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.
