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() 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.  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.
  
  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(), 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 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.
  
  		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() and call_rcu_sched().

  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(), 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 you do not have
  	rcu_read_lock()-protected read-side critical sections, 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().

  
  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(), and synchronize_srcu_expedited()) 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.

  	If you need to enter your read-side critical section in a
  	hardirq or exception handler, and then exit that same read-side
  	critical section in the task that was interrupted, then you need
  	to srcu_read_lock_raw() and srcu_read_unlock_raw(), which avoid
  	the lockdep checking that would otherwise this practice illegal.

  	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(), and synchronize_srcu_expedited().  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 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 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 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.