Documentation / RCU / whatisRCU.txt

Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

	Please note that the "What is RCU?" LWN series is an excellent place
	to start learning about RCU:
	
	1.	What is RCU, Fundamentally?  http://lwn.net/Articles/262464/
	2.	What is RCU? Part 2: Usage   http://lwn.net/Articles/263130/
	3.	RCU part 3: the RCU API      http://lwn.net/Articles/264090/
	4.	The RCU API, 2010 Edition    http://lwn.net/Articles/418853/
		2010 Big API Table           http://lwn.net/Articles/419086/
	5.	The RCU API, 2014 Edition    http://lwn.net/Articles/609904/
		2014 Big API Table           http://lwn.net/Articles/609973/
	
	
	What is RCU?
	
	RCU is a synchronization mechanism that was added to the Linux kernel
	during the 2.5 development effort that is optimized for read-mostly
	situations.  Although RCU is actually quite simple once you understand it,
	getting there can sometimes be a challenge.  Part of the problem is that
	most of the past descriptions of RCU have been written with the mistaken
	assumption that there is "one true way" to describe RCU.  Instead,
	the experience has been that different people must take different paths
	to arrive at an understanding of RCU.  This document provides several
	different paths, as follows:
	
	1.	RCU OVERVIEW
	2.	WHAT IS RCU'S CORE API?
	3.	WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
	4.	WHAT IF MY UPDATING THREAD CANNOT BLOCK?
	5.	WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
	6.	ANALOGY WITH READER-WRITER LOCKING
	7.	FULL LIST OF RCU APIs
	8.	ANSWERS TO QUICK QUIZZES
	
	People who prefer starting with a conceptual overview should focus on
	Section 1, though most readers will profit by reading this section at
	some point.  People who prefer to start with an API that they can then
	experiment with should focus on Section 2.  People who prefer to start
	with example uses should focus on Sections 3 and 4.  People who need to
	understand the RCU implementation should focus on Section 5, then dive
	into the kernel source code.  People who reason best by analogy should
	focus on Section 6.  Section 7 serves as an index to the docbook API
	documentation, and Section 8 is the traditional answer key.
	
	So, start with the section that makes the most sense to you and your
	preferred method of learning.  If you need to know everything about
	everything, feel free to read the whole thing -- but if you are really
	that type of person, you have perused the source code and will therefore
	never need this document anyway.  ;-)
	
	
	1.  RCU OVERVIEW
	
	The basic idea behind RCU is to split updates into "removal" and
	"reclamation" phases.  The removal phase removes references to data items
	within a data structure (possibly by replacing them with references to
	new versions of these data items), and can run concurrently with readers.
	The reason that it is safe to run the removal phase concurrently with
	readers is the semantics of modern CPUs guarantee that readers will see
	either the old or the new version of the data structure rather than a
	partially updated reference.  The reclamation phase does the work of reclaiming
	(e.g., freeing) the data items removed from the data structure during the
	removal phase.  Because reclaiming data items can disrupt any readers
	concurrently referencing those data items, the reclamation phase must
	not start until readers no longer hold references to those data items.
	
	Splitting the update into removal and reclamation phases permits the
	updater to perform the removal phase immediately, and to defer the
	reclamation phase until all readers active during the removal phase have
	completed, either by blocking until they finish or by registering a
	callback that is invoked after they finish.  Only readers that are active
	during the removal phase need be considered, because any reader starting
	after the removal phase will be unable to gain a reference to the removed
	data items, and therefore cannot be disrupted by the reclamation phase.
	
	So the typical RCU update sequence goes something like the following:
	
	a.	Remove pointers to a data structure, so that subsequent
		readers cannot gain a reference to it.
	
	b.	Wait for all previous readers to complete their RCU read-side
		critical sections.
	
	c.	At this point, there cannot be any readers who hold references
		to the data structure, so it now may safely be reclaimed
		(e.g., kfree()d).
	
	Step (b) above is the key idea underlying RCU's deferred destruction.
	The ability to wait until all readers are done allows RCU readers to
	use much lighter-weight synchronization, in some cases, absolutely no
	synchronization at all.  In contrast, in more conventional lock-based
	schemes, readers must use heavy-weight synchronization in order to
	prevent an updater from deleting the data structure out from under them.
	This is because lock-based updaters typically update data items in place,
	and must therefore exclude readers.  In contrast, RCU-based updaters
	typically take advantage of the fact that writes to single aligned
	pointers are atomic on modern CPUs, allowing atomic insertion, removal,
	and replacement of data items in a linked structure without disrupting
	readers.  Concurrent RCU readers can then continue accessing the old
	versions, and can dispense with the atomic operations, memory barriers,
	and communications cache misses that are so expensive on present-day
	SMP computer systems, even in absence of lock contention.
	
	In the three-step procedure shown above, the updater is performing both
	the removal and the reclamation step, but it is often helpful for an
	entirely different thread to do the reclamation, as is in fact the case
	in the Linux kernel's directory-entry cache (dcache).  Even if the same
	thread performs both the update step (step (a) above) and the reclamation
	step (step (c) above), it is often helpful to think of them separately.
	For example, RCU readers and updaters need not communicate at all,
	but RCU provides implicit low-overhead communication between readers
	and reclaimers, namely, in step (b) above.
	
	So how the heck can a reclaimer tell when a reader is done, given
	that readers are not doing any sort of synchronization operations???
	Read on to learn about how RCU's API makes this easy.
	
	
	2.  WHAT IS RCU'S CORE API?
	
	The core RCU API is quite small:
	
	a.	rcu_read_lock()
	b.	rcu_read_unlock()
	c.	synchronize_rcu() / call_rcu()
	d.	rcu_assign_pointer()
	e.	rcu_dereference()
	
	There are many other members of the RCU API, but the rest can be
	expressed in terms of these five, though most implementations instead
	express synchronize_rcu() in terms of the call_rcu() callback API.
	
	The five core RCU APIs are described below, the other 18 will be enumerated
	later.  See the kernel docbook documentation for more info, or look directly
	at the function header comments.
	
	rcu_read_lock()
	
		void rcu_read_lock(void);
	
		Used by a reader to inform the reclaimer that the reader is
		entering an RCU read-side critical section.  It is illegal
		to block while in an RCU read-side critical section, though
		kernels built with CONFIG_PREEMPT_RCU can preempt RCU
		read-side critical sections.  Any RCU-protected data structure
		accessed during an RCU read-side critical section is guaranteed to
		remain unreclaimed for the full duration of that critical section.
		Reference counts may be used in conjunction with RCU to maintain
		longer-term references to data structures.
	
	rcu_read_unlock()
	
		void rcu_read_unlock(void);
	
		Used by a reader to inform the reclaimer that the reader is
		exiting an RCU read-side critical section.  Note that RCU
		read-side critical sections may be nested and/or overlapping.
	
	synchronize_rcu()
	
		void synchronize_rcu(void);
	
		Marks the end of updater code and the beginning of reclaimer
		code.  It does this by blocking until all pre-existing RCU
		read-side critical sections on all CPUs have completed.
		Note that synchronize_rcu() will -not- necessarily wait for
		any subsequent RCU read-side critical sections to complete.
		For example, consider the following sequence of events:
	
		         CPU 0                  CPU 1                 CPU 2
		     ----------------- ------------------------- ---------------
		 1.  rcu_read_lock()
		 2.                    enters synchronize_rcu()
		 3.                                               rcu_read_lock()
		 4.  rcu_read_unlock()
		 5.                     exits synchronize_rcu()
		 6.                                              rcu_read_unlock()
	
		To reiterate, synchronize_rcu() waits only for ongoing RCU
		read-side critical sections to complete, not necessarily for
		any that begin after synchronize_rcu() is invoked.
	
		Of course, synchronize_rcu() does not necessarily return
		-immediately- after the last pre-existing RCU read-side critical
		section completes.  For one thing, there might well be scheduling
		delays.  For another thing, many RCU implementations process
		requests in batches in order to improve efficiencies, which can
		further delay synchronize_rcu().
	
		Since synchronize_rcu() is the API that must figure out when
		readers are done, its implementation is key to RCU.  For RCU
		to be useful in all but the most read-intensive situations,
		synchronize_rcu()'s overhead must also be quite small.
	
		The call_rcu() API is a callback form of synchronize_rcu(),
		and is described in more detail in a later section.  Instead of
		blocking, it registers a function and argument which are invoked
		after all ongoing RCU read-side critical sections have completed.
		This callback variant is particularly useful in situations where
		it is illegal to block or where update-side performance is
		critically important.
	
		However, the call_rcu() API should not be used lightly, as use
		of the synchronize_rcu() API generally results in simpler code.
		In addition, the synchronize_rcu() API has the nice property
		of automatically limiting update rate should grace periods
		be delayed.  This property results in system resilience in face
		of denial-of-service attacks.  Code using call_rcu() should limit
		update rate in order to gain this same sort of resilience.  See
		checklist.txt for some approaches to limiting the update rate.
	
	rcu_assign_pointer()
	
		typeof(p) rcu_assign_pointer(p, typeof(p) v);
	
		Yes, rcu_assign_pointer() -is- implemented as a macro, though it
		would be cool to be able to declare a function in this manner.
		(Compiler experts will no doubt disagree.)
	
		The updater uses this function to assign a new value to an
		RCU-protected pointer, in order to safely communicate the change
		in value from the updater to the reader.  This function returns
		the new value, and also executes any memory-barrier instructions
		required for a given CPU architecture.
	
		Perhaps just as important, it serves to document (1) which
		pointers are protected by RCU and (2) the point at which a
		given structure becomes accessible to other CPUs.  That said,
		rcu_assign_pointer() is most frequently used indirectly, via
		the _rcu list-manipulation primitives such as list_add_rcu().
	
	rcu_dereference()
	
		typeof(p) rcu_dereference(p);
	
		Like rcu_assign_pointer(), rcu_dereference() must be implemented
		as a macro.
	
		The reader uses rcu_dereference() to fetch an RCU-protected
		pointer, which returns a value that may then be safely
		dereferenced.  Note that rcu_dereference() does not actually
		dereference the pointer, instead, it protects the pointer for
		later dereferencing.  It also executes any needed memory-barrier
		instructions for a given CPU architecture.  Currently, only Alpha
		needs memory barriers within rcu_dereference() -- on other CPUs,
		it compiles to nothing, not even a compiler directive.
	
		Common coding practice uses rcu_dereference() to copy an
		RCU-protected pointer to a local variable, then dereferences
		this local variable, for example as follows:
	
			p = rcu_dereference(head.next);
			return p->data;
	
		However, in this case, one could just as easily combine these
		into one statement:
	
			return rcu_dereference(head.next)->data;
	
		If you are going to be fetching multiple fields from the
		RCU-protected structure, using the local variable is of
		course preferred.  Repeated rcu_dereference() calls look
		ugly, do not guarantee that the same pointer will be returned
		if an update happened while in the critical section, and incur
		unnecessary overhead on Alpha CPUs.
	
		Note that the value returned by rcu_dereference() is valid
		only within the enclosing RCU read-side critical section.
		For example, the following is -not- legal:
	
			rcu_read_lock();
			p = rcu_dereference(head.next);
			rcu_read_unlock();
			x = p->address;	/* BUG!!! */
			rcu_read_lock();
			y = p->data;	/* BUG!!! */
			rcu_read_unlock();
	
		Holding a reference from one RCU read-side critical section
		to another is just as illegal as holding a reference from
		one lock-based critical section to another!  Similarly,
		using a reference outside of the critical section in which
		it was acquired is just as illegal as doing so with normal
		locking.
	
		As with rcu_assign_pointer(), an important function of
		rcu_dereference() is to document which pointers are protected by
		RCU, in particular, flagging a pointer that is subject to changing
		at any time, including immediately after the rcu_dereference().
		And, again like rcu_assign_pointer(), rcu_dereference() is
		typically used indirectly, via the _rcu list-manipulation
		primitives, such as list_for_each_entry_rcu().
	
	The following diagram shows how each API communicates among the
	reader, updater, and reclaimer.
	
	
		    rcu_assign_pointer()
		    			    +--------+
		    +---------------------->| reader |---------+
		    |                       +--------+         |
		    |                           |              |
		    |                           |              | Protect:
		    |                           |              | rcu_read_lock()
		    |                           |              | rcu_read_unlock()
		    |        rcu_dereference()  |              |
	       +---------+                      |              |
	       | updater |<---------------------+              |
	       +---------+                                     V
		    |                                    +-----------+
		    +----------------------------------->| reclaimer |
		    				         +-----------+
		      Defer:
		      synchronize_rcu() & call_rcu()
	
	
	The RCU infrastructure observes the time sequence of rcu_read_lock(),
	rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
	order to determine when (1) synchronize_rcu() invocations may return
	to their callers and (2) call_rcu() callbacks may be invoked.  Efficient
	implementations of the RCU infrastructure make heavy use of batching in
	order to amortize their overhead over many uses of the corresponding APIs.
	
	There are no fewer than three RCU mechanisms in the Linux kernel; the
	diagram above shows the first one, which is by far the most commonly used.
	The rcu_dereference() and rcu_assign_pointer() primitives are used for
	all three mechanisms, but different defer and protect primitives are
	used as follows:
	
		Defer			Protect
	
	a.	synchronize_rcu()	rcu_read_lock() / rcu_read_unlock()
		call_rcu()		rcu_dereference()
	
	b.	synchronize_rcu_bh()	rcu_read_lock_bh() / rcu_read_unlock_bh()
		call_rcu_bh()		rcu_dereference_bh()
	
	c.	synchronize_sched()	rcu_read_lock_sched() / rcu_read_unlock_sched()
		call_rcu_sched()	preempt_disable() / preempt_enable()
					local_irq_save() / local_irq_restore()
					hardirq enter / hardirq exit
					NMI enter / NMI exit
					rcu_dereference_sched()
	
	These three mechanisms are used as follows:
	
	a.	RCU applied to normal data structures.
	
	b.	RCU applied to networking data structures that may be subjected
		to remote denial-of-service attacks.
	
	c.	RCU applied to scheduler and interrupt/NMI-handler tasks.
	
	Again, most uses will be of (a).  The (b) and (c) cases are important
	for specialized uses, but are relatively uncommon.
	
	
	3.  WHAT ARE SOME EXAMPLE USES OF CORE RCU API?
	
	This section shows a simple use of the core RCU API to protect a
	global pointer to a dynamically allocated structure.  More-typical
	uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt.
	
		struct foo {
			int a;
			char b;
			long c;
		};
		DEFINE_SPINLOCK(foo_mutex);
	
		struct foo __rcu *gbl_foo;
	
		/*
		 * Create a new struct foo that is the same as the one currently
		 * pointed to by gbl_foo, except that field "a" is replaced
		 * with "new_a".  Points gbl_foo to the new structure, and
		 * frees up the old structure after a grace period.
		 *
		 * Uses rcu_assign_pointer() to ensure that concurrent readers
		 * see the initialized version of the new structure.
		 *
		 * Uses synchronize_rcu() to ensure that any readers that might
		 * have references to the old structure complete before freeing
		 * the old structure.
		 */
		void foo_update_a(int new_a)
		{
			struct foo *new_fp;
			struct foo *old_fp;
	
			new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
			spin_lock(&foo_mutex);
			old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
			*new_fp = *old_fp;
			new_fp->a = new_a;
			rcu_assign_pointer(gbl_foo, new_fp);
			spin_unlock(&foo_mutex);
			synchronize_rcu();
			kfree(old_fp);
		}
	
		/*
		 * Return the value of field "a" of the current gbl_foo
		 * structure.  Use rcu_read_lock() and rcu_read_unlock()
		 * to ensure that the structure does not get deleted out
		 * from under us, and use rcu_dereference() to ensure that
		 * we see the initialized version of the structure (important
		 * for DEC Alpha and for people reading the code).
		 */
		int foo_get_a(void)
		{
			int retval;
	
			rcu_read_lock();
			retval = rcu_dereference(gbl_foo)->a;
			rcu_read_unlock();
			return retval;
		}
	
	So, to sum up:
	
	o	Use rcu_read_lock() and rcu_read_unlock() to guard RCU
		read-side critical sections.
	
	o	Within an RCU read-side critical section, use rcu_dereference()
		to dereference RCU-protected pointers.
	
	o	Use some solid scheme (such as locks or semaphores) to
		keep concurrent updates from interfering with each other.
	
	o	Use rcu_assign_pointer() to update an RCU-protected pointer.
		This primitive protects concurrent readers from the updater,
		-not- concurrent updates from each other!  You therefore still
		need to use locking (or something similar) to keep concurrent
		rcu_assign_pointer() primitives from interfering with each other.
	
	o	Use synchronize_rcu() -after- removing a data element from an
		RCU-protected data structure, but -before- reclaiming/freeing
		the data element, in order to wait for the completion of all
		RCU read-side critical sections that might be referencing that
		data item.
	
	See checklist.txt for additional rules to follow when using RCU.
	And again, more-typical uses of RCU may be found in listRCU.txt,
	arrayRCU.txt, and NMI-RCU.txt.
	
	
	4.  WHAT IF MY UPDATING THREAD CANNOT BLOCK?
	
	In the example above, foo_update_a() blocks until a grace period elapses.
	This is quite simple, but in some cases one cannot afford to wait so
	long -- there might be other high-priority work to be done.
	
	In such cases, one uses call_rcu() rather than synchronize_rcu().
	The call_rcu() API is as follows:
	
		void call_rcu(struct rcu_head * head,
			      void (*func)(struct rcu_head *head));
	
	This function invokes func(head) after a grace period has elapsed.
	This invocation might happen from either softirq or process context,
	so the function is not permitted to block.  The foo struct needs to
	have an rcu_head structure added, perhaps as follows:
	
		struct foo {
			int a;
			char b;
			long c;
			struct rcu_head rcu;
		};
	
	The foo_update_a() function might then be written as follows:
	
		/*
		 * Create a new struct foo that is the same as the one currently
		 * pointed to by gbl_foo, except that field "a" is replaced
		 * with "new_a".  Points gbl_foo to the new structure, and
		 * frees up the old structure after a grace period.
		 *
		 * Uses rcu_assign_pointer() to ensure that concurrent readers
		 * see the initialized version of the new structure.
		 *
		 * Uses call_rcu() to ensure that any readers that might have
		 * references to the old structure complete before freeing the
		 * old structure.
		 */
		void foo_update_a(int new_a)
		{
			struct foo *new_fp;
			struct foo *old_fp;
	
			new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL);
			spin_lock(&foo_mutex);
			old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
			*new_fp = *old_fp;
			new_fp->a = new_a;
			rcu_assign_pointer(gbl_foo, new_fp);
			spin_unlock(&foo_mutex);
			call_rcu(&old_fp->rcu, foo_reclaim);
		}
	
	The foo_reclaim() function might appear as follows:
	
		void foo_reclaim(struct rcu_head *rp)
		{
			struct foo *fp = container_of(rp, struct foo, rcu);
	
			foo_cleanup(fp->a);
	
			kfree(fp);
		}
	
	The container_of() primitive is a macro that, given a pointer into a
	struct, the type of the struct, and the pointed-to field within the
	struct, returns a pointer to the beginning of the struct.
	
	The use of call_rcu() permits the caller of foo_update_a() to
	immediately regain control, without needing to worry further about the
	old version of the newly updated element.  It also clearly shows the
	RCU distinction between updater, namely foo_update_a(), and reclaimer,
	namely foo_reclaim().
	
	The summary of advice is the same as for the previous section, except
	that we are now using call_rcu() rather than synchronize_rcu():
	
	o	Use call_rcu() -after- removing a data element from an
		RCU-protected data structure in order to register a callback
		function that will be invoked after the completion of all RCU
		read-side critical sections that might be referencing that
		data item.
	
	If the callback for call_rcu() is not doing anything more than calling
	kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
	to avoid having to write your own callback:
	
		kfree_rcu(old_fp, rcu);
	
	Again, see checklist.txt for additional rules governing the use of RCU.
	
	
	5.  WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?
	
	One of the nice things about RCU is that it has extremely simple "toy"
	implementations that are a good first step towards understanding the
	production-quality implementations in the Linux kernel.  This section
	presents two such "toy" implementations of RCU, one that is implemented
	in terms of familiar locking primitives, and another that more closely
	resembles "classic" RCU.  Both are way too simple for real-world use,
	lacking both functionality and performance.  However, they are useful
	in getting a feel for how RCU works.  See kernel/rcupdate.c for a
	production-quality implementation, and see:
	
		http://www.rdrop.com/users/paulmck/RCU
	
	for papers describing the Linux kernel RCU implementation.  The OLS'01
	and OLS'02 papers are a good introduction, and the dissertation provides
	more details on the current implementation as of early 2004.
	
	
	5A.  "TOY" IMPLEMENTATION #1: LOCKING
	
	This section presents a "toy" RCU implementation that is based on
	familiar locking primitives.  Its overhead makes it a non-starter for
	real-life use, as does its lack of scalability.  It is also unsuitable
	for realtime use, since it allows scheduling latency to "bleed" from
	one read-side critical section to another.  It also assumes recursive
	reader-writer locks:  If you try this with non-recursive locks, and
	you allow nested rcu_read_lock() calls, you can deadlock.
	
	However, it is probably the easiest implementation to relate to, so is
	a good starting point.
	
	It is extremely simple:
	
		static DEFINE_RWLOCK(rcu_gp_mutex);
	
		void rcu_read_lock(void)
		{
			read_lock(&rcu_gp_mutex);
		}
	
		void rcu_read_unlock(void)
		{
			read_unlock(&rcu_gp_mutex);
		}
	
		void synchronize_rcu(void)
		{
			write_lock(&rcu_gp_mutex);
			write_unlock(&rcu_gp_mutex);
		}
	
	[You can ignore rcu_assign_pointer() and rcu_dereference() without missing
	much.  But here are simplified versions anyway.  And whatever you do,
	don't forget about them when submitting patches making use of RCU!]
	
		#define rcu_assign_pointer(p, v) \
		({ \
			smp_store_release(&(p), (v)); \
		})
	
		#define rcu_dereference(p) \
		({ \
			typeof(p) _________p1 = READ_ONCE(p); \
			(_________p1); \
		})
	
	
	The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
	and release a global reader-writer lock.  The synchronize_rcu()
	primitive write-acquires this same lock, then immediately releases
	it.  This means that once synchronize_rcu() exits, all RCU read-side
	critical sections that were in progress before synchronize_rcu() was
	called are guaranteed to have completed -- there is no way that
	synchronize_rcu() would have been able to write-acquire the lock
	otherwise.
	
	It is possible to nest rcu_read_lock(), since reader-writer locks may
	be recursively acquired.  Note also that rcu_read_lock() is immune
	from deadlock (an important property of RCU).  The reason for this is
	that the only thing that can block rcu_read_lock() is a synchronize_rcu().
	But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
	so there can be no deadlock cycle.
	
	Quick Quiz #1:	Why is this argument naive?  How could a deadlock
			occur when using this algorithm in a real-world Linux
			kernel?  How could this deadlock be avoided?
	
	
	5B.  "TOY" EXAMPLE #2: CLASSIC RCU
	
	This section presents a "toy" RCU implementation that is based on
	"classic RCU".  It is also short on performance (but only for updates) and
	on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT
	kernels.  The definitions of rcu_dereference() and rcu_assign_pointer()
	are the same as those shown in the preceding section, so they are omitted.
	
		void rcu_read_lock(void) { }
	
		void rcu_read_unlock(void) { }
	
		void synchronize_rcu(void)
		{
			int cpu;
	
			for_each_possible_cpu(cpu)
				run_on(cpu);
		}
	
	Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
	This is the great strength of classic RCU in a non-preemptive kernel:
	read-side overhead is precisely zero, at least on non-Alpha CPUs.
	And there is absolutely no way that rcu_read_lock() can possibly
	participate in a deadlock cycle!
	
	The implementation of synchronize_rcu() simply schedules itself on each
	CPU in turn.  The run_on() primitive can be implemented straightforwardly
	in terms of the sched_setaffinity() primitive.  Of course, a somewhat less
	"toy" implementation would restore the affinity upon completion rather
	than just leaving all tasks running on the last CPU, but when I said
	"toy", I meant -toy-!
	
	So how the heck is this supposed to work???
	
	Remember that it is illegal to block while in an RCU read-side critical
	section.  Therefore, if a given CPU executes a context switch, we know
	that it must have completed all preceding RCU read-side critical sections.
	Once -all- CPUs have executed a context switch, then -all- preceding
	RCU read-side critical sections will have completed.
	
	So, suppose that we remove a data item from its structure and then invoke
	synchronize_rcu().  Once synchronize_rcu() returns, we are guaranteed
	that there are no RCU read-side critical sections holding a reference
	to that data item, so we can safely reclaim it.
	
	Quick Quiz #2:	Give an example where Classic RCU's read-side
			overhead is -negative-.
	
	Quick Quiz #3:  If it is illegal to block in an RCU read-side
			critical section, what the heck do you do in
			PREEMPT_RT, where normal spinlocks can block???
	
	
	6.  ANALOGY WITH READER-WRITER LOCKING
	
	Although RCU can be used in many different ways, a very common use of
	RCU is analogous to reader-writer locking.  The following unified
	diff shows how closely related RCU and reader-writer locking can be.
	
		@@ -5,5 +5,5 @@ struct el {
		 	int data;
		 	/* Other data fields */
		 };
		-rwlock_t listmutex;
		+spinlock_t listmutex;
		 struct el head;
	
		@@ -13,15 +14,15 @@
			struct list_head *lp;
			struct el *p;
	
		-	read_lock(&listmutex);
		-	list_for_each_entry(p, head, lp) {
		+	rcu_read_lock();
		+	list_for_each_entry_rcu(p, head, lp) {
				if (p->key == key) {
					*result = p->data;
		-			read_unlock(&listmutex);
		+			rcu_read_unlock();
					return 1;
				}
			}
		-	read_unlock(&listmutex);
		+	rcu_read_unlock();
			return 0;
		 }
	
		@@ -29,15 +30,16 @@
		 {
			struct el *p;
	
		-	write_lock(&listmutex);
		+	spin_lock(&listmutex);
			list_for_each_entry(p, head, lp) {
				if (p->key == key) {
		-			list_del(&p->list);
		-			write_unlock(&listmutex);
		+			list_del_rcu(&p->list);
		+			spin_unlock(&listmutex);
		+			synchronize_rcu();
					kfree(p);
					return 1;
				}
			}
		-	write_unlock(&listmutex);
		+	spin_unlock(&listmutex);
			return 0;
		 }
	
	Or, for those who prefer a side-by-side listing:
	
	 1 struct el {                          1 struct el {
	 2   struct list_head list;             2   struct list_head list;
	 3   long key;                          3   long key;
	 4   spinlock_t mutex;                  4   spinlock_t mutex;
	 5   int data;                          5   int data;
	 6   /* Other data fields */            6   /* Other data fields */
	 7 };                                   7 };
	 8 rwlock_t listmutex;                  8 spinlock_t listmutex;
	 9 struct el head;                      9 struct el head;
	
	 1 int search(long key, int *result)    1 int search(long key, int *result)
	 2 {                                    2 {
	 3   struct list_head *lp;              3   struct list_head *lp;
	 4   struct el *p;                      4   struct el *p;
	 5                                      5
	 6   read_lock(&listmutex);             6   rcu_read_lock();
	 7   list_for_each_entry(p, head, lp) { 7   list_for_each_entry_rcu(p, head, lp) {
	 8     if (p->key == key) {             8     if (p->key == key) {
	 9       *result = p->data;             9       *result = p->data;
	10       read_unlock(&listmutex);      10       rcu_read_unlock();
	11       return 1;                     11       return 1;
	12     }                               12     }
	13   }                                 13   }
	14   read_unlock(&listmutex);          14   rcu_read_unlock();
	15   return 0;                         15   return 0;
	16 }                                   16 }
	
	 1 int delete(long key)                 1 int delete(long key)
	 2 {                                    2 {
	 3   struct el *p;                      3   struct el *p;
	 4                                      4
	 5   write_lock(&listmutex);            5   spin_lock(&listmutex);
	 6   list_for_each_entry(p, head, lp) { 6   list_for_each_entry(p, head, lp) {
	 7     if (p->key == key) {             7     if (p->key == key) {
	 8       list_del(&p->list);            8       list_del_rcu(&p->list);
	 9       write_unlock(&listmutex);      9       spin_unlock(&listmutex);
	                                       10       synchronize_rcu();
	10       kfree(p);                     11       kfree(p);
	11       return 1;                     12       return 1;
	12     }                               13     }
	13   }                                 14   }
	14   write_unlock(&listmutex);         15   spin_unlock(&listmutex);
	15   return 0;                         16   return 0;
	16 }                                   17 }
	
	Either way, the differences are quite small.  Read-side locking moves
	to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
	a reader-writer lock to a simple spinlock, and a synchronize_rcu()
	precedes the kfree().
	
	However, there is one potential catch: the read-side and update-side
	critical sections can now run concurrently.  In many cases, this will
	not be a problem, but it is necessary to check carefully regardless.
	For example, if multiple independent list updates must be seen as
	a single atomic update, converting to RCU will require special care.
	
	Also, the presence of synchronize_rcu() means that the RCU version of
	delete() can now block.  If this is a problem, there is a callback-based
	mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
	be used in place of synchronize_rcu().
	
	
	7.  FULL LIST OF RCU APIs
	
	The RCU APIs are documented in docbook-format header comments in the
	Linux-kernel source code, but it helps to have a full list of the
	APIs, since there does not appear to be a way to categorize them
	in docbook.  Here is the list, by category.
	
	RCU list traversal:
	
		list_entry_rcu
		list_first_entry_rcu
		list_next_rcu
		list_for_each_entry_rcu
		list_for_each_entry_continue_rcu
		hlist_first_rcu
		hlist_next_rcu
		hlist_pprev_rcu
		hlist_for_each_entry_rcu
		hlist_for_each_entry_rcu_bh
		hlist_for_each_entry_continue_rcu
		hlist_for_each_entry_continue_rcu_bh
		hlist_nulls_first_rcu
		hlist_nulls_for_each_entry_rcu
		hlist_bl_first_rcu
		hlist_bl_for_each_entry_rcu
	
	RCU pointer/list update:
	
		rcu_assign_pointer
		list_add_rcu
		list_add_tail_rcu
		list_del_rcu
		list_replace_rcu
		hlist_add_behind_rcu
		hlist_add_before_rcu
		hlist_add_head_rcu
		hlist_del_rcu
		hlist_del_init_rcu
		hlist_replace_rcu
		list_splice_init_rcu()
		hlist_nulls_del_init_rcu
		hlist_nulls_del_rcu
		hlist_nulls_add_head_rcu
		hlist_bl_add_head_rcu
		hlist_bl_del_init_rcu
		hlist_bl_del_rcu
		hlist_bl_set_first_rcu
	
	RCU:	Critical sections	Grace period		Barrier
	
		rcu_read_lock		synchronize_net		rcu_barrier
		rcu_read_unlock		synchronize_rcu
		rcu_dereference		synchronize_rcu_expedited
		rcu_read_lock_held	call_rcu
		rcu_dereference_check	kfree_rcu
		rcu_dereference_protected
	
	bh:	Critical sections	Grace period		Barrier
	
		rcu_read_lock_bh	call_rcu_bh		rcu_barrier_bh
		rcu_read_unlock_bh	synchronize_rcu_bh
		rcu_dereference_bh	synchronize_rcu_bh_expedited
		rcu_dereference_bh_check
		rcu_dereference_bh_protected
		rcu_read_lock_bh_held
	
	sched:	Critical sections	Grace period		Barrier
	
		rcu_read_lock_sched	synchronize_sched	rcu_barrier_sched
		rcu_read_unlock_sched	call_rcu_sched
		[preempt_disable]	synchronize_sched_expedited
		[and friends]
		rcu_read_lock_sched_notrace
		rcu_read_unlock_sched_notrace
		rcu_dereference_sched
		rcu_dereference_sched_check
		rcu_dereference_sched_protected
		rcu_read_lock_sched_held
	
	
	SRCU:	Critical sections	Grace period		Barrier
	
		srcu_read_lock		synchronize_srcu	srcu_barrier
		srcu_read_unlock	call_srcu
		srcu_dereference	synchronize_srcu_expedited
		srcu_dereference_check
		srcu_read_lock_held
	
	SRCU:	Initialization/cleanup
		DEFINE_SRCU
		DEFINE_STATIC_SRCU
		init_srcu_struct
		cleanup_srcu_struct
	
	All:  lockdep-checked RCU-protected pointer access
	
		rcu_access_pointer
		rcu_dereference_raw
		RCU_LOCKDEP_WARN
		rcu_sleep_check
		RCU_NONIDLE
	
	See the comment headers in the source code (or the docbook generated
	from them) for more information.
	
	However, given that there are no fewer than four families of RCU APIs
	in the Linux kernel, how do you choose which one to use?  The following
	list can be helpful:
	
	a.	Will readers need to block?  If so, you need SRCU.
	
	b.	What about the -rt patchset?  If readers would need to block
		in an non-rt kernel, you need SRCU.  If readers would block
		in a -rt kernel, but not in a non-rt kernel, SRCU is not
		necessary.  (The -rt patchset turns spinlocks into sleeplocks,
		hence this distinction.)
	
	c.	Do you need to treat NMI handlers, hardirq handlers,
		and code segments with preemption disabled (whether
		via preempt_disable(), local_irq_save(), local_bh_disable(),
		or some other mechanism) as if they were explicit RCU readers?
		If so, RCU-sched is the only choice that will work for you.
	
	d.	Do you need RCU grace periods to complete even in the face
		of softirq monopolization of one or more of the CPUs?  For
		example, is your code subject to network-based denial-of-service
		attacks?  If so, you need RCU-bh.
	
	e.	Is your workload too update-intensive for normal use of
		RCU, but inappropriate for other synchronization mechanisms?
		If so, consider SLAB_TYPESAFE_BY_RCU (which was originally
		named SLAB_DESTROY_BY_RCU).  But please be careful!
	
	f.	Do you need read-side critical sections that are respected
		even though they are in the middle of the idle loop, during
		user-mode execution, or on an offlined CPU?  If so, SRCU is the
		only choice that will work for you.
	
	g.	Otherwise, use RCU.
	
	Of course, this all assumes that you have determined that RCU is in fact
	the right tool for your job.
	
	
	8.  ANSWERS TO QUICK QUIZZES
	
	Quick Quiz #1:	Why is this argument naive?  How could a deadlock
			occur when using this algorithm in a real-world Linux
			kernel?  [Referring to the lock-based "toy" RCU
			algorithm.]
	
	Answer:		Consider the following sequence of events:
	
			1.	CPU 0 acquires some unrelated lock, call it
				"problematic_lock", disabling irq via
				spin_lock_irqsave().
	
			2.	CPU 1 enters synchronize_rcu(), write-acquiring
				rcu_gp_mutex.
	
			3.	CPU 0 enters rcu_read_lock(), but must wait
				because CPU 1 holds rcu_gp_mutex.
	
			4.	CPU 1 is interrupted, and the irq handler
				attempts to acquire problematic_lock.
	
			The system is now deadlocked.
	
			One way to avoid this deadlock is to use an approach like
			that of CONFIG_PREEMPT_RT, where all normal spinlocks
			become blocking locks, and all irq handlers execute in
			the context of special tasks.  In this case, in step 4
			above, the irq handler would block, allowing CPU 1 to
			release rcu_gp_mutex, avoiding the deadlock.
	
			Even in the absence of deadlock, this RCU implementation
			allows latency to "bleed" from readers to other
			readers through synchronize_rcu().  To see this,
			consider task A in an RCU read-side critical section
			(thus read-holding rcu_gp_mutex), task B blocked
			attempting to write-acquire rcu_gp_mutex, and
			task C blocked in rcu_read_lock() attempting to
			read_acquire rcu_gp_mutex.  Task A's RCU read-side
			latency is holding up task C, albeit indirectly via
			task B.
	
			Realtime RCU implementations therefore use a counter-based
			approach where tasks in RCU read-side critical sections
			cannot be blocked by tasks executing synchronize_rcu().
	
	Quick Quiz #2:	Give an example where Classic RCU's read-side
			overhead is -negative-.
	
	Answer:		Imagine a single-CPU system with a non-CONFIG_PREEMPT
			kernel where a routing table is used by process-context
			code, but can be updated by irq-context code (for example,
			by an "ICMP REDIRECT" packet).	The usual way of handling
			this would be to have the process-context code disable
			interrupts while searching the routing table.  Use of
			RCU allows such interrupt-disabling to be dispensed with.
			Thus, without RCU, you pay the cost of disabling interrupts,
			and with RCU you don't.
	
			One can argue that the overhead of RCU in this
			case is negative with respect to the single-CPU
			interrupt-disabling approach.  Others might argue that
			the overhead of RCU is merely zero, and that replacing
			the positive overhead of the interrupt-disabling scheme
			with the zero-overhead RCU scheme does not constitute
			negative overhead.
	
			In real life, of course, things are more complex.  But
			even the theoretical possibility of negative overhead for
			a synchronization primitive is a bit unexpected.  ;-)
	
	Quick Quiz #3:  If it is illegal to block in an RCU read-side
			critical section, what the heck do you do in
			PREEMPT_RT, where normal spinlocks can block???
	
	Answer:		Just as PREEMPT_RT permits preemption of spinlock
			critical sections, it permits preemption of RCU
			read-side critical sections.  It also permits
			spinlocks blocking while in RCU read-side critical
			sections.
	
			Why the apparent inconsistency?  Because it is it
			possible to use priority boosting to keep the RCU
			grace periods short if need be (for example, if running
			short of memory).  In contrast, if blocking waiting
			for (say) network reception, there is no way to know
			what should be boosted.  Especially given that the
			process we need to boost might well be a human being
			who just went out for a pizza or something.  And although
			a computer-operated cattle prod might arouse serious
			interest, it might also provoke serious objections.
			Besides, how does the computer know what pizza parlor
			the human being went to???
	
	
	ACKNOWLEDGEMENTS
	
	My thanks to the people who helped make this human-readable, including
	Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
	
	
	For more information, see http://www.rdrop.com/users/paulmck/RCU.

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

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