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 ================================
 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 provides the same potential simplifications 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
         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).  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.
         Disabling of preemption can serve as rcu_read_lock_sched(), but
         is less readable and prevents lockdep from detecting locking issues.
 
         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.  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 operations 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.
 
                 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 call_rcu() or call_srcu() is used, the callback function will
         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_srcu(), synchronize_rcu_expedited(), 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
         (with the exception of synchronize_srcu_expedited()) 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.
         However, real-time workloads can use 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.
 
 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.
 
         If the updater uses call_rcu_tasks() or synchronize_rcu_tasks(),
         then the readers must refrain from executing voluntary
         context switches, that is, from blocking.  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().  If an updater uses call_rcu_tasks_rude()
         or synchronize_rcu_tasks_rude(), then the corresponding readers
         must use anything that disables interrupts.
 
         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() usually results in even simpler code than does
         synchronize_rcu() without synchronize_rcu()'s multi-millisecond
         latency.  So please take advantage of kfree_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()
         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 synchronize_rcu(), permitting a limited
                 number of updates per grace period.
 
         The same cautions apply to call_srcu() and kfree_rcu().
 
         Note that although these primitives do take action to avoid memory
         exhaustion when any given CPU has too many callbacks, a determined
         user could 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 read your code.
 
 11.     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 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() or call_srcu().
         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.
 
 13.     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 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().
 
         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).
 
         __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 register a callback using call_rcu() or call_srcu(), and
         pass in a function defined within a loadable module, then it in
         necessary to wait for all pending callbacks to be invoked after
         the last invocation and before unloading that module.  Note that
         it is absolutely *not* sufficient to wait for a grace period!
         The current (say) synchronize_rcu() implementation is *not*
         guaranteed to wait for callbacks registered on other CPUs.
         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()
 
         However, these barrier functions are absolutely *not* guaranteed
         to wait for a grace period.  In fact, if there are no call_rcu()
         callbacks waiting anywhere in the system, rcu_barrier() is within
         its rights to return immediately.
 
         So if you need to wait for both an RCU grace period and for
         all pre-existing call_rcu() callbacks, you will need to execute
         both rcu_barrier() and synchronize_rcu(), if necessary, using
         something like workqueues to to execute them concurrently.
 
         See rcubarrier.rst for more information.

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