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diff --git a/Documentation/RCU/rcu.txt b/Documentation/RCU/rcu.txt new file mode 100644 index 000000000..745f429fd --- /dev/null +++ b/Documentation/RCU/rcu.txt @@ -0,0 +1,96 @@ +RCU Concepts + + +The basic idea behind RCU (read-copy update) is to split destructive +operations into two parts, one that prevents anyone from seeing the data +item being destroyed, and one that actually carries out the destruction. +A "grace period" must elapse between the two parts, and this grace period +must be long enough that any readers accessing the item being deleted have +since dropped their references. For example, an RCU-protected deletion +from a linked list would first remove the item from the list, wait for +a grace period to elapse, then free the element. See the listRCU.txt +file for more information on using RCU with linked lists. + + +Frequently Asked Questions + +o Why would anyone want to use RCU? + + The advantage of RCU's two-part approach is that RCU readers need + not acquire any locks, perform any atomic instructions, write to + shared memory, or (on CPUs other than Alpha) execute any memory + barriers. The fact that these operations are quite expensive + on modern CPUs is what gives RCU its performance advantages + in read-mostly situations. The fact that RCU readers need not + acquire locks can also greatly simplify deadlock-avoidance code. + +o How can the updater tell when a grace period has completed + if the RCU readers give no indication when they are done? + + Just as with spinlocks, RCU readers are not permitted to + block, switch to user-mode execution, or enter the idle loop. + Therefore, as soon as a CPU is seen passing through any of these + three states, we know that that CPU has exited any previous RCU + read-side critical sections. So, if we remove an item from a + linked list, and then wait until all CPUs have switched context, + executed in user mode, or executed in the idle loop, we can + safely free up that item. + + Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the + same effect, but require that the readers manipulate CPU-local + counters. These counters allow limited types of blocking within + RCU read-side critical sections. SRCU also uses CPU-local + counters, and permits general blocking within RCU read-side + critical sections. These variants of RCU detect grace periods + by sampling these counters. + +o If I am running on a uniprocessor kernel, which can only do one + thing at a time, why should I wait for a grace period? + + See the UP.txt file in this directory. + +o How can I see where RCU is currently used in the Linux kernel? + + Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu", + "rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh", + "srcu_read_lock", "srcu_read_unlock", "synchronize_rcu", + "synchronize_net", "synchronize_srcu", and the other RCU + primitives. Or grab one of the cscope databases from: + + http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html + +o What guidelines should I follow when writing code that uses RCU? + + See the checklist.txt file in this directory. + +o Why the name "RCU"? + + "RCU" stands for "read-copy update". The file listRCU.txt has + more information on where this name came from, search for + "read-copy update" to find it. + +o I hear that RCU is patented? What is with that? + + Yes, it is. There are several known patents related to RCU, + search for the string "Patent" in RTFP.txt to find them. + Of these, one was allowed to lapse by the assignee, and the + others have been contributed to the Linux kernel under GPL. + There are now also LGPL implementations of user-level RCU + available (http://lttng.org/?q=node/18). + +o I hear that RCU needs work in order to support realtime kernels? + + This work is largely completed. Realtime-friendly RCU can be + enabled via the CONFIG_PREEMPT_RCU kernel configuration + parameter. However, work is in progress for enabling priority + boosting of preempted RCU read-side critical sections. This is + needed if you have CPU-bound realtime threads. + +o Where can I find more information on RCU? + + See the RTFP.txt file in this directory. + Or point your browser at http://www.rdrop.com/users/paulmck/RCU/. + +o What are all these files in this directory? + + See 00-INDEX for the list. |