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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.

        The other exception would be 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.

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 reads to be reordered), 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 suppress preemption (or bottom halves, in the case of

        rcu_read_lock_bh()) in the read-side critical sections,

        and are also an excellent aid to readability.

        As a rough rule of thumb, any dereference of an RCU-protected

        pointer must be covered by rcu_read_lock() or rcu_read_lock_bh()

        or by the appropriate update-side lock.

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.      Make updates appear atomic to readers.  For example,

                pointer updates to properly aligned fields will appear

                atomic, as will individual atomic primitives.  Operations

                performed under a lock and sequences of multiple atomic

                primitives will -not- appear to be atomic.

                This is almost always the best approach.

        b.      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 i386 CPUs allow reads to be reordered.

        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.

                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.

        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() primitive may be used to

                replace an old structure with a new one in an

                RCU-protected list.

        d.      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(),

        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.

7.      If the updater uses call_rcu(), then the corresponding readers

        must use rcu_read_lock() and rcu_read_unlock().  If the updater

        uses call_rcu_bh(), then the corresponding readers must use

        rcu_read_lock_bh() and rcu_read_unlock_bh().  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 a bit 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.

9.      All RCU list-traversal primitives, which include

        list_for_each_rcu(), list_for_each_entry_rcu(),

        list_for_each_continue_rcu(), and list_for_each_safe_rcu(),

        must be within an RCU read-side critical section.  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().

        Use of the _rcu() list-traversal primitives outside of an

        RCU read-side critical section causes no harm other than

        a slight performance degradation on Alpha CPUs.  It can

        also 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,

        you -must- use the "_rcu()" variants of the list macros.

        Failing to do so will break Alpha and confuse people reading

        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().

        If you want to wait for some of these other things, you might

        instead need to use synchronize_irq() or synchronize_sched().

12.     Any lock acquired by an RCU callback must be acquired elsewhere

        with irq disabled, e.g., via spin_lock_irqsave().  Failing to

        disable irq on a given acquisition of that lock will result in

        deadlock as soon as the RCU callback happens to interrupt 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.

14.     SRCU (srcu_read_lock(), srcu_read_unlock(), and synchronize_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.

        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()

        and synchronize_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 passd 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.