![Joel Fernandes (Google)](/assets/img/avatar_default.png)
The checklist suggests rcu_barrier_bh() for RCU-bh and similarly for sched, however these APIs are now implemented as rcu_barrier() itself due to the RCU consolidation. This commit therefore corrects checklist.txt to encourage use of the underlying rcu_barrier() API. Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
460 lines
21 KiB
Plaintext
460 lines
21 KiB
Plaintext
Review Checklist for RCU Patches
|
|
|
|
|
|
This document contains a checklist for producing and reviewing patches
|
|
that make use of RCU. Violating any of the rules listed below will
|
|
result in the same sorts of problems that leaving out a locking primitive
|
|
would cause. This list is based on experiences reviewing such patches
|
|
over a rather long period of time, but improvements are always welcome!
|
|
|
|
0. Is RCU being applied to a read-mostly situation? If the data
|
|
structure is updated more than about 10% of the time, then you
|
|
should strongly consider some other approach, unless detailed
|
|
performance measurements show that RCU is nonetheless the right
|
|
tool for the job. Yes, RCU does reduce read-side overhead by
|
|
increasing write-side overhead, which is exactly why normal uses
|
|
of RCU will do much more reading than updating.
|
|
|
|
Another exception is where performance is not an issue, and RCU
|
|
provides a simpler implementation. An example of this situation
|
|
is the dynamic NMI code in the Linux 2.6 kernel, at least on
|
|
architectures where NMIs are rare.
|
|
|
|
Yet another exception is where the low real-time latency of RCU's
|
|
read-side primitives is critically important.
|
|
|
|
One final exception is where RCU readers are used to prevent
|
|
the ABA problem (https://en.wikipedia.org/wiki/ABA_problem)
|
|
for lockless updates. This does result in the mildly
|
|
counter-intuitive situation where rcu_read_lock() and
|
|
rcu_read_unlock() are used to protect updates, however, this
|
|
approach provides the same potential simplifications that garbage
|
|
collectors do.
|
|
|
|
1. Does the update code have proper mutual exclusion?
|
|
|
|
RCU does allow -readers- to run (almost) naked, but -writers- must
|
|
still use some sort of mutual exclusion, such as:
|
|
|
|
a. locking,
|
|
b. atomic operations, or
|
|
c. restricting updates to a single task.
|
|
|
|
If you choose #b, be prepared to describe how you have handled
|
|
memory barriers on weakly ordered machines (pretty much all of
|
|
them -- even x86 allows later loads to be reordered to precede
|
|
earlier stores), and be prepared to explain why this added
|
|
complexity is worthwhile. If you choose #c, be prepared to
|
|
explain how this single task does not become a major bottleneck on
|
|
big multiprocessor machines (for example, if the task is updating
|
|
information relating to itself that other tasks can read, there
|
|
by definition can be no bottleneck). Note that the definition
|
|
of "large" has changed significantly: Eight CPUs was "large"
|
|
in the year 2000, but a hundred CPUs was unremarkable in 2017.
|
|
|
|
2. Do the RCU read-side critical sections make proper use of
|
|
rcu_read_lock() and friends? These primitives are needed
|
|
to prevent grace periods from ending prematurely, which
|
|
could result in data being unceremoniously freed out from
|
|
under your read-side code, which can greatly increase the
|
|
actuarial risk of your kernel.
|
|
|
|
As a rough rule of thumb, any dereference of an RCU-protected
|
|
pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(),
|
|
rcu_read_lock_sched(), or by the appropriate update-side lock.
|
|
Disabling of preemption can serve as rcu_read_lock_sched(), but
|
|
is less readable and prevents lockdep from detecting locking issues.
|
|
|
|
Letting RCU-protected pointers "leak" out of an RCU read-side
|
|
critical section is every bid as bad as letting them leak out
|
|
from under a lock. Unless, of course, you have arranged some
|
|
other means of protection, such as a lock or a reference count
|
|
-before- letting them out of the RCU read-side critical section.
|
|
|
|
3. Does the update code tolerate concurrent accesses?
|
|
|
|
The whole point of RCU is to permit readers to run without
|
|
any locks or atomic operations. This means that readers will
|
|
be running while updates are in progress. There are a number
|
|
of ways to handle this concurrency, depending on the situation:
|
|
|
|
a. Use the RCU variants of the list and hlist update
|
|
primitives to add, remove, and replace elements on
|
|
an RCU-protected list. Alternatively, use the other
|
|
RCU-protected data structures that have been added to
|
|
the Linux kernel.
|
|
|
|
This is almost always the best approach.
|
|
|
|
b. Proceed as in (a) above, but also maintain per-element
|
|
locks (that are acquired by both readers and writers)
|
|
that guard per-element state. Of course, fields that
|
|
the readers refrain from accessing can be guarded by
|
|
some other lock acquired only by updaters, if desired.
|
|
|
|
This works quite well, also.
|
|
|
|
c. Make updates appear atomic to readers. For example,
|
|
pointer updates to properly aligned fields will
|
|
appear atomic, as will individual atomic primitives.
|
|
Sequences of operations performed under a lock will -not-
|
|
appear to be atomic to RCU readers, nor will sequences
|
|
of multiple atomic primitives.
|
|
|
|
This can work, but is starting to get a bit tricky.
|
|
|
|
d. Carefully order the updates and the reads so that
|
|
readers see valid data at all phases of the update.
|
|
This is often more difficult than it sounds, especially
|
|
given modern CPUs' tendency to reorder memory references.
|
|
One must usually liberally sprinkle memory barriers
|
|
(smp_wmb(), smp_rmb(), smp_mb()) through the code,
|
|
making it difficult to understand and to test.
|
|
|
|
It is usually better to group the changing data into
|
|
a separate structure, so that the change may be made
|
|
to appear atomic by updating a pointer to reference
|
|
a new structure containing updated values.
|
|
|
|
4. Weakly ordered CPUs pose special challenges. Almost all CPUs
|
|
are weakly ordered -- even x86 CPUs allow later loads to be
|
|
reordered to precede earlier stores. RCU code must take all of
|
|
the following measures to prevent memory-corruption problems:
|
|
|
|
a. Readers must maintain proper ordering of their memory
|
|
accesses. The rcu_dereference() primitive ensures that
|
|
the CPU picks up the pointer before it picks up the data
|
|
that the pointer points to. This really is necessary
|
|
on Alpha CPUs. If you don't believe me, see:
|
|
|
|
http://www.openvms.compaq.com/wizard/wiz_2637.html
|
|
|
|
The rcu_dereference() primitive is also an excellent
|
|
documentation aid, letting the person reading the
|
|
code know exactly which pointers are protected by RCU.
|
|
Please note that compilers can also reorder code, and
|
|
they are becoming increasingly aggressive about doing
|
|
just that. The rcu_dereference() primitive therefore also
|
|
prevents destructive compiler optimizations. However,
|
|
with a bit of devious creativity, it is possible to
|
|
mishandle the return value from rcu_dereference().
|
|
Please see rcu_dereference.txt in this directory for
|
|
more information.
|
|
|
|
The rcu_dereference() primitive is used by the
|
|
various "_rcu()" list-traversal primitives, such
|
|
as the list_for_each_entry_rcu(). Note that it is
|
|
perfectly legal (if redundant) for update-side code to
|
|
use rcu_dereference() and the "_rcu()" list-traversal
|
|
primitives. This is particularly useful in code that
|
|
is common to readers and updaters. However, lockdep
|
|
will complain if you access rcu_dereference() outside
|
|
of an RCU read-side critical section. See lockdep.txt
|
|
to learn what to do about this.
|
|
|
|
Of course, neither rcu_dereference() nor the "_rcu()"
|
|
list-traversal primitives can substitute for a good
|
|
concurrency design coordinating among multiple updaters.
|
|
|
|
b. If the list macros are being used, the list_add_tail_rcu()
|
|
and list_add_rcu() primitives must be used in order
|
|
to prevent weakly ordered machines from misordering
|
|
structure initialization and pointer planting.
|
|
Similarly, if the hlist macros are being used, the
|
|
hlist_add_head_rcu() primitive is required.
|
|
|
|
c. If the list macros are being used, the list_del_rcu()
|
|
primitive must be used to keep list_del()'s pointer
|
|
poisoning from inflicting toxic effects on concurrent
|
|
readers. Similarly, if the hlist macros are being used,
|
|
the hlist_del_rcu() primitive is required.
|
|
|
|
The list_replace_rcu() and hlist_replace_rcu() primitives
|
|
may be used to replace an old structure with a new one
|
|
in their respective types of RCU-protected lists.
|
|
|
|
d. Rules similar to (4b) and (4c) apply to the "hlist_nulls"
|
|
type of RCU-protected linked lists.
|
|
|
|
e. Updates must ensure that initialization of a given
|
|
structure happens before pointers to that structure are
|
|
publicized. Use the rcu_assign_pointer() primitive
|
|
when publicizing a pointer to a structure that can
|
|
be traversed by an RCU read-side critical section.
|
|
|
|
5. If call_rcu(), or a related primitive such as call_rcu_bh(),
|
|
call_rcu_sched(), or call_srcu() is used, the callback function
|
|
will be called from softirq context. In particular, it cannot
|
|
block.
|
|
|
|
6. Since synchronize_rcu() can block, it cannot be called from
|
|
any sort of irq context. The same rule applies for
|
|
synchronize_rcu_bh(), synchronize_sched(), synchronize_srcu(),
|
|
synchronize_rcu_expedited(), synchronize_rcu_bh_expedited(),
|
|
synchronize_sched_expedite(), and synchronize_srcu_expedited().
|
|
|
|
The expedited forms of these primitives have the same semantics
|
|
as the non-expedited forms, but expediting is both expensive and
|
|
(with the exception of synchronize_srcu_expedited()) 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.
|
|
However, real-time workloads can use rcupdate.rcu_normal kernel
|
|
boot parameter to completely disable expedited grace periods,
|
|
though this might have performance implications.
|
|
|
|
In particular, if you find yourself invoking one of the expedited
|
|
primitives repeatedly in a loop, please do everyone a favor:
|
|
Restructure your code so that it batches the updates, allowing
|
|
a single non-expedited primitive to cover the entire batch.
|
|
This will very likely be faster than the loop containing the
|
|
expedited primitive, and will be much much easier on the rest
|
|
of the system, especially to real-time workloads running on
|
|
the rest of the system.
|
|
|
|
7. If the updater uses call_rcu() or synchronize_rcu(), then the
|
|
corresponding readers must use rcu_read_lock() and
|
|
rcu_read_unlock(). If the updater uses call_rcu_bh() or
|
|
synchronize_rcu_bh(), then the corresponding readers must
|
|
use rcu_read_lock_bh() and rcu_read_unlock_bh(). If the
|
|
updater uses call_rcu_sched() or synchronize_sched(), then
|
|
the corresponding readers must disable preemption, possibly
|
|
by calling rcu_read_lock_sched() and rcu_read_unlock_sched().
|
|
If the updater uses synchronize_srcu() or call_srcu(), then
|
|
the corresponding readers must use srcu_read_lock() and
|
|
srcu_read_unlock(), and with the same srcu_struct. The rules for
|
|
the expedited primitives are the same as for their non-expedited
|
|
counterparts. Mixing things up will result in confusion and
|
|
broken kernels.
|
|
|
|
One exception to this rule: rcu_read_lock() and rcu_read_unlock()
|
|
may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
|
|
in cases where local bottom halves are already known to be
|
|
disabled, for example, in irq or softirq context. Commenting
|
|
such cases is a must, of course! And the jury is still out on
|
|
whether the increased speed is worth it.
|
|
|
|
8. Although synchronize_rcu() is slower than is call_rcu(), it
|
|
usually results in simpler code. So, unless update performance is
|
|
critically important, the updaters cannot block, or the latency of
|
|
synchronize_rcu() is visible from userspace, synchronize_rcu()
|
|
should be used in preference to call_rcu(). Furthermore,
|
|
kfree_rcu() usually results in even simpler code than does
|
|
synchronize_rcu() without synchronize_rcu()'s multi-millisecond
|
|
latency. So please take advantage of kfree_rcu()'s "fire and
|
|
forget" memory-freeing capabilities where it applies.
|
|
|
|
An especially important property of the synchronize_rcu()
|
|
primitive is that it automatically self-limits: if grace periods
|
|
are delayed for whatever reason, then the synchronize_rcu()
|
|
primitive will correspondingly delay updates. In contrast,
|
|
code using call_rcu() should explicitly limit update rate in
|
|
cases where grace periods are delayed, as failing to do so can
|
|
result in excessive realtime latencies or even OOM conditions.
|
|
|
|
Ways of gaining this self-limiting property when using call_rcu()
|
|
include:
|
|
|
|
a. Keeping a count of the number of data-structure elements
|
|
used by the RCU-protected data structure, including
|
|
those waiting for a grace period to elapse. Enforce a
|
|
limit on this number, stalling updates as needed to allow
|
|
previously deferred frees to complete. Alternatively,
|
|
limit only the number awaiting deferred free rather than
|
|
the total number of elements.
|
|
|
|
One way to stall the updates is to acquire the update-side
|
|
mutex. (Don't try this with a spinlock -- other CPUs
|
|
spinning on the lock could prevent the grace period
|
|
from ever ending.) Another way to stall the updates
|
|
is for the updates to use a wrapper function around
|
|
the memory allocator, so that this wrapper function
|
|
simulates OOM when there is too much memory awaiting an
|
|
RCU grace period. There are of course many other
|
|
variations on this theme.
|
|
|
|
b. Limiting update rate. For example, if updates occur only
|
|
once per hour, then no explicit rate limiting is
|
|
required, unless your system is already badly broken.
|
|
Older versions of the dcache subsystem take this approach,
|
|
guarding updates with a global lock, limiting their rate.
|
|
|
|
c. Trusted update -- if updates can only be done manually by
|
|
superuser or some other trusted user, then it might not
|
|
be necessary to automatically limit them. The theory
|
|
here is that superuser already has lots of ways to crash
|
|
the machine.
|
|
|
|
d. Periodically invoke synchronize_rcu(), permitting a limited
|
|
number of updates per grace period.
|
|
|
|
The same cautions apply to call_rcu_bh(), call_rcu_sched(),
|
|
call_srcu(), and kfree_rcu().
|
|
|
|
Note that although these primitives do take action to avoid memory
|
|
exhaustion when any given CPU has too many callbacks, a determined
|
|
user could still exhaust memory. This is especially the case
|
|
if a system with a large number of CPUs has been configured to
|
|
offload all of its RCU callbacks onto a single CPU, or if the
|
|
system has relatively little free memory.
|
|
|
|
9. All RCU list-traversal primitives, which include
|
|
rcu_dereference(), list_for_each_entry_rcu(), and
|
|
list_for_each_safe_rcu(), must be either within an RCU read-side
|
|
critical section or must be protected by appropriate update-side
|
|
locks. RCU read-side critical sections are delimited by
|
|
rcu_read_lock() and rcu_read_unlock(), or by similar primitives
|
|
such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which
|
|
case the matching rcu_dereference() primitive must be used in
|
|
order to keep lockdep happy, in this case, rcu_dereference_bh().
|
|
|
|
The reason that it is permissible to use RCU list-traversal
|
|
primitives when the update-side lock is held is that doing so
|
|
can be quite helpful in reducing code bloat when common code is
|
|
shared between readers and updaters. Additional primitives
|
|
are provided for this case, as discussed in lockdep.txt.
|
|
|
|
10. Conversely, if you are in an RCU read-side critical section,
|
|
and you don't hold the appropriate update-side lock, you -must-
|
|
use the "_rcu()" variants of the list macros. Failing to do so
|
|
will break Alpha, cause aggressive compilers to generate bad code,
|
|
and confuse people trying to read your code.
|
|
|
|
11. Any lock acquired by an RCU callback must be acquired elsewhere
|
|
with softirq disabled, e.g., via spin_lock_irqsave(),
|
|
spin_lock_bh(), etc. Failing to disable irq on a given
|
|
acquisition of that lock will result in deadlock as soon as
|
|
the RCU softirq handler happens to run your RCU callback while
|
|
interrupting that acquisition's critical section.
|
|
|
|
12. 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.)
|
|
|
|
13. 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 either at build time via DEFINE_SRCU()
|
|
or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct()
|
|
and cleanup_srcu_struct(). These last two are passed a
|
|
"struct srcu_struct" that defines the scope of a given
|
|
SRCU domain. Once initialized, the srcu_struct is passed
|
|
to srcu_read_lock(), srcu_read_unlock() synchronize_srcu(),
|
|
synchronize_srcu_expedited(), and call_srcu(). A given
|
|
synchronize_srcu() waits only for SRCU read-side critical
|
|
sections governed by srcu_read_lock() and srcu_read_unlock()
|
|
calls that have been passed the same srcu_struct. This property
|
|
is what makes sleeping read-side critical sections tolerable --
|
|
a given subsystem delays only its own updates, not those of other
|
|
subsystems using SRCU. Therefore, SRCU is less prone to OOM the
|
|
system than RCU would be if RCU's read-side critical sections
|
|
were permitted to sleep.
|
|
|
|
The ability to sleep in read-side critical sections does not
|
|
come for free. First, corresponding srcu_read_lock() and
|
|
srcu_read_unlock() calls must be passed the same srcu_struct.
|
|
Second, grace-period-detection overhead is amortized only
|
|
over those updates sharing a given srcu_struct, rather than
|
|
being globally amortized as they are for other forms of RCU.
|
|
Therefore, SRCU should be used in preference to rw_semaphore
|
|
only in extremely read-intensive situations, or in situations
|
|
requiring SRCU's read-side deadlock immunity or low read-side
|
|
realtime latency. You should also consider percpu_rw_semaphore
|
|
when you need lightweight readers.
|
|
|
|
SRCU's expedited primitive (synchronize_srcu_expedited())
|
|
never sends IPIs to other CPUs, so it is easier on
|
|
real-time workloads than is synchronize_rcu_expedited(),
|
|
synchronize_rcu_bh_expedited() or synchronize_sched_expedited().
|
|
|
|
Note that rcu_dereference() and rcu_assign_pointer() relate to
|
|
SRCU just as they do to other forms of RCU.
|
|
|
|
14. 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.
|
|
|
|
15. 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.
|
|
|
|
16. Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
|
|
__rcu sparse checks to validate your RCU code. These can help
|
|
find problems as follows:
|
|
|
|
CONFIG_PROVE_LOCKING: check that accesses to RCU-protected data
|
|
structures are carried out under the proper RCU
|
|
read-side critical section, while holding the right
|
|
combination of locks, or whatever other conditions
|
|
are appropriate.
|
|
|
|
CONFIG_DEBUG_OBJECTS_RCU_HEAD: check that you don't pass the
|
|
same object to call_rcu() (or friends) before an RCU
|
|
grace period has elapsed since the last time that you
|
|
passed that same object to call_rcu() (or friends).
|
|
|
|
__rcu sparse checks: tag the pointer to the RCU-protected data
|
|
structure with __rcu, and sparse will warn you if you
|
|
access that pointer without the services of one of the
|
|
variants of rcu_dereference().
|
|
|
|
These debugging aids can help you find problems that are
|
|
otherwise extremely difficult to spot.
|
|
|
|
17. If you register a callback using call_rcu(), call_rcu_bh(),
|
|
call_rcu_sched(), or call_srcu(), and 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 waits only for all previous
|
|
callbacks registered on the CPU that synchronize_rcu() is running
|
|
on, but it is -not- guaranteed to wait for callbacks registered
|
|
on other CPUs.
|
|
|
|
You instead need to use one of the barrier functions:
|
|
|
|
o call_rcu() -> rcu_barrier()
|
|
o call_rcu_bh() -> rcu_barrier()
|
|
o call_rcu_sched() -> rcu_barrier()
|
|
o call_srcu() -> srcu_barrier()
|
|
|
|
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.txt for more information.
|