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. 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", "synchronize_rcu", and "synchronize_net". 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. o Where can I find more information on RCU? See the RTFP.txt file in this directory.