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.
+ structure is updated more than about 10% of the time, then you
+ should strongly consider some other approach, unless detailed
+ performance measurements show that RCU is nonetheless the right
+ tool for the job. Yes, RCU does reduce read-side overhead by
+ increasing write-side overhead, which is exactly why normal uses
+ of RCU will do much more reading than updating.
- 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.
+ Another exception is where performance is not an issue, and RCU
+ provides a simpler implementation. An example of this situation
+ is the dynamic NMI code in the Linux 2.6 kernel, at least on
+ architectures where NMIs are rare.
+
+ Yet another exception is where the low real-time latency of RCU's
+ read-side primitives is critically important.
1. Does the update code have proper mutual exclusion?
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).
+ them -- even x86 allows later loads to be reordered to precede
+ earlier stores), and be prepared to explain why this added
+ complexity is worthwhile. If you choose #c, be prepared to
+ explain how this single task does not become a major bottleneck on
+ big multiprocessor machines (for example, if the task is updating
+ information relating to itself that other tasks can read, there
+ by definition can be no bottleneck).
2. Do the RCU read-side critical sections make proper use of
rcu_read_lock() and friends? These primitives are needed
- to 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.
+ to prevent grace periods from ending prematurely, which
+ could result in data being unceremoniously freed out from
+ under your read-side code, which can greatly increase the
+ actuarial risk of your kernel.
As a rough rule of thumb, any dereference of an RCU-protected
- pointer must be covered by rcu_read_lock() or rcu_read_lock_bh()
- or by the appropriate update-side lock.
+ pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(),
+ rcu_read_lock_sched(), or by the appropriate update-side lock.
+ Disabling of preemption can serve as rcu_read_lock_sched(), but
+ is less readable.
3. Does the update code tolerate concurrent accesses?
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.
+ a. Use the RCU variants of the list and hlist update
+ primitives to add, remove, and replace elements on
+ an RCU-protected list. Alternatively, use the other
+ RCU-protected data structures that have been added to
+ the Linux kernel.
This is almost always the best approach.
- b. Carefully order the updates and the reads so that
+ b. Proceed as in (a) above, but also maintain per-element
+ locks (that are acquired by both readers and writers)
+ that guard per-element state. Of course, fields that
+ the readers refrain from accessing can be guarded by
+ some other lock acquired only by updaters, if desired.
+
+ This works quite well, also.
+
+ c. Make updates appear atomic to readers. For example,
+ pointer updates to properly aligned fields will
+ appear atomic, as will individual atomic primitives.
+ Sequences of perations performed under a lock will -not-
+ appear to be atomic to RCU readers, nor will sequences
+ of multiple atomic primitives.
+
+ This can work, but is starting to get a bit tricky.
+
+ d. Carefully order the updates and the reads so that
readers see valid data at all phases of the update.
This is often more difficult than it sounds, especially
given modern CPUs' tendency to reorder memory references.
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:
+ are weakly ordered -- even x86 CPUs allow later loads to be
+ reordered to precede earlier stores. RCU code must take all of
+ the following measures to prevent memory-corruption problems:
a. Readers must maintain proper ordering of their memory
accesses. The rcu_dereference() primitive ensures that
The 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.
+ Please note that compilers can also reorder code, and
+ they are becoming increasingly aggressive about doing
+ just that. The rcu_dereference() primitive therefore
+ also prevents destructive compiler optimizations.
+
+ The rcu_dereference() primitive is used by the
+ various "_rcu()" list-traversal primitives, such
+ as the list_for_each_entry_rcu(). Note that it is
+ perfectly legal (if redundant) for update-side code to
+ use rcu_dereference() and the "_rcu()" list-traversal
+ primitives. This is particularly useful in code that
+ is common to readers and updaters. However, lockdep
+ will complain if you access rcu_dereference() outside
+ of an RCU read-side critical section. See lockdep.txt
+ to learn what to do about this.
+
+ Of course, neither rcu_dereference() nor the "_rcu()"
+ list-traversal primitives can substitute for a good
+ concurrency design coordinating among multiple updaters.
b. If the list macros are being used, the list_add_tail_rcu()
and list_add_rcu() primitives must be used in order
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.
+ The list_replace_rcu() and hlist_replace_rcu() primitives
+ may be used to replace an old structure with a new one
+ in their respective types of RCU-protected lists.
+
+ d. Rules similar to (4b) and (4c) apply to the "hlist_nulls"
+ type of RCU-protected linked lists.
- d. Updates must ensure that initialization of a given
+ e. Updates must ensure that initialization of a given
structure happens before pointers to that structure are
publicized. Use the rcu_assign_pointer() primitive
when publicizing a pointer to a structure that can
be traversed by an RCU read-side critical section.
-5. If call_rcu(), or a related primitive such as call_rcu_bh(),
- is used, the callback function must be written to be called
- from softirq context. In particular, it cannot block.
+5. If call_rcu(), or a related primitive such as call_rcu_bh() or
+ call_rcu_sched(), 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.
+ any sort of irq context. The same rule applies for
+ synchronize_rcu_bh(), synchronize_sched(), synchronize_srcu(),
+ synchronize_rcu_expedited(), synchronize_rcu_bh_expedited(),
+ synchronize_sched_expedite(), and synchronize_srcu_expedited().
+
+ The expedited forms of these primitives have the same semantics
+ as the non-expedited forms, but expediting is both expensive
+ and unfriendly to real-time workloads. Use of the expedited
+ primitives should be restricted to rare configuration-change
+ operations that would not normally be undertaken while a real-time
+ workload is running.
+
+7. If the updater uses call_rcu() or synchronize_rcu(), then the
+ corresponding readers must use rcu_read_lock() and
+ rcu_read_unlock(). If the updater uses call_rcu_bh() or
+ synchronize_rcu_bh(), then the corresponding readers must
+ use rcu_read_lock_bh() and rcu_read_unlock_bh(). If the
+ updater uses call_rcu_sched() or synchronize_sched(), then
+ the corresponding readers must disable preemption, possibly
+ by calling rcu_read_lock_sched() and rcu_read_unlock_sched().
+ If the updater uses synchronize_srcu(), the the corresponding
+ readers must use srcu_read_lock() and srcu_read_unlock(),
+ and with the same srcu_struct. The rules for the expedited
+ primitives are the same as for their non-expedited counterparts.
+ Mixing things up will result in confusion and broken kernels.
One exception to this rule: rcu_read_lock() and rcu_read_unlock()
may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
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,
+8. Although synchronize_rcu() is slower than is call_rcu(), it
+ usually results in simpler code. So, unless update performance
+ is critically important or the updaters cannot block,
synchronize_rcu() should be used in preference to call_rcu().
An especially important property of the synchronize_rcu()
e. Periodically invoke synchronize_rcu(), permitting a limited
number of updates per grace period.
+ The same cautions apply to call_rcu_bh() and call_rcu_sched().
+
9. All RCU list-traversal primitives, which include
- list_for_each_rcu(), list_for_each_entry_rcu(),
+ rcu_dereference(), 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
+ must be either within an RCU read-side critical section or
+ must be protected by appropriate update-side locks. RCU
read-side critical sections are delimited by rcu_read_lock()
and rcu_read_unlock(), or by similar primitives such as
- rcu_read_lock_bh() and rcu_read_unlock_bh().
+ rcu_read_lock_bh() and rcu_read_unlock_bh(), in which case
+ the matching rcu_dereference() primitive must be used in order
+ to keep lockdep happy, in this case, rcu_dereference_bh().
- 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.
+ The reason that it is permissible to use RCU list-traversal
+ primitives when the update-side lock is held is that doing so
+ can be quite helpful in reducing code bloat when common code is
+ shared between readers and updaters. Additional primitives
+ are provided for this case, as discussed in lockdep.txt.
10. Conversely, if you are in an RCU read-side critical section,
- you -must- use the "_rcu()" variants of the list macros.
- Failing to do so will break Alpha and confuse people reading
- your code.
+ and you don't hold the appropriate update-side lock, you -must-
+ use the "_rcu()" variants of the list macros. Failing to do so
+ will break Alpha, cause aggressive compilers to generate bad code,
+ and confuse people trying to read your code.
11. Note that synchronize_rcu() -only- guarantees to wait until
all currently executing rcu_read_lock()-protected RCU read-side
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().
+ Similarly, disabling preemption is not an acceptable substitute
+ for rcu_read_lock(). Code that attempts to use preemption
+ disabling where it should be using rcu_read_lock() will break
+ in real-time kernel builds.
+
+ If you want to wait for interrupt handlers, NMI handlers, and
+ code under the influence of preempt_disable(), you instead
+ need to use synchronize_irq() or synchronize_sched().
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. 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.
+ with softirq disabled, e.g., via spin_lock_irqsave(),
+ spin_lock_bh(), etc. Failing to disable irq on a given
+ acquisition of that lock will result in deadlock as soon as
+ the RCU softirq handler happens to run your RCU callback while
+ interrupting that acquisition's critical section.
+
+13. RCU callbacks can be and are executed in parallel. In many cases,
+ the callback code simply wrappers around kfree(), so that this
+ is not an issue (or, more accurately, to the extent that it is
+ an issue, the memory-allocator locking handles it). However,
+ if the callbacks do manipulate a shared data structure, they
+ must use whatever locking or other synchronization is required
+ to safely access and/or modify that data structure.
+
+ RCU callbacks are -usually- executed on the same CPU that executed
+ the corresponding call_rcu(), call_rcu_bh(), or call_rcu_sched(),
+ but are by -no- means guaranteed to be. For example, if a given
+ CPU goes offline while having an RCU callback pending, then that
+ RCU callback will execute on some surviving CPU. (If this was
+ not the case, a self-spawning RCU callback would prevent the
+ victim CPU from ever going offline.)
+
+14. SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
+ synchronize_srcu(), and synchronize_srcu_expedited()) 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.
+ synchronize_srcu(), and synchronize_srcu_expedited(). A given
+ synchronize_srcu() waits only for SRCU read-side critical
+ sections governed by srcu_read_lock() and srcu_read_unlock()
+ calls that have been passed the same srcu_struct. This property
+ is what makes sleeping read-side critical sections tolerable --
+ a given subsystem delays only its own updates, not those of other
+ subsystems using SRCU. Therefore, SRCU is less prone to OOM the
+ system than RCU would be if RCU's read-side critical sections
+ were permitted to sleep.
The ability to sleep in read-side critical sections does not
come for free. First, corresponding srcu_read_lock() and
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.
+ Note that, rcu_assign_pointer() relates to SRCU just as they do
+ to other forms of RCU.
+
+15. The whole point of call_rcu(), synchronize_rcu(), and friends
+ is to wait until all pre-existing readers have finished before
+ carrying out some otherwise-destructive operation. It is
+ therefore critically important to -first- remove any path
+ that readers can follow that could be affected by the
+ destructive operation, and -only- -then- invoke call_rcu(),
+ synchronize_rcu(), or friends.
+
+ Because these primitives only wait for pre-existing readers, it
+ is the caller's responsibility to guarantee that any subsequent
+ readers will execute safely.
+
+16. The various RCU read-side primitives do -not- necessarily contain
+ memory barriers. You should therefore plan for the CPU
+ and the compiler to freely reorder code into and out of RCU
+ read-side critical sections. It is the responsibility of the
+ RCU update-side primitives to deal with this.
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff