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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 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? 41 42 See the UP.txt file in this directory. 43 44o How can I see where RCU is currently used in the Linux kernel? 45 46 Search for "rcu_read_lock", "call_rcu", and "synchronize_kernel". 47 48o What guidelines should I follow when writing code that uses RCU? 49 50 See the checklist.txt file in this directory. 51 52o Why the name "RCU"? 53 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. 57 58o I hear that RCU is patented? What is with that? 59 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. 64 65o Where can I find more information on RCU? 66 67 See the RTFP.txt file in this directory.