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>
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RCU Concepts
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.. _rcu_doc:
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RCU Concepts
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============
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The basic idea behind RCU (read-copy update) is to split destructive
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operations into two parts, one that prevents anyone from seeing the data
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must be long enough that any readers accessing the item being deleted have
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since dropped their references. For example, an RCU-protected deletion
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from a linked list would first remove the item from the list, wait for
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a grace period to elapse, then free the element. See the listRCU.txt
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file for more information on using RCU with linked lists.
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a grace period to elapse, then free the element. See the
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Documentation/RCU/listRCU.rst file for more information on using RCU with
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linked lists.
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Frequently Asked Questions
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--------------------------
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o Why would anyone want to use RCU?
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- Why would anyone want to use RCU?
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The advantage of RCU's two-part approach is that RCU readers need
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not acquire any locks, perform any atomic instructions, write to
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in read-mostly situations. The fact that RCU readers need not
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acquire locks can also greatly simplify deadlock-avoidance code.
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o How can the updater tell when a grace period has completed
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- How can the updater tell when a grace period has completed
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if the RCU readers give no indication when they are done?
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Just as with spinlocks, RCU readers are not permitted to
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critical sections. These variants of RCU detect grace periods
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by sampling these counters.
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o If I am running on a uniprocessor kernel, which can only do one
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- If I am running on a uniprocessor kernel, which can only do one
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thing at a time, why should I wait for a grace period?
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See the UP.txt file in this directory.
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See the Documentation/RCU/UP.rst file for more information.
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o How can I see where RCU is currently used in the Linux kernel?
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- How can I see where RCU is currently used in the Linux kernel?
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Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
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"rcu_read_lock_bh", "rcu_read_unlock_bh", "srcu_read_lock",
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"synchronize_srcu", and the other RCU primitives. Or grab one
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of the cscope databases from:
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http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html
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(http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html).
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o What guidelines should I follow when writing code that uses RCU?
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- What guidelines should I follow when writing code that uses RCU?
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See the checklist.txt file in this directory.
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o Why the name "RCU"?
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- Why the name "RCU"?
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"RCU" stands for "read-copy update". The file listRCU.txt has
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more information on where this name came from, search for
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"RCU" stands for "read-copy update". The file Documentation/RCU/listRCU.rst
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has more information on where this name came from, search for
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"read-copy update" to find it.
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o I hear that RCU is patented? What is with that?
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- I hear that RCU is patented? What is with that?
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Yes, it is. There are several known patents related to RCU,
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search for the string "Patent" in RTFP.txt to find them.
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There are now also LGPL implementations of user-level RCU
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available (http://liburcu.org/).
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o I hear that RCU needs work in order to support realtime kernels?
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- I hear that RCU needs work in order to support realtime kernels?
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Realtime-friendly RCU can be enabled via the CONFIG_PREEMPT_RCU
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kernel configuration parameter.
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o Where can I find more information on RCU?
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- Where can I find more information on RCU?
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See the RTFP.txt file in this directory.
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Or point your browser at http://www.rdrop.com/users/paulmck/RCU/.
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Or point your browser at (http://www.rdrop.com/users/paulmck/RCU/).
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