Documentation / RCU / checklist.txt

Custom Search

Based on kernel version 2.6.35.4. Page generated on 2010-09-02 21:40 EST.

	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.
	
		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.
	
		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.

• [ RCU ]
• 00-INDEX
• arrayRCU.txt
• checklist.txt
• listRCU.txt
• lockdep.txt
• NMI-RCU.txt
• rcu.txt
• rcubarrier.txt
• rculist_nulls.txt
• rcuref.txt
• RTFP.txt
• stallwarn.txt
• torture.txt
• trace.txt
• UP.txt
• whatisRCU.txt
•