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[releases.git] / Documentation / RCU / checklist.rst

.. SPDX-License-Identifier: GPL-2.0

================================
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.

        One final exception is where RCU readers are used to prevent
        the ABA problem (https://en.wikipedia.org/wiki/ABA_problem)
        for lockless updates.  This does result in the mildly
        counter-intuitive situation where rcu_read_lock() and
        rcu_read_unlock() are used to protect updates, however, this
        approach can provide the same simplifications to certain types
        of lockless algorithms that garbage collectors do.

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 large systems (for example, if the task is updating information
        relating to itself that other tasks can read, there by definition
        can be no bottleneck).  Note that the definition of "large" has
        changed significantly:  Eight CPUs was "large" in the year 2000,
        but a hundred CPUs was unremarkable in 2017.

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.
        Explicit disabling of preemption (preempt_disable(), for example)
        can serve as rcu_read_lock_sched(), but is less readable and
        prevents lockdep from detecting locking issues.  Acquiring a
        spinlock also enters an RCU read-side critical section.

        Please note that you *cannot* rely on code known to be built
        only in non-preemptible kernels.  Such code can and will break,
        especially in kernels built with CONFIG_PREEMPT_COUNT=y.

        Letting RCU-protected pointers "leak" out of an RCU read-side
        critical section is every bit as bad as letting them leak out
        from under a lock.  Unless, of course, you have arranged some
        other means of protection, such as a lock or a reference count
        *before* letting them out of the RCU read-side critical section.

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.  Fields that the readers
                refrain from accessing can be guarded by some other lock
                acquired only by updaters, if desired.

                This also works quite well.

        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 operations performed under a lock will *not*
                appear to be atomic to RCU readers, nor will sequences
                of multiple atomic primitives.  One alternative is to
                move multiple individual fields to a separate structure,
                thus solving the multiple-field problem by imposing an
                additional level of indirection.

                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-ordering operations
                through the code, making it difficult to understand and
                to test.  Where it works, it is better to use things
                like smp_store_release() and smp_load_acquire(), but in
                some cases the smp_mb() full memory barrier is required.

                As noted earlier, 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.

                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.  However,
                with a bit of devious creativity, it is possible to
                mishandle the return value from rcu_dereference().
                Please see rcu_dereference.rst for more information.

                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, lockdep
                will complain if you access rcu_dereference() outside
                of an RCU read-side critical section.  See lockdep.rst
                to learn what to do about this.

                Of course, 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 any of call_rcu(), call_srcu(), call_rcu_tasks(),
        call_rcu_tasks_rude(), or call_rcu_tasks_trace() is used,
        the callback function may be invoked from softirq context,
        and in any case with bottom halves disabled.  In particular,
        this callback function cannot block.  If you need the callback
        to block, run that code in a workqueue handler scheduled from
        the callback.  The queue_rcu_work() function does this for you
        in the case of call_rcu().

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

        The expedited forms of these primitives have the same semantics
        as the non-expedited forms, but expediting is more CPU intensive.
        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.  Note that
        IPI-sensitive real-time workloads can use the rcupdate.rcu_normal
        kernel boot parameter to completely disable expedited grace
        periods, though this might have performance implications.

        In particular, if you find yourself invoking one of the expedited
        primitives repeatedly in a loop, please do everyone a favor:
        Restructure your code so that it batches the updates, allowing
        a single non-expedited primitive to cover the entire batch.
        This will very likely be faster than the loop containing the
        expedited primitive, and will be much much easier on the rest
        of the system, especially to real-time workloads running on the
        rest of the system.  Alternatively, instead use asynchronous
        primitives such as call_rcu().

7.      As of v4.20, a given kernel implements only one RCU flavor, which
        is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y.
        If the updater uses call_rcu() or synchronize_rcu(), then
        the corresponding readers may use:  (1) rcu_read_lock() and
        rcu_read_unlock(), (2) any pair of primitives that disables
        and re-enables softirq, for example, rcu_read_lock_bh() and
        rcu_read_unlock_bh(), or (3) any pair of primitives that disables
        and re-enables preemption, for example, rcu_read_lock_sched() and
        rcu_read_unlock_sched().  If the updater uses synchronize_srcu()
        or call_srcu(), then the corresponding readers must use
        srcu_read_lock() and srcu_read_unlock(), and with the same
        srcu_struct.  The rules for the expedited RCU grace-period-wait
        primitives are the same as for their non-expedited counterparts.

        Similarly, it is necessary to correctly use the RCU Tasks flavors:

        a.      If the updater uses synchronize_rcu_tasks() or
                call_rcu_tasks(), then the readers must refrain from
                executing voluntary context switches, that is, from
                blocking.

        b.      If the updater uses call_rcu_tasks_trace()
                or synchronize_rcu_tasks_trace(), then the
                corresponding readers must use rcu_read_lock_trace()
                and rcu_read_unlock_trace().

        c.      If an updater uses call_rcu_tasks_rude() or
                synchronize_rcu_tasks_rude(), then the corresponding
                readers must use anything that disables preemption,
                for example, preempt_disable() and preempt_enable().

        Mixing things up will result in confusion and broken kernels, and
        has even resulted in an exploitable security issue.  Therefore,
        when using non-obvious pairs of primitives, commenting is
        of course a must.  One example of non-obvious pairing is
        the XDP feature in networking, which calls BPF programs from
        network-driver NAPI (softirq) context.  BPF relies heavily on RCU
        protection for its data structures, but because the BPF program
        invocation happens entirely within a single local_bh_disable()
        section in a NAPI poll cycle, this usage is safe.  The reason
        that this usage is safe is that readers can use anything that
        disables BH when updaters use call_rcu() or synchronize_rcu().

8.      Although synchronize_rcu() is slower than is call_rcu(),
        it usually results in simpler code.  So, unless update
        performance is critically important, the updaters cannot block,
        or the latency of synchronize_rcu() is visible from userspace,
        synchronize_rcu() should be used in preference to call_rcu().
        Furthermore, kfree_rcu() and kvfree_rcu() usually result
        in even simpler code than does synchronize_rcu() without
        synchronize_rcu()'s multi-millisecond latency.  So please take
        advantage of kfree_rcu()'s and kvfree_rcu()'s "fire and forget"
        memory-freeing capabilities where it applies.

        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(),
        kfree_rcu(), or kvfree_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.

                One way to stall the updates is to acquire the update-side
                mutex.  (Don't try this with a spinlock -- other CPUs
                spinning on the lock could prevent the grace period
                from ever ending.)  Another way to stall the updates
                is for the updates to use a wrapper function around
                the memory allocator, so that this wrapper function
                simulates OOM when there is too much memory awaiting an
                RCU grace period.  There are of course many other
                variations on this theme.

        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.
                Older versions of the dcache subsystem take this approach,
                guarding updates with 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.      Periodically invoke rcu_barrier(), permitting a limited
                number of updates per grace period.

        The same cautions apply to call_srcu(), call_rcu_tasks(),
        call_rcu_tasks_rude(), and call_rcu_tasks_trace().  This is
        why there is an srcu_barrier(), rcu_barrier_tasks(),
        rcu_barrier_tasks_rude(), and rcu_barrier_tasks_rude(),
        respectively.

        Note that although these primitives do take action to avoid
        memory exhaustion when any given CPU has too many callbacks,
        a determined user or administrator can still exhaust memory.
        This is especially the case if a system with a large number of
        CPUs has been configured to offload all of its RCU callbacks onto
        a single CPU, or if the system has relatively little free memory.

9.      All RCU list-traversal primitives, which include
        rcu_dereference(), list_for_each_entry_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(), in which
        case the matching rcu_dereference() primitive must be used in
        order to keep lockdep happy, in this case, rcu_dereference_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.  Additional primitives
        are provided for this case, as discussed in lockdep.rst.

        One exception to this rule is when data is only ever added to
        the linked data structure, and is never removed during any
        time that readers might be accessing that structure.  In such
        cases, READ_ONCE() may be used in place of rcu_dereference()
        and the read-side markers (rcu_read_lock() and rcu_read_unlock(),
        for example) may be omitted.

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 understand your code.

11.     Any lock acquired by an RCU callback must be acquired elsewhere
        with softirq disabled, e.g., via spin_lock_bh().  Failing to
        disable softirq 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.

12.     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.

        Do not assume that RCU callbacks will be executed on
        the same CPU that executed the corresponding call_rcu(),
        call_srcu(), call_rcu_tasks(), call_rcu_tasks_rude(), or
        call_rcu_tasks_trace().  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.)  Furthermore, CPUs designated by rcu_nocbs=
        might well *always* have their RCU callbacks executed on some
        other CPUs, in fact, for some  real-time workloads, this is the
        whole point of using the rcu_nocbs= kernel boot parameter.

        In addition, do not assume that callbacks queued in a given order
        will be invoked in that order, even if they all are queued on the
        same CPU.  Furthermore, do not assume that same-CPU callbacks will
        be invoked serially.  For example, in recent kernels, CPUs can be
        switched between offloaded and de-offloaded callback invocation,
        and while a given CPU is undergoing such a switch, its callbacks
        might be concurrently invoked by that CPU's softirq handler and
        that CPU's rcuo kthread.  At such times, that CPU's callbacks
        might be executed both concurrently and out of order.

13.     Unlike most flavors 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 either at build time via DEFINE_SRCU()
        or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct()
        and cleanup_srcu_struct().  These last two 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(),
        synchronize_srcu_expedited(), and call_srcu().  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.  You should also consider percpu_rw_semaphore
        when you need lightweight readers.

        SRCU's expedited primitive (synchronize_srcu_expedited())
        never sends IPIs to other CPUs, so it is easier on
        real-time workloads than is synchronize_rcu_expedited().

        It is also permissible to sleep in RCU Tasks Trace read-side
        critical section, which are delimited by rcu_read_lock_trace() and
        rcu_read_unlock_trace().  However, this is a specialized flavor
        of RCU, and you should not use it without first checking with
        its current users.  In most cases, you should instead use SRCU.

        Note that rcu_assign_pointer() relates to SRCU just as it does to
        other forms of RCU, but instead of rcu_dereference() you should
        use srcu_dereference() in order to avoid lockdep splats.

14.     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.

15.     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.

        For SRCU readers, you can use smp_mb__after_srcu_read_unlock()
        immediately after an srcu_read_unlock() to get a full barrier.

16.     Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
        __rcu sparse checks to validate your RCU code.  These can help
        find problems as follows:

        CONFIG_PROVE_LOCKING:
                check that accesses to RCU-protected data structures
                are carried out under the proper RCU read-side critical
                section, while holding the right combination of locks,
                or whatever other conditions are appropriate.

        CONFIG_DEBUG_OBJECTS_RCU_HEAD:
                check that you don't pass the same object to call_rcu()
                (or friends) before an RCU grace period has elapsed
                since the last time that you passed that same object to
                call_rcu() (or friends).

        CONFIG_RCU_STRICT_GRACE_PERIOD:
                combine with KASAN to check for pointers leaked out
                of RCU read-side critical sections.  This Kconfig
                option is tough on both performance and scalability,
                and so is limited to four-CPU systems.

        __rcu sparse checks:
                tag the pointer to the RCU-protected data structure
                with __rcu, and sparse will warn you if you access that
                pointer without the services of one of the variants
                of rcu_dereference().

        These debugging aids can help you find problems that are
        otherwise extremely difficult to spot.

17.     If you pass a callback function defined within a module to one of
        call_rcu(), call_srcu(), call_rcu_tasks(), call_rcu_tasks_rude(),
        or call_rcu_tasks_trace(), then it is necessary to wait for all
        pending callbacks to be invoked before unloading that module.
        Note that it is absolutely *not* sufficient to wait for a grace
        period!  For example, synchronize_rcu() implementation is *not*
        guaranteed to wait for callbacks registered on other CPUs via
        call_rcu().  Or even on the current CPU if that CPU recently
        went offline and came back online.

        You instead need to use one of the barrier functions:

        -       call_rcu() -> rcu_barrier()
        -       call_srcu() -> srcu_barrier()
        -       call_rcu_tasks() -> rcu_barrier_tasks()
        -       call_rcu_tasks_rude() -> rcu_barrier_tasks_rude()
        -       call_rcu_tasks_trace() -> rcu_barrier_tasks_trace()

        However, these barrier functions are absolutely *not* guaranteed
        to wait for a grace period.  For example, if there are no
        call_rcu() callbacks queued anywhere in the system, rcu_barrier()
        can and will return immediately.

        So if you need to wait for both a grace period and for all
        pre-existing callbacks, you will need to invoke both functions,
        with the pair depending on the flavor of RCU:

        -       Either synchronize_rcu() or synchronize_rcu_expedited(),
                together with rcu_barrier()
        -       Either synchronize_srcu() or synchronize_srcu_expedited(),
                together with and srcu_barrier()
        -       synchronize_rcu_tasks() and rcu_barrier_tasks()
        -       synchronize_tasks_rude() and rcu_barrier_tasks_rude()
        -       synchronize_tasks_trace() and rcu_barrier_tasks_trace()

        If necessary, you can use something like workqueues to execute
        the requisite pair of functions concurrently.

        See rcubarrier.rst for more information.