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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 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
118			code 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 also
122			prevents destructive compiler optimizations.  However,
123			with a bit of devious creativity, it is possible to
124			mishandle the return value from rcu_dereference().
125			Please see rcu_dereference.txt in this directory for
126			more information.
127	
128			The rcu_dereference() primitive is used by the
129			various "_rcu()" list-traversal primitives, such
130			as the list_for_each_entry_rcu().  Note that it is
131			perfectly legal (if redundant) for update-side code to
132			use rcu_dereference() and the "_rcu()" list-traversal
133			primitives.  This is particularly useful in code that
134			is common to readers and updaters.  However, lockdep
135			will complain if you access rcu_dereference() outside
136			of an RCU read-side critical section.  See lockdep.txt
137			to learn what to do about this.
138	
139			Of course, neither rcu_dereference() nor the "_rcu()"
140			list-traversal primitives can substitute for a good
141			concurrency design coordinating among multiple updaters.
142	
143		b.	If the list macros are being used, the list_add_tail_rcu()
144			and list_add_rcu() primitives must be used in order
145			to prevent weakly ordered machines from misordering
146			structure initialization and pointer planting.
147			Similarly, if the hlist macros are being used, the
148			hlist_add_head_rcu() primitive is required.
149	
150		c.	If the list macros are being used, the list_del_rcu()
151			primitive must be used to keep list_del()'s pointer
152			poisoning from inflicting toxic effects on concurrent
153			readers.  Similarly, if the hlist macros are being used,
154			the hlist_del_rcu() primitive is required.
155	
156			The list_replace_rcu() and hlist_replace_rcu() primitives
157			may be used to replace an old structure with a new one
158			in their respective types of RCU-protected lists.
159	
160		d.	Rules similar to (4b) and (4c) apply to the "hlist_nulls"
161			type of RCU-protected linked lists.
162	
163		e.	Updates must ensure that initialization of a given
164			structure happens before pointers to that structure are
165			publicized.  Use the rcu_assign_pointer() primitive
166			when publicizing a pointer to a structure that can
167			be traversed by an RCU read-side critical section.
168	
169	5.	If call_rcu(), or a related primitive such as call_rcu_bh(),
170		call_rcu_sched(), or call_srcu() is used, the callback function
171		must be written to be called from softirq context.  In particular,
172		it cannot block.
173	
174	6.	Since synchronize_rcu() can block, it cannot be called from
175		any sort of irq context.  The same rule applies for
176		synchronize_rcu_bh(), synchronize_sched(), synchronize_srcu(),
177		synchronize_rcu_expedited(), synchronize_rcu_bh_expedited(),
178		synchronize_sched_expedite(), and synchronize_srcu_expedited().
179	
180		The expedited forms of these primitives have the same semantics
181		as the non-expedited forms, but expediting is both expensive
182		and unfriendly to real-time workloads.	Use of the expedited
183		primitives should be restricted to rare configuration-change
184		operations that would not normally be undertaken while a real-time
185		workload is running.
186	
187		In particular, if you find yourself invoking one of the expedited
188		primitives repeatedly in a loop, please do everyone a favor:
189		Restructure your code so that it batches the updates, allowing
190		a single non-expedited primitive to cover the entire batch.
191		This will very likely be faster than the loop containing the
192		expedited primitive, and will be much much easier on the rest
193		of the system, especially to real-time workloads running on
194		the rest of the system.
195	
196		In addition, it is illegal to call the expedited forms from
197		a CPU-hotplug notifier, or while holding a lock that is acquired
198		by a CPU-hotplug notifier.  Failing to observe this restriction
199		will result in deadlock.
200	
201	7.	If the updater uses call_rcu() or synchronize_rcu(), then the
202		corresponding readers must use rcu_read_lock() and
203		rcu_read_unlock().  If the updater uses call_rcu_bh() or
204		synchronize_rcu_bh(), then the corresponding readers must
205		use rcu_read_lock_bh() and rcu_read_unlock_bh().  If the
206		updater uses call_rcu_sched() or synchronize_sched(), then
207		the corresponding readers must disable preemption, possibly
208		by calling rcu_read_lock_sched() and rcu_read_unlock_sched().
209		If the updater uses synchronize_srcu() or call_srcu(), then
210		the corresponding readers must use srcu_read_lock() and
211		srcu_read_unlock(), and with the same srcu_struct.  The rules for
212		the expedited primitives are the same as for their non-expedited
213		counterparts.  Mixing things up will result in confusion and
214		broken kernels.
215	
216		One exception to this rule: rcu_read_lock() and rcu_read_unlock()
217		may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
218		in cases where local bottom halves are already known to be
219		disabled, for example, in irq or softirq context.  Commenting
220		such cases is a must, of course!  And the jury is still out on
221		whether the increased speed is worth it.
222	
223	8.	Although synchronize_rcu() is slower than is call_rcu(), it
224		usually results in simpler code.  So, unless update performance is
225		critically important, the updaters cannot block, or the latency of
226		synchronize_rcu() is visible from userspace, synchronize_rcu()
227		should be used in preference to call_rcu().  Furthermore,
228		kfree_rcu() usually results in even simpler code than does
229		synchronize_rcu() without synchronize_rcu()'s multi-millisecond
230		latency.  So please take advantage of kfree_rcu()'s "fire and
231		forget" memory-freeing capabilities where it applies.
232	
233		An especially important property of the synchronize_rcu()
234		primitive is that it automatically self-limits: if grace periods
235		are delayed for whatever reason, then the synchronize_rcu()
236		primitive will correspondingly delay updates.  In contrast,
237		code using call_rcu() should explicitly limit update rate in
238		cases where grace periods are delayed, as failing to do so can
239		result in excessive realtime latencies or even OOM conditions.
240	
241		Ways of gaining this self-limiting property when using call_rcu()
242		include:
243	
244		a.	Keeping a count of the number of data-structure elements
245			used by the RCU-protected data structure, including
246			those waiting for a grace period to elapse.  Enforce a
247			limit on this number, stalling updates as needed to allow
248			previously deferred frees to complete.	Alternatively,
249			limit only the number awaiting deferred free rather than
250			the total number of elements.
251	
252			One way to stall the updates is to acquire the update-side
253			mutex.	(Don't try this with a spinlock -- other CPUs
254			spinning on the lock could prevent the grace period
255			from ever ending.)  Another way to stall the updates
256			is for the updates to use a wrapper function around
257			the memory allocator, so that this wrapper function
258			simulates OOM when there is too much memory awaiting an
259			RCU grace period.  There are of course many other
260			variations on this theme.
261	
262		b.	Limiting update rate.  For example, if updates occur only
263			once per hour, then no explicit rate limiting is
264			required, unless your system is already badly broken.
265			Older versions of the dcache subsystem take this approach,
266			guarding updates with a global lock, limiting their rate.
267	
268		c.	Trusted update -- if updates can only be done manually by
269			superuser or some other trusted user, then it might not
270			be necessary to automatically limit them.  The theory
271			here is that superuser already has lots of ways to crash
272			the machine.
273	
274		d.	Use call_rcu_bh() rather than call_rcu(), in order to take
275			advantage of call_rcu_bh()'s faster grace periods.  (This
276			is only a partial solution, though.)
277	
278		e.	Periodically invoke synchronize_rcu(), permitting a limited
279			number of updates per grace period.
280	
281		The same cautions apply to call_rcu_bh(), call_rcu_sched(),
282		call_srcu(), and kfree_rcu().
283	
284		Note that although these primitives do take action to avoid memory
285		exhaustion when any given CPU has too many callbacks, a determined
286		user could still exhaust memory.  This is especially the case
287		if a system with a large number of CPUs has been configured to
288		offload all of its RCU callbacks onto a single CPU, or if the
289		system has relatively little free memory.
290	
291	9.	All RCU list-traversal primitives, which include
292		rcu_dereference(), list_for_each_entry_rcu(), and
293		list_for_each_safe_rcu(), must be either within an RCU read-side
294		critical section or must be protected by appropriate update-side
295		locks.	RCU read-side critical sections are delimited by
296		rcu_read_lock() and rcu_read_unlock(), or by similar primitives
297		such as rcu_read_lock_bh() and rcu_read_unlock_bh(), in which
298		case the matching rcu_dereference() primitive must be used in
299		order to keep lockdep happy, in this case, rcu_dereference_bh().
300	
301		The reason that it is permissible to use RCU list-traversal
302		primitives when the update-side lock is held is that doing so
303		can be quite helpful in reducing code bloat when common code is
304		shared between readers and updaters.  Additional primitives
305		are provided for this case, as discussed in lockdep.txt.
306	
307	10.	Conversely, if you are in an RCU read-side critical section,
308		and you don't hold the appropriate update-side lock, you -must-
309		use the "_rcu()" variants of the list macros.  Failing to do so
310		will break Alpha, cause aggressive compilers to generate bad code,
311		and confuse people trying to read your code.
312	
313	11.	Note that synchronize_rcu() -only- guarantees to wait until
314		all currently executing rcu_read_lock()-protected RCU read-side
315		critical sections complete.  It does -not- necessarily guarantee
316		that all currently running interrupts, NMIs, preempt_disable()
317		code, or idle loops will complete.  Therefore, if your
318		read-side critical sections are protected by something other
319		than rcu_read_lock(), do -not- use synchronize_rcu().
320	
321		Similarly, disabling preemption is not an acceptable substitute
322		for rcu_read_lock().  Code that attempts to use preemption
323		disabling where it should be using rcu_read_lock() will break
324		in real-time kernel builds.
325	
326		If you want to wait for interrupt handlers, NMI handlers, and
327		code under the influence of preempt_disable(), you instead
328		need to use synchronize_irq() or synchronize_sched().
329	
330		This same limitation also applies to synchronize_rcu_bh()
331		and synchronize_srcu(), as well as to the asynchronous and
332		expedited forms of the three primitives, namely call_rcu(),
333		call_rcu_bh(), call_srcu(), synchronize_rcu_expedited(),
334		synchronize_rcu_bh_expedited(), and synchronize_srcu_expedited().
335	
336	12.	Any lock acquired by an RCU callback must be acquired elsewhere
337		with softirq disabled, e.g., via spin_lock_irqsave(),
338		spin_lock_bh(), etc.  Failing to disable irq on a given
339		acquisition of that lock will result in deadlock as soon as
340		the RCU softirq handler happens to run your RCU callback while
341		interrupting that acquisition's critical section.
342	
343	13.	RCU callbacks can be and are executed in parallel.  In many cases,
344		the callback code simply wrappers around kfree(), so that this
345		is not an issue (or, more accurately, to the extent that it is
346		an issue, the memory-allocator locking handles it).  However,
347		if the callbacks do manipulate a shared data structure, they
348		must use whatever locking or other synchronization is required
349		to safely access and/or modify that data structure.
350	
351		RCU callbacks are -usually- executed on the same CPU that executed
352		the corresponding call_rcu(), call_rcu_bh(), or call_rcu_sched(),
353		but are by -no- means guaranteed to be.  For example, if a given
354		CPU goes offline while having an RCU callback pending, then that
355		RCU callback will execute on some surviving CPU.  (If this was
356		not the case, a self-spawning RCU callback would prevent the
357		victim CPU from ever going offline.)
358	
359	14.	SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
360		synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu())
361		may only be invoked from process context.  Unlike other forms of
362		RCU, it -is- permissible to block in an SRCU read-side critical
363		section (demarked by srcu_read_lock() and srcu_read_unlock()),
364		hence the "SRCU": "sleepable RCU".  Please note that if you
365		don't need to sleep in read-side critical sections, you should be
366		using RCU rather than SRCU, because RCU is almost always faster
367		and easier to use than is SRCU.
368	
369		Also unlike other forms of RCU, explicit initialization
370		and cleanup is required via init_srcu_struct() and
371		cleanup_srcu_struct().	These are passed a "struct srcu_struct"
372		that defines the scope of a given SRCU domain.	Once initialized,
373		the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
374		synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu().
375		A given synchronize_srcu() waits only for SRCU read-side critical
376		sections governed by srcu_read_lock() and srcu_read_unlock()
377		calls that have been passed the same srcu_struct.  This property
378		is what makes sleeping read-side critical sections tolerable --
379		a given subsystem delays only its own updates, not those of other
380		subsystems using SRCU.	Therefore, SRCU is less prone to OOM the
381		system than RCU would be if RCU's read-side critical sections
382		were permitted to sleep.
383	
384		The ability to sleep in read-side critical sections does not
385		come for free.	First, corresponding srcu_read_lock() and
386		srcu_read_unlock() calls must be passed the same srcu_struct.
387		Second, grace-period-detection overhead is amortized only
388		over those updates sharing a given srcu_struct, rather than
389		being globally amortized as they are for other forms of RCU.
390		Therefore, SRCU should be used in preference to rw_semaphore
391		only in extremely read-intensive situations, or in situations
392		requiring SRCU's read-side deadlock immunity or low read-side
393		realtime latency.
394	
395		Note that, rcu_assign_pointer() relates to SRCU just as it does
396		to other forms of RCU.
397	
398	15.	The whole point of call_rcu(), synchronize_rcu(), and friends
399		is to wait until all pre-existing readers have finished before
400		carrying out some otherwise-destructive operation.  It is
401		therefore critically important to -first- remove any path
402		that readers can follow that could be affected by the
403		destructive operation, and -only- -then- invoke call_rcu(),
404		synchronize_rcu(), or friends.
405	
406		Because these primitives only wait for pre-existing readers, it
407		is the caller's responsibility to guarantee that any subsequent
408		readers will execute safely.
409	
410	16.	The various RCU read-side primitives do -not- necessarily contain
411		memory barriers.  You should therefore plan for the CPU
412		and the compiler to freely reorder code into and out of RCU
413		read-side critical sections.  It is the responsibility of the
414		RCU update-side primitives to deal with this.
415	
416	17.	Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
417		__rcu sparse checks (enabled by CONFIG_SPARSE_RCU_POINTER) to
418		validate your RCU code.  These can help find problems as follows:
419	
420		CONFIG_PROVE_RCU: check that accesses to RCU-protected data
421			structures are carried out under the proper RCU
422			read-side critical section, while holding the right
423			combination of locks, or whatever other conditions
424			are appropriate.
425	
426		CONFIG_DEBUG_OBJECTS_RCU_HEAD: check that you don't pass the
427			same object to call_rcu() (or friends) before an RCU
428			grace period has elapsed since the last time that you
429			passed that same object to call_rcu() (or friends).
430	
431		__rcu sparse checks: tag the pointer to the RCU-protected data
432			structure with __rcu, and sparse will warn you if you
433			access that pointer without the services of one of the
434			variants of rcu_dereference().
435	
436		These debugging aids can help you find problems that are
437		otherwise extremely difficult to spot.
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