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- .. _rcu_doc:
- 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 listRCU.rst for more
- information on using RCU with linked lists.
- Frequently Asked Questions
- --------------------------
- - 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.
- - 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.
- - 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 UP.rst for more information.
- - 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", "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).
- - What guidelines should I follow when writing code that uses RCU?
- See checklist.rst.
- - Why the name "RCU"?
- "RCU" stands for "read-copy update".
- listRCU.rst has more information on where this name came from, search
- for "read-copy update" to find it.
- - 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 Documentation/RCU/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 (https://liburcu.org/).
- - I hear that RCU needs work in order to support realtime kernels?
- Realtime-friendly RCU can be enabled via the CONFIG_PREEMPT_RCU
- kernel configuration parameter.
- - Where can I find more information on RCU?
- See the Documentation/RCU/RTFP.txt file.
- Or point your browser at (http://www.rdrop.com/users/paulmck/RCU/).
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