Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 533 insertions, 0 deletions

diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst
new file mode 100644
index 0000000000..bd3c58c44b
--- /dev/null
+++ b/Documentation/RCU/checklist.rst
@@ -0,0 +1,533 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+================================
+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.
+
+	One final exception is where RCU readers are used to prevent
+	the ABA problem (https://en.wikipedia.org/wiki/ABA_problem)
+	for lockless updates.  This does result in the mildly
+	counter-intuitive situation where rcu_read_lock() and
+	rcu_read_unlock() are used to protect updates, however, this
+	approach can provide the same simplifications to certain types
+	of lockless algorithms that garbage collectors do.
+
+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 large systems (for example, if the task is updating information
+	relating to itself that other tasks can read, there by definition
+	can be no bottleneck).	Note that the definition of "large" has
+	changed significantly:	Eight CPUs was "large" in the year 2000,
+	but a hundred CPUs was unremarkable in 2017.
+
+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.
+	Explicit disabling of preemption (preempt_disable(), for example)
+	can serve as rcu_read_lock_sched(), but is less readable and
+	prevents lockdep from detecting locking issues.
+
+	Please note that you *cannot* rely on code known to be built
+	only in non-preemptible kernels.  Such code can and will break,
+	especially in kernels built with CONFIG_PREEMPT_COUNT=y.
+
+	Letting RCU-protected pointers "leak" out of an RCU read-side
+	critical section is every bit as bad as letting them leak out
+	from under a lock.  Unless, of course, you have arranged some
+	other means of protection, such as a lock or a reference count
+	*before* letting them out of the RCU read-side critical section.
+
+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.  Fields that the readers
+		refrain from accessing can be guarded by some other lock
+		acquired only by updaters, if desired.
+
+		This also works quite well.
+
+	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 operations performed under a lock will *not*
+		appear to be atomic to RCU readers, nor will sequences
+		of multiple atomic primitives.	One alternative is to
+		move multiple individual fields to a separate structure,
+		thus solving the multiple-field problem by imposing an
+		additional level of indirection.
+
+		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-ordering operations
+		through the code, making it difficult to understand and
+		to test.  Where it works, it is better to use things
+		like smp_store_release() and smp_load_acquire(), but in
+		some cases the smp_mb() full memory barrier is required.
+
+		As noted earlier, 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.
+
+		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.  However,
+		with a bit of devious creativity, it is possible to
+		mishandle the return value from rcu_dereference().
+		Please see rcu_dereference.rst for more information.
+
+		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.rst
+		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 any of call_rcu(), call_srcu(), call_rcu_tasks(),
+	call_rcu_tasks_rude(), or call_rcu_tasks_trace() is used,
+	the callback function may be invoked from softirq context,
+	and in any case with bottom halves disabled.  In particular,
+	this callback function cannot block.  If you need the callback
+	to block, run that code in a workqueue handler scheduled from
+	the callback.  The queue_rcu_work() function does this for you
+	in the case of call_rcu().
+
+6.	Since synchronize_rcu() can block, it cannot be called
+	from any sort of irq context.  The same rule applies
+	for synchronize_srcu(), synchronize_rcu_expedited(),
+	synchronize_srcu_expedited(), synchronize_rcu_tasks(),
+	synchronize_rcu_tasks_rude(), and synchronize_rcu_tasks_trace().
+
+	The expedited forms of these primitives have the same semantics
+	as the non-expedited forms, but expediting is more CPU intensive.
+	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.  Note that
+	IPI-sensitive real-time workloads can use the rcupdate.rcu_normal
+	kernel boot parameter to completely disable expedited grace
+	periods, though this might have performance implications.
+
+	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.  Alternatively, instead use asynchronous
+	primitives such as call_rcu().
+
+7.	As of v4.20, a given kernel implements only one RCU flavor, which
+	is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y.
+	If the updater uses call_rcu() or synchronize_rcu(), then
+	the corresponding readers may use:  (1) rcu_read_lock() and
+	rcu_read_unlock(), (2) any pair of primitives that disables
+	and re-enables softirq, for example, rcu_read_lock_bh() and
+	rcu_read_unlock_bh(), or (3) any pair of primitives that disables
+	and re-enables preemption, for example, rcu_read_lock_sched() and
+	rcu_read_unlock_sched().  If the updater uses synchronize_srcu()
+	or call_srcu(), then the corresponding readers must use
+	srcu_read_lock() and srcu_read_unlock(), and with the same
+	srcu_struct.  The rules for the expedited RCU grace-period-wait
+	primitives are the same as for their non-expedited counterparts.
+
+	If the updater uses call_rcu_tasks() or synchronize_rcu_tasks(),
+	then the readers must refrain from executing voluntary
+	context switches, that is, from blocking.  If the updater uses
+	call_rcu_tasks_trace() or synchronize_rcu_tasks_trace(), then
+	the corresponding readers must use rcu_read_lock_trace() and
+	rcu_read_unlock_trace().  If an updater uses call_rcu_tasks_rude()
+	or synchronize_rcu_tasks_rude(), then the corresponding readers
+	must use anything that disables preemption, for example,
+	preempt_disable() and preempt_enable().
+
+	Mixing things up will result in confusion and broken kernels, and
+	has even resulted in an exploitable security issue.  Therefore,
+	when using non-obvious pairs of primitives, commenting is
+	of course a must.  One example of non-obvious pairing is
+	the XDP feature in networking, which calls BPF programs from
+	network-driver NAPI (softirq) context.	BPF relies heavily on RCU
+	protection for its data structures, but because the BPF program
+	invocation happens entirely within a single local_bh_disable()
+	section in a NAPI poll cycle, this usage is safe.  The reason
+	that this usage is safe is that readers can use anything that
+	disables BH when updaters use call_rcu() or synchronize_rcu().
+
+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() and kvfree_rcu() usually result
+	in even simpler code than does synchronize_rcu() without
+	synchronize_rcu()'s multi-millisecond latency.	So please take
+	advantage of kfree_rcu()'s and kvfree_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(),
+	kfree_rcu(), or kvfree_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.
+		Older versions of the dcache subsystem take this approach,
+		guarding updates with 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.	Periodically invoke rcu_barrier(), permitting a limited
+		number of updates per grace period.
+
+	The same cautions apply to call_srcu(), call_rcu_tasks(),
+	call_rcu_tasks_rude(), and call_rcu_tasks_trace().  This is
+	why there is an srcu_barrier(), rcu_barrier_tasks(),
+	rcu_barrier_tasks_rude(), and rcu_barrier_tasks_rude(),
+	respectively.
+
+	Note that although these primitives do take action to avoid
+	memory exhaustion when any given CPU has too many callbacks,
+	a determined user or administrator can still exhaust memory.
+	This is especially the case if a system with a large number of
+	CPUs has been configured to offload all of its RCU callbacks onto
+	a single CPU, or if the system has relatively little free memory.
+
+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.rst.
+
+	One exception to this rule is when data is only ever added to
+	the linked data structure, and is never removed during any
+	time that readers might be accessing that structure.  In such
+	cases, READ_ONCE() may be used in place of rcu_dereference()
+	and the read-side markers (rcu_read_lock() and rcu_read_unlock(),
+	for example) may be omitted.
+
+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 understand your code.
+
+11.	Any lock acquired by an RCU callback must be acquired elsewhere
+	with softirq disabled, e.g., via spin_lock_bh().  Failing to
+	disable softirq 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.
+
+12.	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.
+
+	Do not assume that RCU callbacks will be executed on the same
+	CPU that executed the corresponding call_rcu() or call_srcu().
+	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.)
+	Furthermore, CPUs designated by rcu_nocbs= might well *always*
+	have their RCU callbacks executed on some other CPUs, in fact,
+	for some  real-time workloads, this is the whole point of using
+	the rcu_nocbs= kernel boot parameter.
+
+	In addition, do not assume that callbacks queued in a given order
+	will be invoked in that order, even if they all are queued on the
+	same CPU.  Furthermore, do not assume that same-CPU callbacks will
+	be invoked serially.  For example, in recent kernels, CPUs can be
+	switched between offloaded and de-offloaded callback invocation,
+	and while a given CPU is undergoing such a switch, its callbacks
+	might be concurrently invoked by that CPU's softirq handler and
+	that CPU's rcuo kthread.  At such times, that CPU's callbacks
+	might be executed both concurrently and out of order.
+
+13.	Unlike most flavors 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 either at build time via DEFINE_SRCU()
+	or DEFINE_STATIC_SRCU() or at runtime via init_srcu_struct()
+	and cleanup_srcu_struct().  These last two 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.  You should also consider percpu_rw_semaphore
+	when you need lightweight readers.
+
+	SRCU's expedited primitive (synchronize_srcu_expedited())
+	never sends IPIs to other CPUs, so it is easier on
+	real-time workloads than is synchronize_rcu_expedited().
+
+	It is also permissible to sleep in RCU Tasks Trace read-side
+	critical, which are delimited by rcu_read_lock_trace() and
+	rcu_read_unlock_trace().  However, this is a specialized flavor
+	of RCU, and you should not use it without first checking with
+	its current users.  In most cases, you should instead use SRCU.
+
+	Note that rcu_assign_pointer() relates to SRCU just as it does to
+	other forms of RCU, but instead of rcu_dereference() you should
+	use srcu_dereference() in order to avoid lockdep splats.
+
+14.	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.
+
+15.	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.
+
+	For SRCU readers, you can use smp_mb__after_srcu_read_unlock()
+	immediately after an srcu_read_unlock() to get a full barrier.
+
+16.	Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
+	__rcu sparse checks to validate your RCU code.	These can help
+	find problems as follows:
+
+	CONFIG_PROVE_LOCKING:
+		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.
+
+17.	If you pass a callback function defined within a module to one of
+	call_rcu(), call_srcu(), call_rcu_tasks(), call_rcu_tasks_rude(),
+	or call_rcu_tasks_trace(), then it is necessary to wait for all
+	pending callbacks to be invoked before unloading that module.
+	Note that it is absolutely *not* sufficient to wait for a grace
+	period!  For example, synchronize_rcu() implementation is *not*
+	guaranteed to wait for callbacks registered on other CPUs via
+	call_rcu().  Or even on the current CPU if that CPU recently
+	went offline and came back online.
+
+	You instead need to use one of the barrier functions:
+
+	-	call_rcu() -> rcu_barrier()
+	-	call_srcu() -> srcu_barrier()
+	-	call_rcu_tasks() -> rcu_barrier_tasks()
+	-	call_rcu_tasks_rude() -> rcu_barrier_tasks_rude()
+	-	call_rcu_tasks_trace() -> rcu_barrier_tasks_trace()
+
+	However, these barrier functions are absolutely *not* guaranteed
+	to wait for a grace period.  For example, if there are no
+	call_rcu() callbacks queued anywhere in the system, rcu_barrier()
+	can and will return immediately.
+
+	So if you need to wait for both a grace period and for all
+	pre-existing callbacks, you will need to invoke both functions,
+	with the pair depending on the flavor of RCU:
+
+	-	Either synchronize_rcu() or synchronize_rcu_expedited(),
+		together with rcu_barrier()
+	-	Either synchronize_srcu() or synchronize_srcu_expedited(),
+		together with and srcu_barrier()
+	-	synchronize_rcu_tasks() and rcu_barrier_tasks()
+	-	synchronize_tasks_rude() and rcu_barrier_tasks_rude()
+	-	synchronize_tasks_trace() and rcu_barrier_tasks_trace()
+
+	If necessary, you can use something like workqueues to execute
+	the requisite pair of functions concurrently.
+
+	See rcubarrier.rst for more information.