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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.
+
+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, lockdep
+ will complain if you access rcu_dereference() outside
+ of an RCU read-side critical section. See lockdep.txt
+ 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 a related primitive such as call_rcu_bh(),
+ call_rcu_sched(), or call_srcu() 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.
+
+ 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.
+
+ In addition, it is illegal to call the expedited forms from
+ a CPU-hotplug notifier, or while holding a lock that is acquired
+ by a CPU-hotplug notifier. Failing to observe this restriction
+ will result in deadlock.
+
+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() or call_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.
+
+ 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. 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(), 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.txt.
+
+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().
+
+ This same limitation also applies to synchronize_rcu_bh()
+ and synchronize_srcu(), as well as to the asynchronous and
+ expedited forms of the three primitives, namely call_rcu(),
+ call_rcu_bh(), call_srcu(), synchronize_rcu_expedited(),
+ synchronize_rcu_bh_expedited(), and synchronize_srcu_expedited().
+
+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(), srcu_dereference(),
+ synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu())
+ 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.
+
+ If you need to enter your read-side critical section in a
+ hardirq or exception handler, and then exit that same read-side
+ critical section in the task that was interrupted, then you need
+ to srcu_read_lock_raw() and srcu_read_unlock_raw(), which avoid
+ the lockdep checking that would otherwise this practice illegal.
+
+ 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(), 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.
+
+ Note that, rcu_assign_pointer() relates to SRCU just as it does
+ 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.
+
+17. Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and
+ the __rcu sparse checks to validate your RCU code. These
+ can help find problems as follows:
+
+ CONFIG_PROVE_RCU: 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.