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Documentation: RCU: Convert RCU basic concepts to reST 

RCU basic concepts reST markup.

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Jiunn Chang <c0d1n61at3@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Jiunn Chang 2019-06-26 15:07:01 -05:00 committed by Jonathan Corbet
parent 7c116d22ad
commit a9f0969cd7
1 changed files with 61 additions and 58 deletions
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119
Documentation/RCU/rcu.txt
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@ -1,5 +1,7 @@
RCU Concepts .. _rcu_doc:
RCU Concepts
============
The basic idea behind RCU (read-copy update) is to split destructive The basic idea behind RCU (read-copy update) is to split destructive
operations into two parts, one that prevents anyone from seeing the data operations into two parts, one that prevents anyone from seeing the data
@ -8,82 +10,83 @@ 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 must be long enough that any readers accessing the item being deleted have
since dropped their references. For example, an RCU-protected deletion since dropped their references. For example, an RCU-protected deletion
from a linked list would first remove the item from the list, wait for 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 a grace period to elapse, then free the element. See the
file for more information on using RCU with linked lists. Documentation/RCU/listRCU.rst file for more information on using RCU with
linked lists.
Frequently Asked Questions Frequently Asked Questions
--------------------------
o Why would anyone want to use RCU? - Why would anyone want to use RCU?
The advantage of RCU's two-part approach is that RCU readers need The advantage of RCU's two-part approach is that RCU readers need
not acquire any locks, perform any atomic instructions, write to not acquire any locks, perform any atomic instructions, write to
shared memory, or (on CPUs other than Alpha) execute any memory shared memory, or (on CPUs other than Alpha) execute any memory
barriers. The fact that these operations are quite expensive barriers. The fact that these operations are quite expensive
on modern CPUs is what gives RCU its performance advantages on modern CPUs is what gives RCU its performance advantages
in read-mostly situations. The fact that RCU readers need not in read-mostly situations. The fact that RCU readers need not
acquire locks can also greatly simplify deadlock-avoidance code. acquire locks can also greatly simplify deadlock-avoidance code.
o How can the updater tell when a grace period has completed - How can the updater tell when a grace period has completed
if the RCU readers give no indication when they are done? if the RCU readers give no indication when they are done?
Just as with spinlocks, RCU readers are not permitted to Just as with spinlocks, RCU readers are not permitted to
block, switch to user-mode execution, or enter the idle loop. block, switch to user-mode execution, or enter the idle loop.
Therefore, as soon as a CPU is seen passing through any of these 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 three states, we know that that CPU has exited any previous RCU
read-side critical sections. So, if we remove an item from a read-side critical sections. So, if we remove an item from a
linked list, and then wait until all CPUs have switched context, linked list, and then wait until all CPUs have switched context,
executed in user mode, or executed in the idle loop, we can executed in user mode, or executed in the idle loop, we can
safely free up that item. safely free up that item.
Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the
same effect, but require that the readers manipulate CPU-local same effect, but require that the readers manipulate CPU-local
counters. These counters allow limited types of blocking within counters. These counters allow limited types of blocking within
RCU read-side critical sections. SRCU also uses CPU-local RCU read-side critical sections. SRCU also uses CPU-local
counters, and permits general blocking within RCU read-side counters, and permits general blocking within RCU read-side
critical sections. These variants of RCU detect grace periods critical sections. These variants of RCU detect grace periods
by sampling these counters. by sampling these counters.
o If I am running on a uniprocessor kernel, which can only do one - 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? thing at a time, why should I wait for a grace period?
See the UP.txt file in this directory. See the Documentation/RCU/UP.rst file for more information.
o How can I see where RCU is currently used in the Linux kernel? - How can I see where RCU is currently used in the Linux kernel?
Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu", Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
"rcu_read_lock_bh", "rcu_read_unlock_bh", "srcu_read_lock", "rcu_read_lock_bh", "rcu_read_unlock_bh", "srcu_read_lock",
"srcu_read_unlock", "synchronize_rcu", "synchronize_net", "srcu_read_unlock", "synchronize_rcu", "synchronize_net",
"synchronize_srcu", and the other RCU primitives. Or grab one "synchronize_srcu", and the other RCU primitives. Or grab one
of the cscope databases from: of the cscope databases from:
http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html (http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html).
o What guidelines should I follow when writing code that uses RCU? - What guidelines should I follow when writing code that uses RCU?
See the checklist.txt file in this directory. See the checklist.txt file in this directory.
o Why the name "RCU"? - Why the name "RCU"?
"RCU" stands for "read-copy update". The file listRCU.txt has "RCU" stands for "read-copy update". The file Documentation/RCU/listRCU.rst
more information on where this name came from, search for has more information on where this name came from, search for
"read-copy update" to find it. "read-copy update" to find it.
o I hear that RCU is patented? What is with that? - I hear that RCU is patented? What is with that?
Yes, it is. There are several known patents related to RCU, Yes, it is. There are several known patents related to RCU,
search for the string "Patent" in RTFP.txt to find them. search for the string "Patent" in RTFP.txt to find them.
Of these, one was allowed to lapse by the assignee, and the Of these, one was allowed to lapse by the assignee, and the
others have been contributed to the Linux kernel under GPL. others have been contributed to the Linux kernel under GPL.
There are now also LGPL implementations of user-level RCU There are now also LGPL implementations of user-level RCU
available (http://liburcu.org/). available (http://liburcu.org/).
o I hear that RCU needs work in order to support realtime kernels? - I hear that RCU needs work in order to support realtime kernels?
Realtime-friendly RCU can be enabled via the CONFIG_PREEMPT_RCU Realtime-friendly RCU can be enabled via the CONFIG_PREEMPT_RCU
kernel configuration parameter. kernel configuration parameter.
o Where can I find more information on RCU? - Where can I find more information on RCU?
See the RTFP.txt file in this directory. See the RTFP.txt file in this directory.
Or point your browser at http://www.rdrop.com/users/paulmck/RCU/. Or point your browser at (http://www.rdrop.com/users/paulmck/RCU/).