1RCU 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
16
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
39	Preemptible variants of RCU (CONFIG_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
47o	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
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
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	There are now also LGPL implementations of user-level RCU
79	available (http://lttng.org/?q=node/18).
80
81o	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_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
89o	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
94o	What are all these files in this directory?
95
96	See 00-INDEX for the list.
97