Documentation/RCU/checklist.txt - maze/linux - Git at Google

 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.

 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).

 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.

 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 perations 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.

 		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
 	must be written to 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 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.

 	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.

 	In addition, it is illegal to call the expedited forms from
 	a CPU-hotplug notifier, or while holding a lock that is acquired
 	by a CPU-hotplug notifier.  Failing to observe this restriction
 	will result in deadlock.

 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(),
 	the 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.  The dcache
 		subsystem takes this approach -- updates are guarded
 		by 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.

 	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().

 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 real-time 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.	SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
 	synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu())
 	may only be invoked from process context.  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 via init_srcu_struct() and
 	cleanup_srcu_struct().	These 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.

 	Note that, rcu_assign_pointer() relates to SRCU just as it does
 	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_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
 	__rcu sparse checks (enabled by CONFIG_SPARSE_RCU_POINTER) to
 	validate your RCU code.  These can help find problems as follows:

 	CONFIG_PROVE_RCU: 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.
	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.

	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).

	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.

	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 perations 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.

	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
	must be written to 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 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.

	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.

	In addition, it is illegal to call the expedited forms from
	a CPU-hotplug notifier, or while holding a lock that is acquired
	by a CPU-hotplug notifier. Failing to observe this restriction
	will result in deadlock.

	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(),
	the 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. The dcache
	subsystem takes this approach -- updates are guarded
	by 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.

	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().

	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 real-time 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. SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
	synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu())
	may only be invoked from process context. 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 via init_srcu_struct() and
	cleanup_srcu_struct(). These 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.

	Note that, rcu_assign_pointer() relates to SRCU just as it does
	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_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
	__rcu sparse checks (enabled by CONFIG_SPARSE_RCU_POINTER) to
	validate your RCU code. These can help find problems as follows:

	CONFIG_PROVE_RCU: 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.