483 lines
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ReStructuredText
483 lines
23 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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================================
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Review Checklist for RCU Patches
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================================
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This document contains a checklist for producing and reviewing patches
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that make use of RCU. Violating any of the rules listed below will
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result in the same sorts of problems that leaving out a locking primitive
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would cause. This list is based on experiences reviewing such patches
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over a rather long period of time, but improvements are always welcome!
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0. Is RCU being applied to a read-mostly situation? If the data
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structure is updated more than about 10% of the time, then you
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should strongly consider some other approach, unless detailed
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performance measurements show that RCU is nonetheless the right
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tool for the job. Yes, RCU does reduce read-side overhead by
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increasing write-side overhead, which is exactly why normal uses
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of RCU will do much more reading than updating.
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Another exception is where performance is not an issue, and RCU
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provides a simpler implementation. An example of this situation
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is the dynamic NMI code in the Linux 2.6 kernel, at least on
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architectures where NMIs are rare.
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Yet another exception is where the low real-time latency of RCU's
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read-side primitives is critically important.
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One final exception is where RCU readers are used to prevent
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the ABA problem (https://en.wikipedia.org/wiki/ABA_problem)
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for lockless updates. This does result in the mildly
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counter-intuitive situation where rcu_read_lock() and
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rcu_read_unlock() are used to protect updates, however, this
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approach provides the same potential simplifications that garbage
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collectors do.
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1. Does the update code have proper mutual exclusion?
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RCU does allow *readers* to run (almost) naked, but *writers* must
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still use some sort of mutual exclusion, such as:
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a. locking,
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b. atomic operations, or
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c. restricting updates to a single task.
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If you choose #b, be prepared to describe how you have handled
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memory barriers on weakly ordered machines (pretty much all of
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them -- even x86 allows later loads to be reordered to precede
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earlier stores), and be prepared to explain why this added
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complexity is worthwhile. If you choose #c, be prepared to
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explain how this single task does not become a major bottleneck on
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big multiprocessor machines (for example, if the task is updating
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information relating to itself that other tasks can read, there
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by definition can be no bottleneck). Note that the definition
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of "large" has changed significantly: Eight CPUs was "large"
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in the year 2000, but a hundred CPUs was unremarkable in 2017.
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2. Do the RCU read-side critical sections make proper use of
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rcu_read_lock() and friends? These primitives are needed
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to prevent grace periods from ending prematurely, which
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could result in data being unceremoniously freed out from
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under your read-side code, which can greatly increase the
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actuarial risk of your kernel.
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As a rough rule of thumb, any dereference of an RCU-protected
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pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(),
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rcu_read_lock_sched(), or by the appropriate update-side lock.
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Disabling of preemption can serve as rcu_read_lock_sched(), but
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is less readable and prevents lockdep from detecting locking issues.
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Letting RCU-protected pointers "leak" out of an RCU read-side
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critical section is every bit as bad as letting them leak out
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from under a lock. Unless, of course, you have arranged some
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other means of protection, such as a lock or a reference count
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*before* letting them out of the RCU read-side critical section.
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3. Does the update code tolerate concurrent accesses?
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The whole point of RCU is to permit readers to run without
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any locks or atomic operations. This means that readers will
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be running while updates are in progress. There are a number
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of ways to handle this concurrency, depending on the situation:
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a. Use the RCU variants of the list and hlist update
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primitives to add, remove, and replace elements on
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an RCU-protected list. Alternatively, use the other
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RCU-protected data structures that have been added to
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the Linux kernel.
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This is almost always the best approach.
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b. Proceed as in (a) above, but also maintain per-element
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locks (that are acquired by both readers and writers)
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that guard per-element state. Of course, fields that
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the readers refrain from accessing can be guarded by
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some other lock acquired only by updaters, if desired.
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This works quite well, also.
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c. Make updates appear atomic to readers. For example,
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pointer updates to properly aligned fields will
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appear atomic, as will individual atomic primitives.
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Sequences of operations performed under a lock will *not*
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appear to be atomic to RCU readers, nor will sequences
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of multiple atomic primitives.
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This can work, but is starting to get a bit tricky.
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d. Carefully order the updates and the reads so that
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readers see valid data at all phases of the update.
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This is often more difficult than it sounds, especially
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given modern CPUs' tendency to reorder memory references.
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One must usually liberally sprinkle memory barriers
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(smp_wmb(), smp_rmb(), smp_mb()) through the code,
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making it difficult to understand and to test.
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It is usually better to group the changing data into
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a separate structure, so that the change may be made
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to appear atomic by updating a pointer to reference
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a new structure containing updated values.
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4. Weakly ordered CPUs pose special challenges. Almost all CPUs
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are weakly ordered -- even x86 CPUs allow later loads to be
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reordered to precede earlier stores. RCU code must take all of
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the following measures to prevent memory-corruption problems:
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a. Readers must maintain proper ordering of their memory
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accesses. The rcu_dereference() primitive ensures that
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the CPU picks up the pointer before it picks up the data
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that the pointer points to. This really is necessary
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on Alpha CPUs.
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The rcu_dereference() primitive is also an excellent
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documentation aid, letting the person reading the
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code know exactly which pointers are protected by RCU.
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Please note that compilers can also reorder code, and
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they are becoming increasingly aggressive about doing
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just that. The rcu_dereference() primitive therefore also
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prevents destructive compiler optimizations. However,
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with a bit of devious creativity, it is possible to
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mishandle the return value from rcu_dereference().
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Please see rcu_dereference.rst for more information.
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The rcu_dereference() primitive is used by the
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various "_rcu()" list-traversal primitives, such
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as the list_for_each_entry_rcu(). Note that it is
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perfectly legal (if redundant) for update-side code to
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use rcu_dereference() and the "_rcu()" list-traversal
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primitives. This is particularly useful in code that
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is common to readers and updaters. However, lockdep
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will complain if you access rcu_dereference() outside
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of an RCU read-side critical section. See lockdep.rst
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to learn what to do about this.
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Of course, neither rcu_dereference() nor the "_rcu()"
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list-traversal primitives can substitute for a good
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concurrency design coordinating among multiple updaters.
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b. If the list macros are being used, the list_add_tail_rcu()
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and list_add_rcu() primitives must be used in order
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to prevent weakly ordered machines from misordering
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structure initialization and pointer planting.
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Similarly, if the hlist macros are being used, the
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hlist_add_head_rcu() primitive is required.
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c. If the list macros are being used, the list_del_rcu()
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primitive must be used to keep list_del()'s pointer
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poisoning from inflicting toxic effects on concurrent
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readers. Similarly, if the hlist macros are being used,
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the hlist_del_rcu() primitive is required.
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The list_replace_rcu() and hlist_replace_rcu() primitives
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may be used to replace an old structure with a new one
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in their respective types of RCU-protected lists.
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d. Rules similar to (4b) and (4c) apply to the "hlist_nulls"
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type of RCU-protected linked lists.
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e. Updates must ensure that initialization of a given
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structure happens before pointers to that structure are
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publicized. Use the rcu_assign_pointer() primitive
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when publicizing a pointer to a structure that can
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be traversed by an RCU read-side critical section.
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5. If call_rcu() or call_srcu() is used, the callback function will
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be called from softirq context. In particular, it cannot block.
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6. Since synchronize_rcu() can block, it cannot be called
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from any sort of irq context. The same rule applies
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for synchronize_srcu(), synchronize_rcu_expedited(), and
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synchronize_srcu_expedited().
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The expedited forms of these primitives have the same semantics
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as the non-expedited forms, but expediting is both expensive and
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(with the exception of synchronize_srcu_expedited()) unfriendly
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to real-time workloads. Use of the expedited primitives should
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be restricted to rare configuration-change operations that would
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not normally be undertaken while a real-time workload is running.
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However, real-time workloads can use rcupdate.rcu_normal kernel
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boot parameter to completely disable expedited grace periods,
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though this might have performance implications.
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In particular, if you find yourself invoking one of the expedited
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primitives repeatedly in a loop, please do everyone a favor:
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Restructure your code so that it batches the updates, allowing
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a single non-expedited primitive to cover the entire batch.
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This will very likely be faster than the loop containing the
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expedited primitive, and will be much much easier on the rest
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of the system, especially to real-time workloads running on
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the rest of the system.
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7. As of v4.20, a given kernel implements only one RCU flavor, which
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is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y.
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If the updater uses call_rcu() or synchronize_rcu(), then
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the corresponding readers may use: (1) rcu_read_lock() and
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rcu_read_unlock(), (2) any pair of primitives that disables
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and re-enables softirq, for example, rcu_read_lock_bh() and
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rcu_read_unlock_bh(), or (3) any pair of primitives that disables
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and re-enables preemption, for example, rcu_read_lock_sched() and
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rcu_read_unlock_sched(). If the updater uses synchronize_srcu()
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or call_srcu(), then the corresponding readers must use
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srcu_read_lock() and srcu_read_unlock(), and with the same
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srcu_struct. The rules for the expedited RCU grace-period-wait
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primitives are the same as for their non-expedited counterparts.
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If the updater uses call_rcu_tasks() or synchronize_rcu_tasks(),
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then the readers must refrain from executing voluntary
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context switches, that is, from blocking. If the updater uses
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call_rcu_tasks_trace() or synchronize_rcu_tasks_trace(), then
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the corresponding readers must use rcu_read_lock_trace() and
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rcu_read_unlock_trace(). If an updater uses call_rcu_tasks_rude()
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or synchronize_rcu_tasks_rude(), then the corresponding readers
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must use anything that disables interrupts.
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Mixing things up will result in confusion and broken kernels, and
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has even resulted in an exploitable security issue. Therefore,
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when using non-obvious pairs of primitives, commenting is
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of course a must. One example of non-obvious pairing is
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the XDP feature in networking, which calls BPF programs from
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network-driver NAPI (softirq) context. BPF relies heavily on RCU
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protection for its data structures, but because the BPF program
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invocation happens entirely within a single local_bh_disable()
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section in a NAPI poll cycle, this usage is safe. The reason
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that this usage is safe is that readers can use anything that
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disables BH when updaters use call_rcu() or synchronize_rcu().
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8. Although synchronize_rcu() is slower than is call_rcu(), it
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usually results in simpler code. So, unless update performance is
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critically important, the updaters cannot block, or the latency of
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synchronize_rcu() is visible from userspace, synchronize_rcu()
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should be used in preference to call_rcu(). Furthermore,
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kfree_rcu() usually results in even simpler code than does
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synchronize_rcu() without synchronize_rcu()'s multi-millisecond
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latency. So please take advantage of kfree_rcu()'s "fire and
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forget" memory-freeing capabilities where it applies.
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An especially important property of the synchronize_rcu()
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primitive is that it automatically self-limits: if grace periods
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are delayed for whatever reason, then the synchronize_rcu()
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primitive will correspondingly delay updates. In contrast,
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code using call_rcu() should explicitly limit update rate in
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cases where grace periods are delayed, as failing to do so can
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result in excessive realtime latencies or even OOM conditions.
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Ways of gaining this self-limiting property when using call_rcu()
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include:
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a. Keeping a count of the number of data-structure elements
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used by the RCU-protected data structure, including
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those waiting for a grace period to elapse. Enforce a
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limit on this number, stalling updates as needed to allow
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previously deferred frees to complete. Alternatively,
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limit only the number awaiting deferred free rather than
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the total number of elements.
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One way to stall the updates is to acquire the update-side
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mutex. (Don't try this with a spinlock -- other CPUs
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spinning on the lock could prevent the grace period
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from ever ending.) Another way to stall the updates
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is for the updates to use a wrapper function around
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the memory allocator, so that this wrapper function
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simulates OOM when there is too much memory awaiting an
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RCU grace period. There are of course many other
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variations on this theme.
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b. Limiting update rate. For example, if updates occur only
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once per hour, then no explicit rate limiting is
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required, unless your system is already badly broken.
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Older versions of the dcache subsystem take this approach,
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guarding updates with a global lock, limiting their rate.
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c. Trusted update -- if updates can only be done manually by
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superuser or some other trusted user, then it might not
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be necessary to automatically limit them. The theory
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here is that superuser already has lots of ways to crash
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the machine.
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d. Periodically invoke synchronize_rcu(), permitting a limited
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number of updates per grace period.
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The same cautions apply to call_srcu() and kfree_rcu().
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Note that although these primitives do take action to avoid memory
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exhaustion when any given CPU has too many callbacks, a determined
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user could still exhaust memory. This is especially the case
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if a system with a large number of CPUs has been configured to
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offload all of its RCU callbacks onto a single CPU, or if the
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system has relatively little free memory.
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9. All RCU list-traversal primitives, which include
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rcu_dereference(), list_for_each_entry_rcu(), and
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list_for_each_safe_rcu(), must be either within an RCU read-side
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critical section or must be protected by appropriate update-side
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locks. RCU read-side critical sections are delimited by
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rcu_read_lock() and rcu_read_unlock(), or by similar primitives
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such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which
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case the matching rcu_dereference() primitive must be used in
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order to keep lockdep happy, in this case, rcu_dereference_bh().
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The reason that it is permissible to use RCU list-traversal
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primitives when the update-side lock is held is that doing so
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can be quite helpful in reducing code bloat when common code is
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shared between readers and updaters. Additional primitives
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are provided for this case, as discussed in lockdep.rst.
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One exception to this rule is when data is only ever added to
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the linked data structure, and is never removed during any
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time that readers might be accessing that structure. In such
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cases, READ_ONCE() may be used in place of rcu_dereference()
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and the read-side markers (rcu_read_lock() and rcu_read_unlock(),
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for example) may be omitted.
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10. Conversely, if you are in an RCU read-side critical section,
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and you don't hold the appropriate update-side lock, you *must*
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use the "_rcu()" variants of the list macros. Failing to do so
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will break Alpha, cause aggressive compilers to generate bad code,
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and confuse people trying to read your code.
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11. Any lock acquired by an RCU callback must be acquired elsewhere
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with softirq disabled, e.g., via spin_lock_irqsave(),
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spin_lock_bh(), etc. Failing to disable softirq on a given
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acquisition of that lock will result in deadlock as soon as
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the RCU softirq handler happens to run your RCU callback while
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interrupting that acquisition's critical section.
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12. RCU callbacks can be and are executed in parallel. In many cases,
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the callback code simply wrappers around kfree(), so that this
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is not an issue (or, more accurately, to the extent that it is
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an issue, the memory-allocator locking handles it). However,
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if the callbacks do manipulate a shared data structure, they
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must use whatever locking or other synchronization is required
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to safely access and/or modify that data structure.
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Do not assume that RCU callbacks will be executed on the same
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CPU that executed the corresponding call_rcu() or call_srcu().
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For example, if a given CPU goes offline while having an RCU
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callback pending, then that RCU callback will execute on some
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surviving CPU. (If this was not the case, a self-spawning RCU
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callback would prevent the victim CPU from ever going offline.)
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Furthermore, CPUs designated by rcu_nocbs= might well *always*
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have their RCU callbacks executed on some other CPUs, in fact,
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for some real-time workloads, this is the whole point of using
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the rcu_nocbs= kernel boot parameter.
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13. Unlike other forms of RCU, it *is* permissible to block in an
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SRCU read-side critical section (demarked by srcu_read_lock()
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and srcu_read_unlock()), hence the "SRCU": "sleepable RCU".
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Please note that if you don't need to sleep in read-side critical
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sections, you should be using RCU rather than SRCU, because RCU
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is almost always faster and easier to use than is SRCU.
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Also unlike other forms of RCU, explicit initialization and
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cleanup is required either at build time via DEFINE_SRCU()
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or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct()
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and cleanup_srcu_struct(). These last two are passed a
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"struct srcu_struct" that defines the scope of a given
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SRCU domain. Once initialized, the srcu_struct is passed
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to srcu_read_lock(), srcu_read_unlock() synchronize_srcu(),
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synchronize_srcu_expedited(), and call_srcu(). A given
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synchronize_srcu() waits only for SRCU read-side critical
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sections governed by srcu_read_lock() and srcu_read_unlock()
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calls that have been passed the same srcu_struct. This property
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is what makes sleeping read-side critical sections tolerable --
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a given subsystem delays only its own updates, not those of other
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subsystems using SRCU. Therefore, SRCU is less prone to OOM the
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system than RCU would be if RCU's read-side critical sections
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were permitted to sleep.
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The ability to sleep in read-side critical sections does not
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come for free. First, corresponding srcu_read_lock() and
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srcu_read_unlock() calls must be passed the same srcu_struct.
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Second, grace-period-detection overhead is amortized only
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over those updates sharing a given srcu_struct, rather than
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being globally amortized as they are for other forms of RCU.
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Therefore, SRCU should be used in preference to rw_semaphore
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only in extremely read-intensive situations, or in situations
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requiring SRCU's read-side deadlock immunity or low read-side
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realtime latency. You should also consider percpu_rw_semaphore
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when you need lightweight readers.
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SRCU's expedited primitive (synchronize_srcu_expedited())
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never sends IPIs to other CPUs, so it is easier on
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real-time workloads than is synchronize_rcu_expedited().
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Note that rcu_assign_pointer() relates to SRCU just as it does to
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other forms of RCU, but instead of rcu_dereference() you should
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use srcu_dereference() in order to avoid lockdep splats.
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14. The whole point of call_rcu(), synchronize_rcu(), and friends
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is to wait until all pre-existing readers have finished before
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carrying out some otherwise-destructive operation. It is
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therefore critically important to *first* remove any path
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that readers can follow that could be affected by the
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destructive operation, and *only then* invoke call_rcu(),
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synchronize_rcu(), or friends.
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Because these primitives only wait for pre-existing readers, it
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is the caller's responsibility to guarantee that any subsequent
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readers will execute safely.
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15. The various RCU read-side primitives do *not* necessarily contain
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memory barriers. You should therefore plan for the CPU
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and the compiler to freely reorder code into and out of RCU
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read-side critical sections. It is the responsibility of the
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RCU update-side primitives to deal with this.
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For SRCU readers, you can use smp_mb__after_srcu_read_unlock()
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immediately after an srcu_read_unlock() to get a full barrier.
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16. Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
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__rcu sparse checks to validate your RCU code. These can help
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find problems as follows:
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CONFIG_PROVE_LOCKING:
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check that accesses to RCU-protected data
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|
structures are carried out under the proper RCU
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|
read-side critical section, while holding the right
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|
combination of locks, or whatever other conditions
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|
are appropriate.
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CONFIG_DEBUG_OBJECTS_RCU_HEAD:
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|
check that you don't pass the
|
|
same object to call_rcu() (or friends) before an RCU
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|
grace period has elapsed since the last time that you
|
|
passed that same object to call_rcu() (or friends).
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|
|
|
__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().
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|
|
|
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
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|
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()
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|
|
|
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.
|