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:
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?
Describes how to use RCU to implement dynamic
NMI handlers, which can be revectored on the fly,
List of RCU-related publications and web sites.
Discussion of RCU usage in UP kernels.
Describes how to use RCU to protect arrays, with
resizeable arrays whose elements reference other
data structures being of the most interest.
Lists things to check for when inspecting code that
Describes how to use RCU to protect linked lists.
This is the simplest and most common use of RCU
in the Linux kernel.
You are reading it!
Describes how to combine use of reference counts
Overview of how the RCU implementation works. Along
the way, presents a conceptual view of RCU.