Documentation/RCU/checklist.txt - linux/kernel/git/klassert/ipsec - 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.

 	The other exception would be 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.

 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 suppress preemption (or bottom halves, in the case of
 	rcu_read_lock_bh()) in the read-side critical sections,
 	and are also an excellent aid to readability.

 	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.	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 is almost always the best approach.

 	b.	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(),
 	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.

 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().  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 a bit slower than is call_rcu(),
 	it usually results in simpler code.  So, unless update performance
 	is important or the updaters cannot block, synchronize_rcu()
 	should be used in preference to call_rcu().

 9.	All RCU list-traversal primitives, which include
 	list_for_each_rcu(), list_for_each_entry_rcu(),
 	list_for_each_continue_rcu(), and list_for_each_safe_rcu(),
 	must be within an RCU read-side critical section.  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().

 	Use of the _rcu() list-traversal primitives outside of an
 	RCU read-side critical section causes no harm other than
 	a slight performance degradation on Alpha CPUs.  It can
 	also 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,
 	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 irq disabled, e.g., via spin_lock_irqsave().  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.
	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.

	The other exception would be 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.

	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 suppress preemption (or bottom halves, in the case of
	rcu_read_lock_bh()) in the read-side critical sections,
	and are also an excellent aid to readability.

	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. 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 is almost always the best approach.

	b. 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(),
	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.

	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(). 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 a bit slower than is call_rcu(),
	it usually results in simpler code. So, unless update performance
	is important or the updaters cannot block, synchronize_rcu()
	should be used in preference to call_rcu().

	9. All RCU list-traversal primitives, which include
	list_for_each_rcu(), list_for_each_entry_rcu(),
	list_for_each_continue_rcu(), and list_for_each_safe_rcu(),
	must be within an RCU read-side critical section. 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().

	Use of the _rcu() list-traversal primitives outside of an
	RCU read-side critical section causes no harm other than
	a slight performance degradation on Alpha CPUs. It can
	also 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,
	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 irq disabled, e.g., via spin_lock_irqsave(). 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.