Documentation: RCU: Convert RCU basic concepts to reST

RCU Concepts

.. _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

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.

a grace period to elapse, then free the element.  See the

Documentation/RCU/listRCU.rst file for more information on using RCU with

linked lists.

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

  not acquire any locks, perform any atomic instructions, write to

  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

- 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

  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

- 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.

  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",

  "rcu_read_lock_bh", "rcu_read_unlock_bh", "srcu_read_lock",

  "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

  (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.

o	Why the name "RCU"?

- 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

  "RCU" stands for "read-copy update".  The file Documentation/RCU/listRCU.rst

  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?

- 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.

  There are now also LGPL implementations of user-level RCU

  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

  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.

	Or point your browser at http://www.rdrop.com/users/paulmck/RCU/.

  Or point your browser at (http://www.rdrop.com/users/paulmck/RCU/).