rcu: 1Q2010 update for RCU documentation

[net-next-2.6.git] / Documentation / RCU / checklist.txt

Review Checklist for RCU Patches


This document contains a checklist for producing and reviewing patches
that make use of RCU.  Violating any of the rules listed below will
result in the same sorts of problems that leaving out a locking primitive
would cause.  This list is based on experiences reviewing such patches
over a rather long period of time, but improvements are always welcome!

0.      Is RCU being applied to a read-mostly situation?  If the data
        structure is updated more than about 10% of the time, then you
        should strongly consider some other approach, unless detailed
        performance measurements show that RCU is nonetheless the right
        tool for the job.  Yes, RCU does reduce read-side overhead by
        increasing write-side overhead, which is exactly why normal uses
        of RCU will do much more reading than updating.

        Another exception is where performance is not an issue, and RCU
        provides a simpler implementation.  An example of this situation
        is the dynamic NMI code in the Linux 2.6 kernel, at least on
        architectures where NMIs are rare.

        Yet another exception is where the low real-time latency of RCU's
        read-side primitives is critically important.

1.      Does the update code have proper mutual exclusion?

        RCU does allow -readers- to run (almost) naked, but -writers- must
        still use some sort of mutual exclusion, such as:

        a.      locking,
        b.      atomic operations, or
        c.      restricting updates to a single task.

        If you choose #b, be prepared to describe how you have handled
        memory barriers on weakly ordered machines (pretty much all of
        them -- even x86 allows later loads to be reordered to precede
        earlier stores), and be prepared to explain why this added
        complexity is worthwhile.  If you choose #c, be prepared to
        explain how this single task does not become a major bottleneck on
        big multiprocessor machines (for example, if the task is updating
        information relating to itself that other tasks can read, there
        by definition can be no bottleneck).

2.      Do the RCU read-side critical sections make proper use of
        rcu_read_lock() and friends?  These primitives are needed
        to prevent grace periods from ending prematurely, which
        could result in data being unceremoniously freed out from
        under your read-side code, which can greatly increase the
        actuarial risk of your kernel.

        As a rough rule of thumb, any dereference of an RCU-protected
        pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(),
        rcu_read_lock_sched(), or by the appropriate update-side lock.
        Disabling of preemption can serve as rcu_read_lock_sched(), but
        is less readable.

3.      Does the update code tolerate concurrent accesses?

        The whole point of RCU is to permit readers to run without
        any locks or atomic operations.  This means that readers will
        be running while updates are in progress.  There are a number
        of ways to handle this concurrency, depending on the situation:

        a.      Use the RCU variants of the list and hlist update
                primitives to add, remove, and replace elements on
                an RCU-protected list.  Alternatively, use the other
                RCU-protected data structures that have been added to
                the Linux kernel.

                This is almost always the best approach.

        b.      Proceed as in (a) above, but also maintain per-element
                locks (that are acquired by both readers and writers)
                that guard per-element state.  Of course, fields that
                the readers refrain from accessing can be guarded by
                some other lock acquired only by updaters, if desired.

                This works quite well, also.

        c.      Make updates appear atomic to readers.  For example,
                pointer updates to properly aligned fields will
                appear atomic, as will individual atomic primitives.
                Sequences of perations performed under a lock will -not-
                appear to be atomic to RCU readers, nor will sequences
                of multiple atomic primitives.

                This can work, but is starting to get a bit tricky.

        d.      Carefully order the updates and the reads so that
                readers see valid data at all phases of the update.
                This is often more difficult than it sounds, especially
                given modern CPUs' tendency to reorder memory references.
                One must usually liberally sprinkle memory barriers
                (smp_wmb(), smp_rmb(), smp_mb()) through the code,
                making it difficult to understand and to test.

                It is usually better to group the changing data into
                a separate structure, so that the change may be made
                to appear atomic by updating a pointer to reference
                a new structure containing updated values.

4.      Weakly ordered CPUs pose special challenges.  Almost all CPUs
        are weakly ordered -- even x86 CPUs allow later loads to be
        reordered to precede earlier stores.  RCU code must take all of
        the following measures to prevent memory-corruption problems:

        a.      Readers must maintain proper ordering of their memory
                accesses.  The rcu_dereference() primitive ensures that
                the CPU picks up the pointer before it picks up the data
                that the pointer points to.  This really is necessary
                on Alpha CPUs.  If you don't believe me, see:

                        http://www.openvms.compaq.com/wizard/wiz_2637.html

                The rcu_dereference() primitive is also an excellent
                documentation aid, letting the person reading the code
                know exactly which pointers are protected by RCU.
                Please note that compilers can also reorder code, and
                they are becoming increasingly aggressive about doing
                just that.  The rcu_dereference() primitive therefore
                also prevents destructive compiler optimizations.

                The rcu_dereference() primitive is used by the
                various "_rcu()" list-traversal primitives, such
                as the list_for_each_entry_rcu().  Note that it is
                perfectly legal (if redundant) for update-side code to
                use rcu_dereference() and the "_rcu()" list-traversal
                primitives.  This is particularly useful in code that
                is common to readers and updaters.  However, neither
                rcu_dereference() nor the "_rcu()" list-traversal
                primitives can substitute for a good concurrency design
                coordinating among multiple updaters.

        b.      If the list macros are being used, the list_add_tail_rcu()
                and list_add_rcu() primitives must be used in order
                to prevent weakly ordered machines from misordering
                structure initialization and pointer planting.
                Similarly, if the hlist macros are being used, the
                hlist_add_head_rcu() primitive is required.

        c.      If the list macros are being used, the list_del_rcu()
                primitive must be used to keep list_del()'s pointer
                poisoning from inflicting toxic effects on concurrent
                readers.  Similarly, if the hlist macros are being used,
                the hlist_del_rcu() primitive is required.

                The list_replace_rcu() and hlist_replace_rcu() primitives
                may be used to replace an old structure with a new one
                in their respective types of RCU-protected lists.

        d.      Rules similar to (4b) and (4c) apply to the "hlist_nulls"
                type of RCU-protected linked lists.

        e.      Updates must ensure that initialization of a given
                structure happens before pointers to that structure are
                publicized.  Use the rcu_assign_pointer() primitive
                when publicizing a pointer to a structure that can
                be traversed by an RCU read-side critical section.

5.      If call_rcu(), or a related primitive such as call_rcu_bh() or
        call_rcu_sched(), is used, the callback function must be
        written to be called from softirq context.  In particular,
        it cannot block.

6.      Since synchronize_rcu() can block, it cannot be called from
        any sort of irq context.  The same rule applies for
        synchronize_rcu_bh(), synchronize_sched(), synchronize_srcu(),
        synchronize_rcu_expedited(), synchronize_rcu_bh_expedited(),
        synchronize_sched_expedite(), and synchronize_srcu_expedited().

        The expedited forms of these primitives have the same semantics
        as the non-expedited forms, but expediting is both expensive
        and unfriendly to real-time workloads.  Use of the expedited
        primitives should be restricted to rare configuration-change
        operations that would not normally be undertaken while a real-time
        workload is running.

7.      If the updater uses call_rcu() or synchronize_rcu(), then the
        corresponding readers must use rcu_read_lock() and
        rcu_read_unlock().  If the updater uses call_rcu_bh() or
        synchronize_rcu_bh(), then the corresponding readers must
        use rcu_read_lock_bh() and rcu_read_unlock_bh().  If the
        updater uses call_rcu_sched() or synchronize_sched(), then
        the corresponding readers must disable preemption, possibly
        by calling rcu_read_lock_sched() and rcu_read_unlock_sched().
        If the updater uses synchronize_srcu(), the the corresponding
        readers must use srcu_read_lock() and srcu_read_unlock(),
        and with the same srcu_struct.  The rules for the expedited
        primitives are the same as for their non-expedited counterparts.
        Mixing things up will result in confusion and broken kernels.

        One exception to this rule: rcu_read_lock() and rcu_read_unlock()
        may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
        in cases where local bottom halves are already known to be
        disabled, for example, in irq or softirq context.  Commenting
        such cases is a must, of course!  And the jury is still out on
        whether the increased speed is worth it.

8.      Although synchronize_rcu() is slower than is call_rcu(), it
        usually results in simpler code.  So, unless update performance
        is critically important or the updaters cannot block,
        synchronize_rcu() should be used in preference to call_rcu().

        An especially important property of the synchronize_rcu()
        primitive is that it automatically self-limits: if grace periods
        are delayed for whatever reason, then the synchronize_rcu()
        primitive will correspondingly delay updates.  In contrast,
        code using call_rcu() should explicitly limit update rate in
        cases where grace periods are delayed, as failing to do so can
        result in excessive realtime latencies or even OOM conditions.

        Ways of gaining this self-limiting property when using call_rcu()
        include:

        a.      Keeping a count of the number of data-structure elements
                used by the RCU-protected data structure, including those
                waiting for a grace period to elapse.  Enforce a limit
                on this number, stalling updates as needed to allow
                previously deferred frees to complete.

                Alternatively, limit only the number awaiting deferred
                free rather than the total number of elements.

        b.      Limiting update rate.  For example, if updates occur only
                once per hour, then no explicit rate limiting is required,
                unless your system is already badly broken.  The dcache
                subsystem takes this approach -- updates are guarded
                by a global lock, limiting their rate.

        c.      Trusted update -- if updates can only be done manually by
                superuser or some other trusted user, then it might not
                be necessary to automatically limit them.  The theory
                here is that superuser already has lots of ways to crash
                the machine.

        d.      Use call_rcu_bh() rather than call_rcu(), in order to take
                advantage of call_rcu_bh()'s faster grace periods.

        e.      Periodically invoke synchronize_rcu(), permitting a limited
                number of updates per grace period.

        The same cautions apply to call_rcu_bh() and call_rcu_sched().

9.      All RCU list-traversal primitives, which include
        rcu_dereference(), list_for_each_entry_rcu(),
        list_for_each_continue_rcu(), and list_for_each_safe_rcu(),
        must be either within an RCU read-side critical section or
        must be protected by appropriate update-side locks.  RCU
        read-side critical sections are delimited by rcu_read_lock()
        and rcu_read_unlock(), or by similar primitives such as
        rcu_read_lock_bh() and rcu_read_unlock_bh().

        The reason that it is permissible to use RCU list-traversal
        primitives when the update-side lock is held is that doing so
        can be quite helpful in reducing code bloat when common code is
        shared between readers and updaters.

10.     Conversely, if you are in an RCU read-side critical section,
        and you don't hold the appropriate update-side lock, you -must-
        use the "_rcu()" variants of the list macros.  Failing to do so
        will break Alpha, cause aggressive compilers to generate bad code,
        and confuse people trying to read your code.

11.     Note that synchronize_rcu() -only- guarantees to wait until
        all currently executing rcu_read_lock()-protected RCU read-side
        critical sections complete.  It does -not- necessarily guarantee
        that all currently running interrupts, NMIs, preempt_disable()
        code, or idle loops will complete.  Therefore, if you do not have
        rcu_read_lock()-protected read-side critical sections, do -not-
        use synchronize_rcu().

        Similarly, disabling preemption is not an acceptable substitute
        for rcu_read_lock().  Code that attempts to use preemption
        disabling where it should be using rcu_read_lock() will break
        in real-time kernel builds.

        If you want to wait for interrupt handlers, NMI handlers, and
        code under the influence of preempt_disable(), you instead
        need to use synchronize_irq() or synchronize_sched().

12.     Any lock acquired by an RCU callback must be acquired elsewhere
        with softirq disabled, e.g., via spin_lock_irqsave(),
        spin_lock_bh(), etc.  Failing to disable irq on a given
        acquisition of that lock will result in deadlock as soon as
        the RCU softirq handler happens to run your RCU callback while
        interrupting that acquisition's critical section.

13.     RCU callbacks can be and are executed in parallel.  In many cases,
        the callback code simply wrappers around kfree(), so that this
        is not an issue (or, more accurately, to the extent that it is
        an issue, the memory-allocator locking handles it).  However,
        if the callbacks do manipulate a shared data structure, they
        must use whatever locking or other synchronization is required
        to safely access and/or modify that data structure.

        RCU callbacks are -usually- executed on the same CPU that executed
        the corresponding call_rcu(), call_rcu_bh(), or call_rcu_sched(),
        but are by -no- means guaranteed to be.  For example, if a given
        CPU goes offline while having an RCU callback pending, then that
        RCU callback will execute on some surviving CPU.  (If this was
        not the case, a self-spawning RCU callback would prevent the
        victim CPU from ever going offline.)

14.     SRCU (srcu_read_lock(), srcu_read_unlock(), synchronize_srcu(),
        and synchronize_srcu_expedited()) may only be invoked from
        process context.  Unlike other forms of RCU, it -is- permissible
        to block in an SRCU read-side critical section (demarked by
        srcu_read_lock() and srcu_read_unlock()), hence the "SRCU":
        "sleepable RCU".  Please note that if you don't need to sleep
        in read-side critical sections, you should be using RCU rather
        than SRCU, because RCU is almost always faster and easier to
        use than is SRCU.

        Also unlike other forms of RCU, explicit initialization
        and cleanup is required via init_srcu_struct() and
        cleanup_srcu_struct().  These are passed a "struct srcu_struct"
        that defines the scope of a given SRCU domain.  Once initialized,
        the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
        synchronize_srcu(), and synchronize_srcu_expedited().  A given
        synchronize_srcu() waits only for SRCU read-side critical
        sections governed by srcu_read_lock() and srcu_read_unlock()
        calls that have been passed the same srcu_struct.  This property
        is what makes sleeping read-side critical sections tolerable --
        a given subsystem delays only its own updates, not those of other
        subsystems using SRCU.  Therefore, SRCU is less prone to OOM the
        system than RCU would be if RCU's read-side critical sections
        were permitted to sleep.

        The ability to sleep in read-side critical sections does not
        come for free.  First, corresponding srcu_read_lock() and
        srcu_read_unlock() calls must be passed the same srcu_struct.
        Second, grace-period-detection overhead is amortized only
        over those updates sharing a given srcu_struct, rather than
        being globally amortized as they are for other forms of RCU.
        Therefore, SRCU should be used in preference to rw_semaphore
        only in extremely read-intensive situations, or in situations
        requiring SRCU's read-side deadlock immunity or low read-side
        realtime latency.

        Note that, rcu_assign_pointer() and rcu_dereference() relate to
        SRCU just as they do to other forms of RCU.

15.     The whole point of call_rcu(), synchronize_rcu(), and friends
        is to wait until all pre-existing readers have finished before
        carrying out some otherwise-destructive operation.  It is
        therefore critically important to -first- remove any path
        that readers can follow that could be affected by the
        destructive operation, and -only- -then- invoke call_rcu(),
        synchronize_rcu(), or friends.

        Because these primitives only wait for pre-existing readers, it
        is the caller's responsibility to guarantee that any subsequent
        readers will execute safely.

16.     The various RCU read-side primitives do -not- necessarily contain
        memory barriers.  You should therefore plan for the CPU
        and the compiler to freely reorder code into and out of RCU
        read-side critical sections.  It is the responsibility of the
        RCU update-side primitives to deal with this.