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authorPaul E. McKenney <paulmck@us.ibm.com>2005-09-10 00:26:24 -0700
committerLinus Torvalds <torvalds@g5.osdl.org>2005-09-10 10:06:24 -0700
commitdd81eca83c8300c95d8a1eaf0d38f56513711535 (patch)
treea1cb1aee24b703b86f7197cfe4f379529a683c5a /Documentation/RCU/whatisRCU.txt
parent12c62c2e9abf8da804fe1def1f5bb44d023f569f (diff)
downloadlinux-linaro-stable-dd81eca83c8300c95d8a1eaf0d38f56513711535.tar.gz
[PATCH] Yet another RCU documentation update
Update RCU documentation based on discussions and review of RCU-based tree patches. Add an introductory whatisRCU.txt file. Signed-off-by: <paulmck@us.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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+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.
+
+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 more important, it serves to document which pointers
+ are protected by RCU. 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. 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()
+
+b. call_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
+
+c. synchronize_sched() preempt_disable() / preempt_enable()
+ local_irq_save() / local_irq_restore()
+ hardirq enter / hardirq exit
+ NMI enter / NMI exit
+
+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(*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.
+
+
+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(*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.
+
+
+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 synchonize_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_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);
+ + 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(&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
+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.
+
+Markers for RCU read-side critical sections:
+
+ rcu_read_lock
+ rcu_read_unlock
+ rcu_read_lock_bh
+ rcu_read_unlock_bh
+
+RCU pointer/list traversal:
+
+ rcu_dereference
+ list_for_each_rcu (to be deprecated in favor of
+ list_for_each_entry_rcu)
+ list_for_each_safe_rcu (deprecated, not used)
+ list_for_each_entry_rcu
+ list_for_each_continue_rcu (to be deprecated in favor of new
+ list_for_each_entry_continue_rcu)
+ hlist_for_each_rcu (to be deprecated in favor of
+ hlist_for_each_entry_rcu)
+ hlist_for_each_entry_rcu
+
+RCU pointer update:
+
+ rcu_assign_pointer
+ list_add_rcu
+ list_add_tail_rcu
+ list_del_rcu
+ list_replace_rcu
+ hlist_del_rcu
+ hlist_add_head_rcu
+
+RCU grace period:
+
+ synchronize_kernel (deprecated)
+ synchronize_net
+ synchronize_sched
+ synchronize_rcu
+ call_rcu
+ call_rcu_bh
+
+See the comment headers in the source code (or the docbook generated
+from them) for more information.
+
+
+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".
+
+ 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, and Suzanne Wood.
+
+
+For more information, see http://www.rdrop.com/users/paulmck/RCU.