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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.

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", and "synchronize_srcu".

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?

        Yes, work in progress.

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.