![Paul E. McKenney](/assets/img/avatar_default.png)
The advent of call_srcu() and srcu_barrier() obsoleted some of the documentation, so this commit brings that up to date. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
421 lines
19 KiB
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421 lines
19 KiB
Plaintext
Review Checklist for RCU Patches
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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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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).
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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.
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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 perations 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. If you don't believe me, see:
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http://www.openvms.compaq.com/wizard/wiz_2637.html
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The rcu_dereference() primitive is also an excellent
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documentation aid, letting the person reading the code
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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
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also prevents destructive compiler optimizations.
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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.txt
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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 a related primitive such as call_rcu_bh(),
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call_rcu_sched(), or call_srcu() is used, the callback function
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must be written to be called from softirq context. In particular,
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it cannot block.
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6. Since synchronize_rcu() can block, it cannot be called from
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any sort of irq context. The same rule applies for
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synchronize_rcu_bh(), synchronize_sched(), synchronize_srcu(),
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synchronize_rcu_expedited(), synchronize_rcu_bh_expedited(),
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synchronize_sched_expedite(), and 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
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and unfriendly to real-time workloads. Use of the expedited
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primitives should be restricted to rare configuration-change
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operations that would not normally be undertaken while a real-time
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workload is running.
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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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In addition, it is illegal to call the expedited forms from
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a CPU-hotplug notifier, or while holding a lock that is acquired
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by a CPU-hotplug notifier. Failing to observe this restriction
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will result in deadlock.
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7. If the updater uses call_rcu() or synchronize_rcu(), then the
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corresponding readers must use rcu_read_lock() and
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rcu_read_unlock(). If the updater uses call_rcu_bh() or
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synchronize_rcu_bh(), then the corresponding readers must
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use rcu_read_lock_bh() and rcu_read_unlock_bh(). If the
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updater uses call_rcu_sched() or synchronize_sched(), then
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the corresponding readers must disable preemption, possibly
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by calling rcu_read_lock_sched() and rcu_read_unlock_sched().
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If the updater uses synchronize_srcu() or call_srcu(),
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the the corresponding readers must use srcu_read_lock() and
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srcu_read_unlock(), and with the same srcu_struct. The rules for
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the expedited primitives are the same as for their non-expedited
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counterparts. Mixing things up will result in confusion and
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broken kernels.
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One exception to this rule: rcu_read_lock() and rcu_read_unlock()
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may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
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in cases where local bottom halves are already known to be
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disabled, for example, in irq or softirq context. Commenting
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such cases is a must, of course! And the jury is still out on
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whether the increased speed is worth it.
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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
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is critically important or the updaters cannot block,
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synchronize_rcu() should be used in preference to call_rcu().
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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 required,
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unless your system is already badly broken. The dcache
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subsystem takes this approach -- updates are guarded
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by 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. Use call_rcu_bh() rather than call_rcu(), in order to take
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advantage of call_rcu_bh()'s faster grace periods.
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e. 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_rcu_bh() and call_rcu_sched().
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9. All RCU list-traversal primitives, which include
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rcu_dereference(), list_for_each_entry_rcu(),
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list_for_each_continue_rcu(), and list_for_each_safe_rcu(),
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must be either within an RCU read-side critical section or
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must be protected by appropriate update-side locks. RCU
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read-side critical sections are delimited by rcu_read_lock()
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and rcu_read_unlock(), or by similar primitives such as
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rcu_read_lock_bh() and rcu_read_unlock_bh(), in which case
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the matching rcu_dereference() primitive must be used in order
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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.txt.
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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. Note that synchronize_rcu() -only- guarantees to wait until
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all currently executing rcu_read_lock()-protected RCU read-side
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critical sections complete. It does -not- necessarily guarantee
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that all currently running interrupts, NMIs, preempt_disable()
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code, or idle loops will complete. Therefore, if you do not have
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rcu_read_lock()-protected read-side critical sections, do -not-
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use synchronize_rcu().
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Similarly, disabling preemption is not an acceptable substitute
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for rcu_read_lock(). Code that attempts to use preemption
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disabling where it should be using rcu_read_lock() will break
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in real-time kernel builds.
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If you want to wait for interrupt handlers, NMI handlers, and
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code under the influence of preempt_disable(), you instead
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need to use synchronize_irq() or synchronize_sched().
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12. 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 irq 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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13. 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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RCU callbacks are -usually- executed on the same CPU that executed
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the corresponding call_rcu(), call_rcu_bh(), or call_rcu_sched(),
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but are by -no- means guaranteed to be. For example, if a given
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CPU goes offline while having an RCU callback pending, then that
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RCU callback will execute on some surviving CPU. (If this was
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not the case, a self-spawning RCU callback would prevent the
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victim CPU from ever going offline.)
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14. SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
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synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu())
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may only be invoked from process context. Unlike other forms of
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RCU, it -is- permissible to block in an SRCU read-side critical
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section (demarked by srcu_read_lock() and srcu_read_unlock()),
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hence the "SRCU": "sleepable RCU". Please note that if you
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don't need to sleep in read-side critical sections, you should be
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using RCU rather than SRCU, because RCU is almost always faster
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and easier to use than is SRCU.
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If you need to enter your read-side critical section in a
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hardirq or exception handler, and then exit that same read-side
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critical section in the task that was interrupted, then you need
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to srcu_read_lock_raw() and srcu_read_unlock_raw(), which avoid
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the lockdep checking that would otherwise this practice illegal.
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Also unlike other forms of RCU, explicit initialization
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and cleanup is required via init_srcu_struct() and
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cleanup_srcu_struct(). These are passed a "struct srcu_struct"
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that defines the scope of a given SRCU domain. Once initialized,
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the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
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synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu().
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A given 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.
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Note that, rcu_assign_pointer() relates to SRCU just as it does
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to other forms of RCU.
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15. 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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16. 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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17. Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and
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the __rcu sparse checks to validate your RCU code. These
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can help find problems as follows:
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CONFIG_PROVE_RCU: 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: check that you don't pass the
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same object to call_rcu() (or friends) before an RCU
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grace period has elapsed since the last time that you
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passed that same object to call_rcu() (or friends).
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__rcu sparse checks: tag the pointer to the RCU-protected data
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structure with __rcu, and sparse will warn you if you
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access that pointer without the services of one of the
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variants of rcu_dereference().
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These debugging aids can help you find problems that are
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otherwise extremely difficult to spot.
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