Documentation/RCU/checklist.txt - kernel/prism - 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, you might think of RCU
 	as simply cutting overhead off of the readers and imposing it
 	on the writers.  That 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 reads to be reordered), 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() or rcu_read_lock_bh()
 	or by the appropriate update-side lock.

 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 RCU-protected
 		trees 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 the
 		update-side lock.

 		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.  Operations
 		performed under a lock and sequences of multiple atomic
 		primitives will -not- appear to be atomic.

 		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 i386 CPUs allow reads to be reordered.
 	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.

 		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.

 	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() primitive may be used to
 		replace an old structure with a new one in an
 		RCU-protected list.

 	d.	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() or
 	call_rcu_sched(), 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.  Ditto for synchronize_sched() and
 	synchronize_srcu().

 7.	If the updater uses call_rcu(), then the corresponding readers
 	must use rcu_read_lock() and rcu_read_unlock().  If the updater
 	uses call_rcu_bh(), then the corresponding readers must use
 	rcu_read_lock_bh() and rcu_read_unlock_bh().  If the updater
 	uses call_rcu_sched(), then the corresponding readers must
 	disable preemption.  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 or the updaters cannot block,
 	synchronize_rcu() should be used in preference to call_rcu().

 	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.

 	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.

 9.	All RCU list-traversal primitives, which include
 	rcu_dereference(), list_for_each_entry_rcu(),
 	list_for_each_continue_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().

 	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.

 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 and confuse people reading 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 you do not have
 	rcu_read_lock()-protected read-side critical sections, do -not-
 	use synchronize_rcu().

 	If you want to wait for some of these other things, you might
 	instead need to use synchronize_irq() or synchronize_sched().

 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 callback happens to interrupt 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(), and synchronize_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()
 	and synchronize_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 passd 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() and rcu_dereference() relate to
 	SRCU just as they do to other forms of RCU.

 15.	The whole point of call_rcu(), synchronize_rcu(), and friends
 	is to wait until all pre-existing readers have finished before
 	carrying out some otherwise-destructive operation.  It is
 	therefore critically important to -first- remove any path
 	that readers can follow that could be affected by the
 	destructive operation, and -only- -then- invoke call_rcu(),
 	synchronize_rcu(), or friends.

 	Because these primitives only wait for pre-existing readers,
 	it is the caller's responsibility to guarantee safety to
 	any subsequent readers.

 16.	The various RCU read-side primitives do -not- contain memory
 	barriers.  The CPU (and in some cases, the compiler) is free
 	to 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.
	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, you might think of RCU
	as simply cutting overhead off of the readers and imposing it
	on the writers. That 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 reads to be reordered), 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() or rcu_read_lock_bh()
	or by the appropriate update-side lock.

	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 RCU-protected
	trees 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 the
	update-side lock.

	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. Operations
	performed under a lock and sequences of multiple atomic
	primitives will -not- appear to be atomic.

	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 i386 CPUs allow reads to be reordered.
	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.

	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.

	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() primitive may be used to
	replace an old structure with a new one in an
	RCU-protected list.

	d. 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() or
	call_rcu_sched(), 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. Ditto for synchronize_sched() and
	synchronize_srcu().

	7. If the updater uses call_rcu(), then the corresponding readers
	must use rcu_read_lock() and rcu_read_unlock(). If the updater
	uses call_rcu_bh(), then the corresponding readers must use
	rcu_read_lock_bh() and rcu_read_unlock_bh(). If the updater
	uses call_rcu_sched(), then the corresponding readers must
	disable preemption. 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 or the updaters cannot block,
	synchronize_rcu() should be used in preference to call_rcu().

	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.

	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.

	9. All RCU list-traversal primitives, which include
	rcu_dereference(), list_for_each_entry_rcu(),
	list_for_each_continue_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().

	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.

	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 and confuse people reading 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 you do not have
	rcu_read_lock()-protected read-side critical sections, do -not-
	use synchronize_rcu().

	If you want to wait for some of these other things, you might
	instead need to use synchronize_irq() or synchronize_sched().

	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 callback happens to interrupt 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(), and synchronize_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()
	and synchronize_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 passd 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() and rcu_dereference() relate to
	SRCU just as they do to other forms of RCU.

	15. The whole point of call_rcu(), synchronize_rcu(), and friends
	is to wait until all pre-existing readers have finished before
	carrying out some otherwise-destructive operation. It is
	therefore critically important to -first- remove any path
	that readers can follow that could be affected by the
	destructive operation, and -only- -then- invoke call_rcu(),
	synchronize_rcu(), or friends.

	Because these primitives only wait for pre-existing readers,
	it is the caller's responsibility to guarantee safety to
	any subsequent readers.

	16. The various RCU read-side primitives do -not- contain memory
	barriers. The CPU (and in some cases, the compiler) is free
	to 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.