1.. SPDX-License-Identifier: GPL-2.0
2
3================================
4Review Checklist for RCU Patches
5================================
6
7
8This document contains a checklist for producing and reviewing patches
9that make use of RCU.  Violating any of the rules listed below will
10result in the same sorts of problems that leaving out a locking primitive
11would cause.  This list is based on experiences reviewing such patches
12over a rather long period of time, but improvements are always welcome!
13
140.	Is RCU being applied to a read-mostly situation?  If the data
15	structure is updated more than about 10% of the time, then you
16	should strongly consider some other approach, unless detailed
17	performance measurements show that RCU is nonetheless the right
18	tool for the job.  Yes, RCU does reduce read-side overhead by
19	increasing write-side overhead, which is exactly why normal uses
20	of RCU will do much more reading than updating.
21
22	Another exception is where performance is not an issue, and RCU
23	provides a simpler implementation.  An example of this situation
24	is the dynamic NMI code in the Linux 2.6 kernel, at least on
25	architectures where NMIs are rare.
26
27	Yet another exception is where the low real-time latency of RCU's
28	read-side primitives is critically important.
29
30	One final exception is where RCU readers are used to prevent
31	the ABA problem (https://en.wikipedia.org/wiki/ABA_problem)
32	for lockless updates.  This does result in the mildly
33	counter-intuitive situation where rcu_read_lock() and
34	rcu_read_unlock() are used to protect updates, however, this
35	approach can provide the same simplifications to certain types
36	of lockless algorithms that garbage collectors do.
37
381.	Does the update code have proper mutual exclusion?
39
40	RCU does allow *readers* to run (almost) naked, but *writers* must
41	still use some sort of mutual exclusion, such as:
42
43	a.	locking,
44	b.	atomic operations, or
45	c.	restricting updates to a single task.
46
47	If you choose #b, be prepared to describe how you have handled
48	memory barriers on weakly ordered machines (pretty much all of
49	them -- even x86 allows later loads to be reordered to precede
50	earlier stores), and be prepared to explain why this added
51	complexity is worthwhile.  If you choose #c, be prepared to
52	explain how this single task does not become a major bottleneck
53	on large systems (for example, if the task is updating information
54	relating to itself that other tasks can read, there by definition
55	can be no bottleneck).	Note that the definition of "large" has
56	changed significantly:	Eight CPUs was "large" in the year 2000,
57	but a hundred CPUs was unremarkable in 2017.
58
592.	Do the RCU read-side critical sections make proper use of
60	rcu_read_lock() and friends?  These primitives are needed
61	to prevent grace periods from ending prematurely, which
62	could result in data being unceremoniously freed out from
63	under your read-side code, which can greatly increase the
64	actuarial risk of your kernel.
65
66	As a rough rule of thumb, any dereference of an RCU-protected
67	pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(),
68	rcu_read_lock_sched(), or by the appropriate update-side lock.
69	Explicit disabling of preemption (preempt_disable(), for example)
70	can serve as rcu_read_lock_sched(), but is less readable and
71	prevents lockdep from detecting locking issues.
72
73	Please not that you *cannot* rely on code known to be built
74	only in non-preemptible kernels.  Such code can and will break,
75	especially in kernels built with CONFIG_PREEMPT_COUNT=y.
76
77	Letting RCU-protected pointers "leak" out of an RCU read-side
78	critical section is every bit as bad as letting them leak out
79	from under a lock.  Unless, of course, you have arranged some
80	other means of protection, such as a lock or a reference count
81	*before* letting them out of the RCU read-side critical section.
82
833.	Does the update code tolerate concurrent accesses?
84
85	The whole point of RCU is to permit readers to run without
86	any locks or atomic operations.  This means that readers will
87	be running while updates are in progress.  There are a number
88	of ways to handle this concurrency, depending on the situation:
89
90	a.	Use the RCU variants of the list and hlist update
91		primitives to add, remove, and replace elements on
92		an RCU-protected list.	Alternatively, use the other
93		RCU-protected data structures that have been added to
94		the Linux kernel.
95
96		This is almost always the best approach.
97
98	b.	Proceed as in (a) above, but also maintain per-element
99		locks (that are acquired by both readers and writers)
100		that guard per-element state.  Fields that the readers
101		refrain from accessing can be guarded by some other lock
102		acquired only by updaters, if desired.
103
104		This also works quite well.
105
106	c.	Make updates appear atomic to readers.	For example,
107		pointer updates to properly aligned fields will
108		appear atomic, as will individual atomic primitives.
109		Sequences of operations performed under a lock will *not*
110		appear to be atomic to RCU readers, nor will sequences
111		of multiple atomic primitives.	One alternative is to
112		move multiple individual fields to a separate structure,
113		thus solving the multiple-field problem by imposing an
114		additional level of indirection.
115
116		This can work, but is starting to get a bit tricky.
117
118	d.	Carefully order the updates and the reads so that readers
119		see valid data at all phases of the update.  This is often
120		more difficult than it sounds, especially given modern
121		CPUs' tendency to reorder memory references.  One must
122		usually liberally sprinkle memory-ordering operations
123		through the code, making it difficult to understand and
124		to test.  Where it works, it is better to use things
125		like smp_store_release() and smp_load_acquire(), but in
126		some cases the smp_mb() full memory barrier is required.
127
128		As noted earlier, it is usually better to group the
129		changing data into a separate structure, so that the
130		change may be made to appear atomic by updating a pointer
131		to reference a new structure containing updated values.
132
1334.	Weakly ordered CPUs pose special challenges.  Almost all CPUs
134	are weakly ordered -- even x86 CPUs allow later loads to be
135	reordered to precede earlier stores.  RCU code must take all of
136	the following measures to prevent memory-corruption problems:
137
138	a.	Readers must maintain proper ordering of their memory
139		accesses.  The rcu_dereference() primitive ensures that
140		the CPU picks up the pointer before it picks up the data
141		that the pointer points to.  This really is necessary
142		on Alpha CPUs.
143
144		The rcu_dereference() primitive is also an excellent
145		documentation aid, letting the person reading the
146		code know exactly which pointers are protected by RCU.
147		Please note that compilers can also reorder code, and
148		they are becoming increasingly aggressive about doing
149		just that.  The rcu_dereference() primitive therefore also
150		prevents destructive compiler optimizations.  However,
151		with a bit of devious creativity, it is possible to
152		mishandle the return value from rcu_dereference().
153		Please see rcu_dereference.rst for more information.
154
155		The rcu_dereference() primitive is used by the
156		various "_rcu()" list-traversal primitives, such
157		as the list_for_each_entry_rcu().  Note that it is
158		perfectly legal (if redundant) for update-side code to
159		use rcu_dereference() and the "_rcu()" list-traversal
160		primitives.  This is particularly useful in code that
161		is common to readers and updaters.  However, lockdep
162		will complain if you access rcu_dereference() outside
163		of an RCU read-side critical section.  See lockdep.rst
164		to learn what to do about this.
165
166		Of course, neither rcu_dereference() nor the "_rcu()"
167		list-traversal primitives can substitute for a good
168		concurrency design coordinating among multiple updaters.
169
170	b.	If the list macros are being used, the list_add_tail_rcu()
171		and list_add_rcu() primitives must be used in order
172		to prevent weakly ordered machines from misordering
173		structure initialization and pointer planting.
174		Similarly, if the hlist macros are being used, the
175		hlist_add_head_rcu() primitive is required.
176
177	c.	If the list macros are being used, the list_del_rcu()
178		primitive must be used to keep list_del()'s pointer
179		poisoning from inflicting toxic effects on concurrent
180		readers.  Similarly, if the hlist macros are being used,
181		the hlist_del_rcu() primitive is required.
182
183		The list_replace_rcu() and hlist_replace_rcu() primitives
184		may be used to replace an old structure with a new one
185		in their respective types of RCU-protected lists.
186
187	d.	Rules similar to (4b) and (4c) apply to the "hlist_nulls"
188		type of RCU-protected linked lists.
189
190	e.	Updates must ensure that initialization of a given
191		structure happens before pointers to that structure are
192		publicized.  Use the rcu_assign_pointer() primitive
193		when publicizing a pointer to a structure that can
194		be traversed by an RCU read-side critical section.
195
1965.	If any of call_rcu(), call_srcu(), call_rcu_tasks(),
197	call_rcu_tasks_rude(), or call_rcu_tasks_trace() is used,
198	the callback function may be invoked from softirq context,
199	and in any case with bottom halves disabled.  In particular,
200	this callback function cannot block.  If you need the callback
201	to block, run that code in a workqueue handler scheduled from
202	the callback.  The queue_rcu_work() function does this for you
203	in the case of call_rcu().
204
2056.	Since synchronize_rcu() can block, it cannot be called
206	from any sort of irq context.  The same rule applies
207	for synchronize_srcu(), synchronize_rcu_expedited(),
208	synchronize_srcu_expedited(), synchronize_rcu_tasks(),
209	synchronize_rcu_tasks_rude(), and synchronize_rcu_tasks_trace().
210
211	The expedited forms of these primitives have the same semantics
212	as the non-expedited forms, but expediting is more CPU intensive.
213	Use of the expedited primitives should be restricted to rare
214	configuration-change operations that would not normally be
215	undertaken while a real-time workload is running.  Note that
216	IPI-sensitive real-time workloads can use the rcupdate.rcu_normal
217	kernel boot parameter to completely disable expedited grace
218	periods, though this might have performance implications.
219
220	In particular, if you find yourself invoking one of the expedited
221	primitives repeatedly in a loop, please do everyone a favor:
222	Restructure your code so that it batches the updates, allowing
223	a single non-expedited primitive to cover the entire batch.
224	This will very likely be faster than the loop containing the
225	expedited primitive, and will be much much easier on the rest
226	of the system, especially to real-time workloads running on the
227	rest of the system.  Alternatively, instead use asynchronous
228	primitives such as call_rcu().
229
2307.	As of v4.20, a given kernel implements only one RCU flavor, which
231	is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y.
232	If the updater uses call_rcu() or synchronize_rcu(), then
233	the corresponding readers may use:  (1) rcu_read_lock() and
234	rcu_read_unlock(), (2) any pair of primitives that disables
235	and re-enables softirq, for example, rcu_read_lock_bh() and
236	rcu_read_unlock_bh(), or (3) any pair of primitives that disables
237	and re-enables preemption, for example, rcu_read_lock_sched() and
238	rcu_read_unlock_sched().  If the updater uses synchronize_srcu()
239	or call_srcu(), then the corresponding readers must use
240	srcu_read_lock() and srcu_read_unlock(), and with the same
241	srcu_struct.  The rules for the expedited RCU grace-period-wait
242	primitives are the same as for their non-expedited counterparts.
243
244	If the updater uses call_rcu_tasks() or synchronize_rcu_tasks(),
245	then the readers must refrain from executing voluntary
246	context switches, that is, from blocking.  If the updater uses
247	call_rcu_tasks_trace() or synchronize_rcu_tasks_trace(), then
248	the corresponding readers must use rcu_read_lock_trace() and
249	rcu_read_unlock_trace().  If an updater uses call_rcu_tasks_rude()
250	or synchronize_rcu_tasks_rude(), then the corresponding readers
251	must use anything that disables preemption, for example,
252	preempt_disable() and preempt_enable().
253
254	Mixing things up will result in confusion and broken kernels, and
255	has even resulted in an exploitable security issue.  Therefore,
256	when using non-obvious pairs of primitives, commenting is
257	of course a must.  One example of non-obvious pairing is
258	the XDP feature in networking, which calls BPF programs from
259	network-driver NAPI (softirq) context.	BPF relies heavily on RCU
260	protection for its data structures, but because the BPF program
261	invocation happens entirely within a single local_bh_disable()
262	section in a NAPI poll cycle, this usage is safe.  The reason
263	that this usage is safe is that readers can use anything that
264	disables BH when updaters use call_rcu() or synchronize_rcu().
265
2668.	Although synchronize_rcu() is slower than is call_rcu(),
267	it usually results in simpler code.  So, unless update
268	performance is critically important, the updaters cannot block,
269	or the latency of synchronize_rcu() is visible from userspace,
270	synchronize_rcu() should be used in preference to call_rcu().
271	Furthermore, kfree_rcu() and kvfree_rcu() usually result
272	in even simpler code than does synchronize_rcu() without
273	synchronize_rcu()'s multi-millisecond latency.	So please take
274	advantage of kfree_rcu()'s and kvfree_rcu()'s "fire and forget"
275	memory-freeing capabilities where it applies.
276
277	An especially important property of the synchronize_rcu()
278	primitive is that it automatically self-limits: if grace periods
279	are delayed for whatever reason, then the synchronize_rcu()
280	primitive will correspondingly delay updates.  In contrast,
281	code using call_rcu() should explicitly limit update rate in
282	cases where grace periods are delayed, as failing to do so can
283	result in excessive realtime latencies or even OOM conditions.
284
285	Ways of gaining this self-limiting property when using call_rcu(),
286	kfree_rcu(), or kvfree_rcu() include:
287
288	a.	Keeping a count of the number of data-structure elements
289		used by the RCU-protected data structure, including
290		those waiting for a grace period to elapse.  Enforce a
291		limit on this number, stalling updates as needed to allow
292		previously deferred frees to complete.	Alternatively,
293		limit only the number awaiting deferred free rather than
294		the total number of elements.
295
296		One way to stall the updates is to acquire the update-side
297		mutex.	(Don't try this with a spinlock -- other CPUs
298		spinning on the lock could prevent the grace period
299		from ever ending.)  Another way to stall the updates
300		is for the updates to use a wrapper function around
301		the memory allocator, so that this wrapper function
302		simulates OOM when there is too much memory awaiting an
303		RCU grace period.  There are of course many other
304		variations on this theme.
305
306	b.	Limiting update rate.  For example, if updates occur only
307		once per hour, then no explicit rate limiting is
308		required, unless your system is already badly broken.
309		Older versions of the dcache subsystem take this approach,
310		guarding updates with a global lock, limiting their rate.
311
312	c.	Trusted update -- if updates can only be done manually by
313		superuser or some other trusted user, then it might not
314		be necessary to automatically limit them.  The theory
315		here is that superuser already has lots of ways to crash
316		the machine.
317
318	d.	Periodically invoke rcu_barrier(), permitting a limited
319		number of updates per grace period.
320
321	The same cautions apply to call_srcu(), call_rcu_tasks(),
322	call_rcu_tasks_rude(), and call_rcu_tasks_trace().  This is
323	why there is an srcu_barrier(), rcu_barrier_tasks(),
324	rcu_barrier_tasks_rude(), and rcu_barrier_tasks_rude(),
325	respectively.
326
327	Note that although these primitives do take action to avoid
328	memory exhaustion when any given CPU has too many callbacks,
329	a determined user or administrator can still exhaust memory.
330	This is especially the case if a system with a large number of
331	CPUs has been configured to offload all of its RCU callbacks onto
332	a single CPU, or if the system has relatively little free memory.
333
3349.	All RCU list-traversal primitives, which include
335	rcu_dereference(), list_for_each_entry_rcu(), and
336	list_for_each_safe_rcu(), must be either within an RCU read-side
337	critical section or must be protected by appropriate update-side
338	locks.	RCU read-side critical sections are delimited by
339	rcu_read_lock() and rcu_read_unlock(), or by similar primitives
340	such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which
341	case the matching rcu_dereference() primitive must be used in
342	order to keep lockdep happy, in this case, rcu_dereference_bh().
343
344	The reason that it is permissible to use RCU list-traversal
345	primitives when the update-side lock is held is that doing so
346	can be quite helpful in reducing code bloat when common code is
347	shared between readers and updaters.  Additional primitives
348	are provided for this case, as discussed in lockdep.rst.
349
350	One exception to this rule is when data is only ever added to
351	the linked data structure, and is never removed during any
352	time that readers might be accessing that structure.  In such
353	cases, READ_ONCE() may be used in place of rcu_dereference()
354	and the read-side markers (rcu_read_lock() and rcu_read_unlock(),
355	for example) may be omitted.
356
35710.	Conversely, if you are in an RCU read-side critical section,
358	and you don't hold the appropriate update-side lock, you *must*
359	use the "_rcu()" variants of the list macros.  Failing to do so
360	will break Alpha, cause aggressive compilers to generate bad code,
361	and confuse people trying to understand your code.
362
36311.	Any lock acquired by an RCU callback must be acquired elsewhere
364	with softirq disabled, e.g., via spin_lock_bh().  Failing to
365	disable softirq on a given acquisition of that lock will result
366	in deadlock as soon as the RCU softirq handler happens to run
367	your RCU callback while interrupting that acquisition's critical
368	section.
369
37012.	RCU callbacks can be and are executed in parallel.  In many cases,
371	the callback code simply wrappers around kfree(), so that this
372	is not an issue (or, more accurately, to the extent that it is
373	an issue, the memory-allocator locking handles it).  However,
374	if the callbacks do manipulate a shared data structure, they
375	must use whatever locking or other synchronization is required
376	to safely access and/or modify that data structure.
377
378	Do not assume that RCU callbacks will be executed on the same
379	CPU that executed the corresponding call_rcu() or call_srcu().
380	For example, if a given CPU goes offline while having an RCU
381	callback pending, then that RCU callback will execute on some
382	surviving CPU.	(If this was not the case, a self-spawning RCU
383	callback would prevent the victim CPU from ever going offline.)
384	Furthermore, CPUs designated by rcu_nocbs= might well *always*
385	have their RCU callbacks executed on some other CPUs, in fact,
386	for some  real-time workloads, this is the whole point of using
387	the rcu_nocbs= kernel boot parameter.
388
389	In addition, do not assume that callbacks queued in a given order
390	will be invoked in that order, even if they all are queued on the
391	same CPU.  Furthermore, do not assume that same-CPU callbacks will
392	be invoked serially.  For example, in recent kernels, CPUs can be
393	switched between offloaded and de-offloaded callback invocation,
394	and while a given CPU is undergoing such a switch, its callbacks
395	might be concurrently invoked by that CPU's softirq handler and
396	that CPU's rcuo kthread.  At such times, that CPU's callbacks
397	might be executed both concurrently and out of order.
398
39913.	Unlike most flavors of RCU, it *is* permissible to block in an
400	SRCU read-side critical section (demarked by srcu_read_lock()
401	and srcu_read_unlock()), hence the "SRCU": "sleepable RCU".
402	Please note that if you don't need to sleep in read-side critical
403	sections, you should be using RCU rather than SRCU, because RCU
404	is almost always faster and easier to use than is SRCU.
405
406	Also unlike other forms of RCU, explicit initialization and
407	cleanup is required either at build time via DEFINE_SRCU()
408	or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct()
409	and cleanup_srcu_struct().  These last two are passed a
410	"struct srcu_struct" that defines the scope of a given
411	SRCU domain.  Once initialized, the srcu_struct is passed
412	to srcu_read_lock(), srcu_read_unlock() synchronize_srcu(),
413	synchronize_srcu_expedited(), and call_srcu().	A given
414	synchronize_srcu() waits only for SRCU read-side critical
415	sections governed by srcu_read_lock() and srcu_read_unlock()
416	calls that have been passed the same srcu_struct.  This property
417	is what makes sleeping read-side critical sections tolerable --
418	a given subsystem delays only its own updates, not those of other
419	subsystems using SRCU.	Therefore, SRCU is less prone to OOM the
420	system than RCU would be if RCU's read-side critical sections
421	were permitted to sleep.
422
423	The ability to sleep in read-side critical sections does not
424	come for free.	First, corresponding srcu_read_lock() and
425	srcu_read_unlock() calls must be passed the same srcu_struct.
426	Second, grace-period-detection overhead is amortized only
427	over those updates sharing a given srcu_struct, rather than
428	being globally amortized as they are for other forms of RCU.
429	Therefore, SRCU should be used in preference to rw_semaphore
430	only in extremely read-intensive situations, or in situations
431	requiring SRCU's read-side deadlock immunity or low read-side
432	realtime latency.  You should also consider percpu_rw_semaphore
433	when you need lightweight readers.
434
435	SRCU's expedited primitive (synchronize_srcu_expedited())
436	never sends IPIs to other CPUs, so it is easier on
437	real-time workloads than is synchronize_rcu_expedited().
438
439	It is also permissible to sleep in RCU Tasks Trace read-side
440	critical, which are delimited by rcu_read_lock_trace() and
441	rcu_read_unlock_trace().  However, this is a specialized flavor
442	of RCU, and you should not use it without first checking with
443	its current users.  In most cases, you should instead use SRCU.
444
445	Note that rcu_assign_pointer() relates to SRCU just as it does to
446	other forms of RCU, but instead of rcu_dereference() you should
447	use srcu_dereference() in order to avoid lockdep splats.
448
44914.	The whole point of call_rcu(), synchronize_rcu(), and friends
450	is to wait until all pre-existing readers have finished before
451	carrying out some otherwise-destructive operation.  It is
452	therefore critically important to *first* remove any path
453	that readers can follow that could be affected by the
454	destructive operation, and *only then* invoke call_rcu(),
455	synchronize_rcu(), or friends.
456
457	Because these primitives only wait for pre-existing readers, it
458	is the caller's responsibility to guarantee that any subsequent
459	readers will execute safely.
460
46115.	The various RCU read-side primitives do *not* necessarily contain
462	memory barriers.  You should therefore plan for the CPU
463	and the compiler to freely reorder code into and out of RCU
464	read-side critical sections.  It is the responsibility of the
465	RCU update-side primitives to deal with this.
466
467	For SRCU readers, you can use smp_mb__after_srcu_read_unlock()
468	immediately after an srcu_read_unlock() to get a full barrier.
469
47016.	Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
471	__rcu sparse checks to validate your RCU code.	These can help
472	find problems as follows:
473
474	CONFIG_PROVE_LOCKING:
475		check that accesses to RCU-protected data structures
476		are carried out under the proper RCU read-side critical
477		section, while holding the right combination of locks,
478		or whatever other conditions are appropriate.
479
480	CONFIG_DEBUG_OBJECTS_RCU_HEAD:
481		check that you don't pass the same object to call_rcu()
482		(or friends) before an RCU grace period has elapsed
483		since the last time that you passed that same object to
484		call_rcu() (or friends).
485
486	__rcu sparse checks:
487		tag the pointer to the RCU-protected data structure
488		with __rcu, and sparse will warn you if you access that
489		pointer without the services of one of the variants
490		of rcu_dereference().
491
492	These debugging aids can help you find problems that are
493	otherwise extremely difficult to spot.
494
49517.	If you pass a callback function defined within a module to one of
496	call_rcu(), call_srcu(), call_rcu_tasks(), call_rcu_tasks_rude(),
497	or call_rcu_tasks_trace(), then it is necessary to wait for all
498	pending callbacks to be invoked before unloading that module.
499	Note that it is absolutely *not* sufficient to wait for a grace
500	period!  For example, synchronize_rcu() implementation is *not*
501	guaranteed to wait for callbacks registered on other CPUs via
502	call_rcu().  Or even on the current CPU if that CPU recently
503	went offline and came back online.
504
505	You instead need to use one of the barrier functions:
506
507	-	call_rcu() -> rcu_barrier()
508	-	call_srcu() -> srcu_barrier()
509	-	call_rcu_tasks() -> rcu_barrier_tasks()
510	-	call_rcu_tasks_rude() -> rcu_barrier_tasks_rude()
511	-	call_rcu_tasks_trace() -> rcu_barrier_tasks_trace()
512
513	However, these barrier functions are absolutely *not* guaranteed
514	to wait for a grace period.  For example, if there are no
515	call_rcu() callbacks queued anywhere in the system, rcu_barrier()
516	can and will return immediately.
517
518	So if you need to wait for both a grace period and for all
519	pre-existing callbacks, you will need to invoke both functions,
520	with the pair depending on the flavor of RCU:
521
522	-	Either synchronize_rcu() or synchronize_rcu_expedited(),
523		together with rcu_barrier()
524	-	Either synchronize_srcu() or synchronize_srcu_expedited(),
525		together with and srcu_barrier()
526	-	synchronize_rcu_tasks() and rcu_barrier_tasks()
527	-	synchronize_tasks_rude() and rcu_barrier_tasks_rude()
528	-	synchronize_tasks_trace() and rcu_barrier_tasks_trace()
529
530	If necessary, you can use something like workqueues to execute
531	the requisite pair of functions concurrently.
532
533	See rcubarrier.rst for more information.
534