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/

  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_TREE_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_deference() 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 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;
  		rcu_read_lock();
  		y = p->data;
  		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.	call_rcu_bh()		rcu_read_lock_bh() / rcu_read_unlock_bh()

  				rcu_dereference_bh()

  c.	synchronize_sched()	rcu_read_lock_sched() / rcu_read_unlock_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 *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 = gbl_foo;
  		*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 = gbl_foo;
  		*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);
  
  		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.
  
  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.
  
  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 they are anyway.  And whatever you do, don't
  forget about them when submitting patches making use of RCU!]
  
  	#define rcu_assign_pointer(p, v)	({ \
  							smp_wmb(); \
  							(p) = (v); \
  						})
  
  	#define rcu_dereference(p)     ({ \
  					typeof(p) _________p1 = p; \
  					smp_read_barrier_depends(); \
  					(_________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.
  
  	@@ -13,15 +14,15 @@
  		struct list_head *lp;
  		struct el *p;
  
  	-	read_lock();
  	-	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();
  	+			rcu_read_unlock();
  				return 1;
  			}
  		}
  	-	read_unlock();
  	+	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 spinlock_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();                       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();                10       rcu_read_unlock();
  11       return 1;                     11       return 1;
  12     }                               12     }
  13   }                                 13   }
  14   read_unlock();                    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(), 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_for_each_entry_rcu
  	hlist_for_each_entry_rcu

  	hlist_nulls_for_each_entry_rcu

  	list_for_each_continue_rcu	(to be deprecated in favor of new
  					 list_for_each_entry_continue_rcu)

  RCU pointer/list update:

  
  	rcu_assign_pointer
  	list_add_rcu
  	list_add_tail_rcu
  	list_del_rcu
  	list_replace_rcu
  	hlist_del_rcu

  	hlist_add_after_rcu
  	hlist_add_before_rcu

  	hlist_add_head_rcu

  	hlist_replace_rcu
  	list_splice_init_rcu()

  RCU:	Critical sections	Grace period		Barrier
  
  	rcu_read_lock		synchronize_net		rcu_barrier
  	rcu_read_unlock		synchronize_rcu

  	rcu_dereference		synchronize_rcu_expedited

  				call_rcu
  
  
  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

  
  
  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_dereference_sched

  
  
  SRCU:	Critical sections	Grace period		Barrier
  
  	srcu_read_lock		synchronize_srcu	N/A

  	srcu_read_unlock	synchronize_srcu_expedited

  	srcu_dereference

  SRCU:	Initialization/cleanup
  	init_srcu_struct
  	cleanup_srcu_struct

  All:  lockdep-checked RCU-protected pointer access
  
  	rcu_dereference_check
  	rcu_dereference_protected
  	rcu_access_pointer

  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.
  
  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, you need RCU-sched.
  
  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_DESTROY_BY_RCU.  But please be careful!
  
  f.	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.