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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

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		Preemptible variants of RCU (CONFIG_TREE_PREEMPT_RCU) get the
40		same effect, but require that the readers manipulate CPU-local
41		counters.  These counters allow limited types of blocking within
42		RCU read-side critical sections.  SRCU also uses CPU-local
43		counters, and permits general blocking within RCU read-side
44		critical sections.  These variants of RCU detect grace periods
45		by sampling these counters.
46	
47	o	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	
52	o	How can I see where RCU is currently used in the Linux kernel?
53	
54		Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
55		"rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
56		"srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
57		"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
61	
62	o	What guidelines should I follow when writing code that uses RCU?
63	
64		See the checklist.txt file in this directory.
65	
66	o	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	
72	o	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		There are now also LGPL implementations of user-level RCU
79		available (http://lttng.org/?q=node/18).
80	
81	o	I hear that RCU needs work in order to support realtime kernels?
82	
83		This work is largely completed.  Realtime-friendly RCU can be
84		enabled via the CONFIG_TREE_PREEMPT_RCU kernel configuration
85		parameter.  However, work is in progress for enabling priority
86		boosting of preempted RCU read-side critical sections.	This is
87		needed if you have CPU-bound realtime threads.
88	
89	o	Where can I find more information on RCU?
90	
91		See the RTFP.txt file in this directory.
92		Or point your browser at http://www.rdrop.com/users/paulmck/RCU/.
93	
94	o	What are all these files in this directory?
95	
96		See 00-INDEX for the list.
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