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.

	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 two variants of
	RCU detect grace periods by sampling these counters.

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",
	"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

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	I hear that RCU needs work in order to support realtime kernels?

	This work is largely completed.  Realtime-friendly RCU can be
	enabled via the CONFIG_PREEMPT_RCU kernel configuration parameter.
	However, work is in progress for enabling priority boosting of
	preempted RCU read-side critical sections.  This is needed if you
	have CPU-bound realtime threads.

o	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/.

o	What are all these files in this directory?


	NMI-RCU.txt

		Describes how to use RCU to implement dynamic
		NMI handlers, which can be revectored on the fly,
		without rebooting.

	RTFP.txt

		List of RCU-related publications and web sites.

	UP.txt

		Discussion of RCU usage in UP kernels.

	arrayRCU.txt

		Describes how to use RCU to protect arrays, with
		resizeable arrays whose elements reference other
		data structures being of the most interest.

	checklist.txt

		Lists things to check for when inspecting code that
		uses RCU.

	listRCU.txt

		Describes how to use RCU to protect linked lists.
		This is the simplest and most common use of RCU
		in the Linux kernel.

	rcu.txt

		You are reading it!

	rcuref.txt

		Describes how to combine use of reference counts
		with RCU.

	whatisRCU.txt

		Overview of how the RCU implementation works.  Along
		the way, presents a conceptual view of RCU.