, and this grace period
10must be long enough that any readers accessing the item being deleted have
11since dropped their references.  For example, an RCU-protected deletion
12from a linked list would first remove the item from the list, wait for
13a grace period to elapse, then free the element.  See listRCU.rst for more
14information on using RCU with linked lists.
15
16Frequently Asked Questions
17--------------------------
18
19- Why would anyone want to use RCU?
20
21  The advantage of RCU's two-part approach is that RCU readers need
22  not acquire any locks, perform any atomic instructions, write to
23  shared memory, or (on CPUs other than Alpha) execute any memory
24  barriers.  The fact that these operations are quite expensive
25  on modern CPUs is what gives RCU its performance advantages
26  in read-mostly situations.  The fact that RCU readers need not
27  acquire locks can also greatly simplify deadlock-avoidance code.
28
29- How can the updater tell when a grace period has completed
30  if the RCU readers give no indication when they are done?
31
32  Just as with spinlocks, RCU readers are not permitted to
33  block, switch to user-mode execution, or enter the idle loop.
34  Therefore, as soon as a CPU is seen passing through any of these
35  three states, we know that that CPU has exited any previous RCU
36  read-side critical sections.  So, if we remove an item from a
37  linked list, and then wait until all CPUs have switched context,
38  executed in user mode, or executed in the idle loop, we can
39  safely free up that item.
40
41  Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the
42  same effect, but require that the readers manipulate CPU-local
43  counters.  These counters allow limited types of blocking within
44  RCU read-side critical sections.  SRCU also uses CPU-local
45  counters, and permits general blocking within RCU read-side
46  critical sections.  These variants of RCU detect grace periods
47  by sampling these counters.
48
49- If I am running on a uniprocessor kernel, which can only do one
50  thing at a time, why should I wait for a grace period?
51
52  See UP.rst for more information.
53
54- How can I see where RCU is currently used in the Linux kernel?
55
56  Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
57  "rcu_read_lock_bh", "rcu_read_unlock_bh", "srcu_read_lock",
58  "srcu_read_unlock", "synchronize_rcu", "synchronize_net",
59  "synchronize_srcu", and the other RCU primitives.  Or grab one
60  of the cscope databases from:
61
62  (http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html).
63
64- What guidelines should I follow when writing code that uses RCU?
65
66  See checklist.rst.
67
68- Why the name "RCU"?
69
70  "RCU" stands for "read-copy update".
71  listRCU.rst has more information on where this name came from, search
72  for "read-copy update" to find it.
73
74- I hear that RCU is patented?  What is with that?
75
76  Yes, it is.  There are several known patents related to RCU,
77  search for the string "Patent" in Documentation/RCU/RTFP.txt to find them.
78  Of these, one was allowed to lapse by the assignee, and the
79  others have been contributed to the Linux kernel under GPL.
80  There are now also LGPL implementations of user-level RCU
81  available (https://liburcu.org/).
82
83- I hear that RCU needs work in order to support realtime kernels?
84
85  Realtime-friendly RCU can be enabled via the CONFIG_PREEMPT_RCU
86  kernel configuration parameter.
87
88- Where can I find more information on RCU?
89
90  See the Documentation/RCU/RTFP.txt file.
91  Or point your browser at (http://www.rdrop.com/users/paulmck/RCU/).
92