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 1RCU Concepts
 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.
15Frequently Asked Questions
17o	Why would anyone want to use RCU?
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.
27o	How can the updater tell when a grace period has completed
28	if the RCU readers give no indication when they are done?
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.
39o	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?
42	See the UP.txt file in this directory.
44o	How can I see where RCU is currently used in the Linux kernel?
46	Search for "rcu_read_lock", "call_rcu", and "synchronize_kernel".
48o	What guidelines should I follow when writing code that uses RCU?
50	See the checklist.txt file in this directory.
52o	Why the name "RCU"?
54	"RCU" stands for "read-copy update".  The file listRCU.txt has
55	more information on where this name came from, search for
56	"read-copy update" to find it.
58o	I hear that RCU is patented?  What is with that?
60	Yes, it is.  There are several known patents related to RCU,
61	search for the string "Patent" in RTFP.txt to find them.
62	Of these, one was allowed to lapse by the assignee, and the
63	others have been contributed to the Linux kernel under GPL.
65o	Where can I find more information on RCU?
67	See the RTFP.txt file in this directory.