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.

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. Use call_rcu_bh() rather than call_rcu(), in order to take

advantage of call_rcu_bh()'s faster grace periods. (This

is only a partial solution, though.)

e. 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. Note that synchronize_rcu() -only- guarantees to wait until

all currently executing rcu_read_lock()-protected RCU read-side

critical sections complete. It does -not- necessarily guarantee

that all currently running interrupts, NMIs, preempt_disable()

code, or idle loops will complete. Therefore, if your

read-side critical sections are protected by something other

than rcu_read_lock(), do -not- use synchronize_rcu().

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 CONFIG_PREEMPT=y 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().

This same limitation also applies to synchronize_rcu_bh()

and synchronize_srcu(), as well as to the asynchronous and

expedited forms of the three primitives, namely call_rcu(),

call_rcu_bh(), call_srcu(), synchronize_rcu_expedited(),

synchronize_rcu_bh_expedited(), and synchronize_srcu_expedited().

12. 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.

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. 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.

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.

17. 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.

18. 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_bh()

o call_rcu_sched() -> rcu_barrier_sched()

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.