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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	Review Checklist for RCU Patches
2	
3	
4	This document contains a checklist for producing and reviewing patches
5	that make use of RCU.  Violating any of the rules listed below will
6	result in the same sorts of problems that leaving out a locking primitive
7	would cause.  This list is based on experiences reviewing such patches
8	over a rather long period of time, but improvements are always welcome!
9	
10	0.	Is RCU being applied to a read-mostly situation?  If the data
11		structure is updated more than about 10% of the time, then you
12		should strongly consider some other approach, unless detailed
13		performance measurements show that RCU is nonetheless the right
14		tool for the job.  Yes, RCU does reduce read-side overhead by
15		increasing write-side overhead, which is exactly why normal uses
16		of RCU will do much more reading than updating.
17	
18		Another exception is where performance is not an issue, and RCU
19		provides a simpler implementation.  An example of this situation
20		is the dynamic NMI code in the Linux 2.6 kernel, at least on
21		architectures where NMIs are rare.
22	
23		Yet another exception is where the low real-time latency of RCU's
24		read-side primitives is critically important.
25	
26	1.	Does the update code have proper mutual exclusion?
27	
28		RCU does allow -readers- to run (almost) naked, but -writers- must
29		still use some sort of mutual exclusion, such as:
30	
31		a.	locking,
32		b.	atomic operations, or
33		c.	restricting updates to a single task.
34	
35		If you choose #b, be prepared to describe how you have handled
36		memory barriers on weakly ordered machines (pretty much all of
37		them -- even x86 allows later loads to be reordered to precede
38		earlier stores), and be prepared to explain why this added
39		complexity is worthwhile.  If you choose #c, be prepared to
40		explain how this single task does not become a major bottleneck on
41		big multiprocessor machines (for example, if the task is updating
42		information relating to itself that other tasks can read, there
43		by definition can be no bottleneck).
44	
45	2.	Do the RCU read-side critical sections make proper use of
46		rcu_read_lock() and friends?  These primitives are needed
47		to prevent grace periods from ending prematurely, which
48		could result in data being unceremoniously freed out from
49		under your read-side code, which can greatly increase the
50		actuarial risk of your kernel.
51	
52		As a rough rule of thumb, any dereference of an RCU-protected
53		pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(),
54		rcu_read_lock_sched(), or by the appropriate update-side lock.
55		Disabling of preemption can serve as rcu_read_lock_sched(), but
56		is less readable.
57	
58	3.	Does the update code tolerate concurrent accesses?
59	
60		The whole point of RCU is to permit readers to run without
61		any locks or atomic operations.  This means that readers will
62		be running while updates are in progress.  There are a number
63		of ways to handle this concurrency, depending on the situation:
64	
65		a.	Use the RCU variants of the list and hlist update
66			primitives to add, remove, and replace elements on
67			an RCU-protected list.	Alternatively, use the other
68			RCU-protected data structures that have been added to
69			the Linux kernel.
70	
71			This is almost always the best approach.
72	
73		b.	Proceed as in (a) above, but also maintain per-element
74			locks (that are acquired by both readers and writers)
75			that guard per-element state.  Of course, fields that
76			the readers refrain from accessing can be guarded by
77			some other lock acquired only by updaters, if desired.
78	
79			This works quite well, also.
80	
81		c.	Make updates appear atomic to readers.  For example,
82			pointer updates to properly aligned fields will
83			appear atomic, as will individual atomic primitives.
84			Sequences of perations performed under a lock will -not-
85			appear to be atomic to RCU readers, nor will sequences
86			of multiple atomic primitives.
87	
88			This can work, but is starting to get a bit tricky.
89	
90		d.	Carefully order the updates and the reads so that
91			readers see valid data at all phases of the update.
92			This is often more difficult than it sounds, especially
93			given modern CPUs' tendency to reorder memory references.
94			One must usually liberally sprinkle memory barriers
95			(smp_wmb(), smp_rmb(), smp_mb()) through the code,
96			making it difficult to understand and to test.
97	
98			It is usually better to group the changing data into
99			a separate structure, so that the change may be made
100			to appear atomic by updating a pointer to reference
101			a new structure containing updated values.
102	
103	4.	Weakly ordered CPUs pose special challenges.  Almost all CPUs
104		are weakly ordered -- even x86 CPUs allow later loads to be
105		reordered to precede earlier stores.  RCU code must take all of
106		the following measures to prevent memory-corruption problems:
107	
108		a.	Readers must maintain proper ordering of their memory
109			accesses.  The rcu_dereference() primitive ensures that
110			the CPU picks up the pointer before it picks up the data
111			that the pointer points to.  This really is necessary
112			on Alpha CPUs.	If you don't believe me, see:
113	
114				http://www.openvms.compaq.com/wizard/wiz_2637.html
115	
116			The rcu_dereference() primitive is also an excellent
117			documentation aid, letting the person reading the code
118			know exactly which pointers are protected by RCU.
119			Please note that compilers can also reorder code, and
120			they are becoming increasingly aggressive about doing
121			just that.  The rcu_dereference() primitive therefore
122			also prevents destructive compiler optimizations.
123	
124			The rcu_dereference() primitive is used by the
125			various "_rcu()" list-traversal primitives, such
126			as the list_for_each_entry_rcu().  Note that it is
127			perfectly legal (if redundant) for update-side code to
128			use rcu_dereference() and the "_rcu()" list-traversal
129			primitives.  This is particularly useful in code that
130			is common to readers and updaters.  However, lockdep
131			will complain if you access rcu_dereference() outside
132			of an RCU read-side critical section.  See lockdep.txt
133			to learn what to do about this.
134	
135			Of course, neither rcu_dereference() nor the "_rcu()"
136			list-traversal primitives can substitute for a good
137			concurrency design coordinating among multiple updaters.
138	
139		b.	If the list macros are being used, the list_add_tail_rcu()
140			and list_add_rcu() primitives must be used in order
141			to prevent weakly ordered machines from misordering
142			structure initialization and pointer planting.
143			Similarly, if the hlist macros are being used, the
144			hlist_add_head_rcu() primitive is required.
145	
146		c.	If the list macros are being used, the list_del_rcu()
147			primitive must be used to keep list_del()'s pointer
148			poisoning from inflicting toxic effects on concurrent
149			readers.  Similarly, if the hlist macros are being used,
150			the hlist_del_rcu() primitive is required.
151	
152			The list_replace_rcu() and hlist_replace_rcu() primitives
153			may be used to replace an old structure with a new one
154			in their respective types of RCU-protected lists.
155	
156		d.	Rules similar to (4b) and (4c) apply to the "hlist_nulls"
157			type of RCU-protected linked lists.
158	
159		e.	Updates must ensure that initialization of a given
160			structure happens before pointers to that structure are
161			publicized.  Use the rcu_assign_pointer() primitive
162			when publicizing a pointer to a structure that can
163			be traversed by an RCU read-side critical section.
164	
165	5.	If call_rcu(), or a related primitive such as call_rcu_bh(),
166		call_rcu_sched(), or call_srcu() is used, the callback function
167		must be written to be called from softirq context.  In particular,
168		it cannot block.
169	
170	6.	Since synchronize_rcu() can block, it cannot be called from
171		any sort of irq context.  The same rule applies for
172		synchronize_rcu_bh(), synchronize_sched(), synchronize_srcu(),
173		synchronize_rcu_expedited(), synchronize_rcu_bh_expedited(),
174		synchronize_sched_expedite(), and synchronize_srcu_expedited().
175	
176		The expedited forms of these primitives have the same semantics
177		as the non-expedited forms, but expediting is both expensive
178		and unfriendly to real-time workloads.	Use of the expedited
179		primitives should be restricted to rare configuration-change
180		operations that would not normally be undertaken while a real-time
181		workload is running.
182	
183		In particular, if you find yourself invoking one of the expedited
184		primitives repeatedly in a loop, please do everyone a favor:
185		Restructure your code so that it batches the updates, allowing
186		a single non-expedited primitive to cover the entire batch.
187		This will very likely be faster than the loop containing the
188		expedited primitive, and will be much much easier on the rest
189		of the system, especially to real-time workloads running on
190		the rest of the system.
191	
192		In addition, it is illegal to call the expedited forms from
193		a CPU-hotplug notifier, or while holding a lock that is acquired
194		by a CPU-hotplug notifier.  Failing to observe this restriction
195		will result in deadlock.
196	
197	7.	If the updater uses call_rcu() or synchronize_rcu(), then the
198		corresponding readers must use rcu_read_lock() and
199		rcu_read_unlock().  If the updater uses call_rcu_bh() or
200		synchronize_rcu_bh(), then the corresponding readers must
201		use rcu_read_lock_bh() and rcu_read_unlock_bh().  If the
202		updater uses call_rcu_sched() or synchronize_sched(), then
203		the corresponding readers must disable preemption, possibly
204		by calling rcu_read_lock_sched() and rcu_read_unlock_sched().
205		If the updater uses synchronize_srcu() or call_srcu(), then
206		the corresponding readers must use srcu_read_lock() and
207		srcu_read_unlock(), and with the same srcu_struct.  The rules for
208		the expedited primitives are the same as for their non-expedited
209		counterparts.  Mixing things up will result in confusion and
210		broken kernels.
211	
212		One exception to this rule: rcu_read_lock() and rcu_read_unlock()
213		may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
214		in cases where local bottom halves are already known to be
215		disabled, for example, in irq or softirq context.  Commenting
216		such cases is a must, of course!  And the jury is still out on
217		whether the increased speed is worth it.
218	
219	8.	Although synchronize_rcu() is slower than is call_rcu(), it
220		usually results in simpler code.  So, unless update performance is
221		critically important, the updaters cannot block, or the latency of
222		synchronize_rcu() is visible from userspace, synchronize_rcu()
223		should be used in preference to call_rcu().  Furthermore,
224		kfree_rcu() usually results in even simpler code than does
225		synchronize_rcu() without synchronize_rcu()'s multi-millisecond
226		latency.  So please take advantage of kfree_rcu()'s "fire and
227		forget" memory-freeing capabilities where it applies.
228	
229		An especially important property of the synchronize_rcu()
230		primitive is that it automatically self-limits: if grace periods
231		are delayed for whatever reason, then the synchronize_rcu()
232		primitive will correspondingly delay updates.  In contrast,
233		code using call_rcu() should explicitly limit update rate in
234		cases where grace periods are delayed, as failing to do so can
235		result in excessive realtime latencies or even OOM conditions.
236	
237		Ways of gaining this self-limiting property when using call_rcu()
238		include:
239	
240		a.	Keeping a count of the number of data-structure elements
241			used by the RCU-protected data structure, including
242			those waiting for a grace period to elapse.  Enforce a
243			limit on this number, stalling updates as needed to allow
244			previously deferred frees to complete.	Alternatively,
245			limit only the number awaiting deferred free rather than
246			the total number of elements.
247	
248			One way to stall the updates is to acquire the update-side
249			mutex.	(Don't try this with a spinlock -- other CPUs
250			spinning on the lock could prevent the grace period
251			from ever ending.)  Another way to stall the updates
252			is for the updates to use a wrapper function around
253			the memory allocator, so that this wrapper function
254			simulates OOM when there is too much memory awaiting an
255			RCU grace period.  There are of course many other
256			variations on this theme.
257	
258		b.	Limiting update rate.  For example, if updates occur only
259			once per hour, then no explicit rate limiting is
260			required, unless your system is already badly broken.
261			Older versions of the dcache subsystem take this approach,
262			guarding updates with a global lock, limiting their rate.
263	
264		c.	Trusted update -- if updates can only be done manually by
265			superuser or some other trusted user, then it might not
266			be necessary to automatically limit them.  The theory
267			here is that superuser already has lots of ways to crash
268			the machine.
269	
270		d.	Use call_rcu_bh() rather than call_rcu(), in order to take
271			advantage of call_rcu_bh()'s faster grace periods.  (This
272			is only a partial solution, though.)
273	
274		e.	Periodically invoke synchronize_rcu(), permitting a limited
275			number of updates per grace period.
276	
277		The same cautions apply to call_rcu_bh(), call_rcu_sched(),
278		call_srcu(), and kfree_rcu().
279	
280		Note that although these primitives do take action to avoid memory
281		exhaustion when any given CPU has too many callbacks, a determined
282		user could still exhaust memory.  This is especially the case
283		if a system with a large number of CPUs has been configured to
284		offload all of its RCU callbacks onto a single CPU, or if the
285		system has relatively little free memory.
286	
287	9.	All RCU list-traversal primitives, which include
288		rcu_dereference(), list_for_each_entry_rcu(), and
289		list_for_each_safe_rcu(), must be either within an RCU read-side
290		critical section or must be protected by appropriate update-side
291		locks.	RCU read-side critical sections are delimited by
292		rcu_read_lock() and rcu_read_unlock(), or by similar primitives
293		such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which
294		case the matching rcu_dereference() primitive must be used in
295		order to keep lockdep happy, in this case, rcu_dereference_bh().
296	
297		The reason that it is permissible to use RCU list-traversal
298		primitives when the update-side lock is held is that doing so
299		can be quite helpful in reducing code bloat when common code is
300		shared between readers and updaters.  Additional primitives
301		are provided for this case, as discussed in lockdep.txt.
302	
303	10.	Conversely, if you are in an RCU read-side critical section,
304		and you don't hold the appropriate update-side lock, you -must-
305		use the "_rcu()" variants of the list macros.  Failing to do so
306		will break Alpha, cause aggressive compilers to generate bad code,
307		and confuse people trying to read your code.
308	
309	11.	Note that synchronize_rcu() -only- guarantees to wait until
310		all currently executing rcu_read_lock()-protected RCU read-side
311		critical sections complete.  It does -not- necessarily guarantee
312		that all currently running interrupts, NMIs, preempt_disable()
313		code, or idle loops will complete.  Therefore, if your
314		read-side critical sections are protected by something other
315		than rcu_read_lock(), do -not- use synchronize_rcu().
316	
317		Similarly, disabling preemption is not an acceptable substitute
318		for rcu_read_lock().  Code that attempts to use preemption
319		disabling where it should be using rcu_read_lock() will break
320		in real-time kernel builds.
321	
322		If you want to wait for interrupt handlers, NMI handlers, and
323		code under the influence of preempt_disable(), you instead
324		need to use synchronize_irq() or synchronize_sched().
325	
326		This same limitation also applies to synchronize_rcu_bh()
327		and synchronize_srcu(), as well as to the asynchronous and
328		expedited forms of the three primitives, namely call_rcu(),
329		call_rcu_bh(), call_srcu(), synchronize_rcu_expedited(),
330		synchronize_rcu_bh_expedited(), and synchronize_srcu_expedited().
331	
332	12.	Any lock acquired by an RCU callback must be acquired elsewhere
333		with softirq disabled, e.g., via spin_lock_irqsave(),
334		spin_lock_bh(), etc.  Failing to disable irq on a given
335		acquisition of that lock will result in deadlock as soon as
336		the RCU softirq handler happens to run your RCU callback while
337		interrupting that acquisition's critical section.
338	
339	13.	RCU callbacks can be and are executed in parallel.  In many cases,
340		the callback code simply wrappers around kfree(), so that this
341		is not an issue (or, more accurately, to the extent that it is
342		an issue, the memory-allocator locking handles it).  However,
343		if the callbacks do manipulate a shared data structure, they
344		must use whatever locking or other synchronization is required
345		to safely access and/or modify that data structure.
346	
347		RCU callbacks are -usually- executed on the same CPU that executed
348		the corresponding call_rcu(), call_rcu_bh(), or call_rcu_sched(),
349		but are by -no- means guaranteed to be.  For example, if a given
350		CPU goes offline while having an RCU callback pending, then that
351		RCU callback will execute on some surviving CPU.  (If this was
352		not the case, a self-spawning RCU callback would prevent the
353		victim CPU from ever going offline.)
354	
355	14.	SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
356		synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu())
357		may only be invoked from process context.  Unlike other forms of
358		RCU, it -is- permissible to block in an SRCU read-side critical
359		section (demarked by srcu_read_lock() and srcu_read_unlock()),
360		hence the "SRCU": "sleepable RCU".  Please note that if you
361		don't need to sleep in read-side critical sections, you should be
362		using RCU rather than SRCU, because RCU is almost always faster
363		and easier to use than is SRCU.
364	
365		Also unlike other forms of RCU, explicit initialization
366		and cleanup is required via init_srcu_struct() and
367		cleanup_srcu_struct().	These are passed a "struct srcu_struct"
368		that defines the scope of a given SRCU domain.	Once initialized,
369		the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
370		synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu().
371		A given synchronize_srcu() waits only for SRCU read-side critical
372		sections governed by srcu_read_lock() and srcu_read_unlock()
373		calls that have been passed the same srcu_struct.  This property
374		is what makes sleeping read-side critical sections tolerable --
375		a given subsystem delays only its own updates, not those of other
376		subsystems using SRCU.	Therefore, SRCU is less prone to OOM the
377		system than RCU would be if RCU's read-side critical sections
378		were permitted to sleep.
379	
380		The ability to sleep in read-side critical sections does not
381		come for free.	First, corresponding srcu_read_lock() and
382		srcu_read_unlock() calls must be passed the same srcu_struct.
383		Second, grace-period-detection overhead is amortized only
384		over those updates sharing a given srcu_struct, rather than
385		being globally amortized as they are for other forms of RCU.
386		Therefore, SRCU should be used in preference to rw_semaphore
387		only in extremely read-intensive situations, or in situations
388		requiring SRCU's read-side deadlock immunity or low read-side
389		realtime latency.
390	
391		Note that, rcu_assign_pointer() relates to SRCU just as it does
392		to other forms of RCU.
393	
394	15.	The whole point of call_rcu(), synchronize_rcu(), and friends
395		is to wait until all pre-existing readers have finished before
396		carrying out some otherwise-destructive operation.  It is
397		therefore critically important to -first- remove any path
398		that readers can follow that could be affected by the
399		destructive operation, and -only- -then- invoke call_rcu(),
400		synchronize_rcu(), or friends.
401	
402		Because these primitives only wait for pre-existing readers, it
403		is the caller's responsibility to guarantee that any subsequent
404		readers will execute safely.
405	
406	16.	The various RCU read-side primitives do -not- necessarily contain
407		memory barriers.  You should therefore plan for the CPU
408		and the compiler to freely reorder code into and out of RCU
409		read-side critical sections.  It is the responsibility of the
410		RCU update-side primitives to deal with this.
411	
412	17.	Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
413		__rcu sparse checks (enabled by CONFIG_SPARSE_RCU_POINTER) to
414		validate your RCU code.  These can help find problems as follows:
415	
416		CONFIG_PROVE_RCU: check that accesses to RCU-protected data
417			structures are carried out under the proper RCU
418			read-side critical section, while holding the right
419			combination of locks, or whatever other conditions
420			are appropriate.
421	
422		CONFIG_DEBUG_OBJECTS_RCU_HEAD: check that you don't pass the
423			same object to call_rcu() (or friends) before an RCU
424			grace period has elapsed since the last time that you
425			passed that same object to call_rcu() (or friends).
426	
427		__rcu sparse checks: tag the pointer to the RCU-protected data
428			structure with __rcu, and sparse will warn you if you
429			access that pointer without the services of one of the
430			variants of rcu_dereference().
431	
432		These debugging aids can help you find problems that are
433		otherwise extremely difficult to spot.
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