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 .. _whatisrcu_doc:
 
 What is RCU?  --  "Read, Copy, Update"
 ======================================
 
 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:
 
 :ref:`1.        RCU OVERVIEW <1_whatisRCU>`
 
 :ref:`2.        WHAT IS RCU'S CORE API? <2_whatisRCU>`
 
 :ref:`3.        WHAT ARE SOME EXAMPLE USES OF CORE RCU API? <3_whatisRCU>`
 
 :ref:`4.        WHAT IF MY UPDATING THREAD CANNOT BLOCK? <4_whatisRCU>`
 
 :ref:`5.        WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? <5_whatisRCU>`
 
 :ref:`6.        ANALOGY WITH READER-WRITER LOCKING <6_whatisRCU>`
 
 :ref:`7.        ANALOGY WITH REFERENCE COUNTING <7_whatisRCU>`
 
 :ref:`8.        FULL LIST OF RCU APIs <8_whatisRCU>`
 
 :ref:`9.        ANSWERS TO QUICK QUIZZES <9_whatisRCU>`
 
 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_whatisRCU:
 
 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_whatisRCU:
 
 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.rst for some approaches to limiting the update rate.
 
 rcu_assign_pointer()
 ^^^^^^^^^^^^^^^^^^^^
         void 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 macro does not
         evaluate to an rvalue, but it does execute 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 [1]_.
         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() [2]_.
 
 ..      [1] The variant rcu_dereference_protected() can be used outside
         of an RCU read-side critical section as long as the usage is
         protected by locks acquired by the update-side code.  This variant
         avoids the lockdep warning that would happen when using (for
         example) rcu_dereference() without rcu_read_lock() protection.
         Using rcu_dereference_protected() also has the advantage
         of permitting compiler optimizations that rcu_dereference()
         must prohibit.  The rcu_dereference_protected() variant takes
         a lockdep expression to indicate which locks must be acquired
         by the caller. If the indicated protection is not provided,
         a lockdep splat is emitted.  See Design/Requirements/Requirements.rst
         and the API's code comments for more details and example usage.
 
 ..      [2] If the list_for_each_entry_rcu() instance might be used by
         update-side code as well as by RCU readers, then an additional
         lockdep expression can be added to its list of arguments.
         For example, given an additional "lock_is_held(&mylock)" argument,
         the RCU lockdep code would complain only if this instance was
         invoked outside of an RCU read-side critical section and without
         the protection of mylock.
 
 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 at least three flavors of RCU usage in the Linux kernel. The diagram
 above shows the most common one. On the updater side, the rcu_assign_pointer(),
 synchronize_rcu() and call_rcu() primitives used are the same for all three
 flavors. However for protection (on the reader side), the primitives used vary
 depending on the flavor:
 
 a.      rcu_read_lock() / rcu_read_unlock()
         rcu_dereference()
 
 b.      rcu_read_lock_bh() / rcu_read_unlock_bh()
         local_bh_disable() / local_bh_enable()
         rcu_dereference_bh()
 
 c.      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 flavors 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_whatisRCU:
 
 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.rst, arrayRCU.rst, and NMI-RCU.rst.
 ::
 
         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:
 
 -       Use rcu_read_lock() and rcu_read_unlock() to guard RCU
         read-side critical sections.
 
 -       Within an RCU read-side critical section, use rcu_dereference()
         to dereference RCU-protected pointers.
 
 -       Use some solid scheme (such as locks or semaphores) to
         keep concurrent updates from interfering with each other.
 
 -       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.
 
 -       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.rst for additional rules to follow when using RCU.
 And again, more-typical uses of RCU may be found in listRCU.rst,
 arrayRCU.rst, and NMI-RCU.rst.
 
 .. _4_whatisRCU:
 
 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, rcu_callback_t func);
 
 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():
 
 -       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.rst for additional rules governing the use of RCU.
 
 .. _5_whatisRCU:
 
 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/rcu/update.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);
                 smp_mb__after_spinlock();
                 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 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.  The smp_mb__after_spinlock()
 promotes synchronize_rcu() to a full memory barrier in compliance with
 the "Memory-Barrier Guarantees" listed in:
 
         Design/Requirements/Requirements.rst
 
 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.
 
 .. _quiz_1:
 
 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?
 
 :ref:`Answers to Quick Quiz <9_whatisRCU>`
 
 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_PREEMPTION
 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.
 
 .. _quiz_2:
 
 Quick Quiz #2:
                 Give an example where Classic RCU's read-side
                 overhead is **negative**.
 
 :ref:`Answers to Quick Quiz <9_whatisRCU>`
 
 .. _quiz_3:
 
 Quick Quiz #3:
                 If it is illegal to block in an RCU read-side
                 critical section, what the heck do you do in
                 CONFIG_PREEMPT_RT, where normal spinlocks can block???
 
 :ref:`Answers to Quick Quiz <9_whatisRCU>`
 
 .. _6_whatisRCU:
 
 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_whatisRCU:
 
 7.  ANALOGY WITH REFERENCE COUNTING
 -----------------------------------
 
 The reader-writer analogy (illustrated by the previous section) is not
 always the best way to think about using RCU.  Another helpful analogy
 considers RCU an effective reference count on everything which is
 protected by RCU.
 
 A reference count typically does not prevent the referenced object's
 values from changing, but does prevent changes to type -- particularly the
 gross change of type that happens when that object's memory is freed and
 re-allocated for some other purpose.  Once a type-safe reference to the
 object is obtained, some other mechanism is needed to ensure consistent
 access to the data in the object.  This could involve taking a spinlock,
 but with RCU the typical approach is to perform reads with SMP-aware
 operations such as smp_load_acquire(), to perform updates with atomic
 read-modify-write operations, and to provide the necessary ordering.
 RCU provides a number of support functions that embed the required
 operations and ordering, such as the list_for_each_entry_rcu() macro
 used in the previous section.
 
 A more focused view of the reference counting behavior is that,
 between rcu_read_lock() and rcu_read_unlock(), any reference taken with
 rcu_dereference() on a pointer marked as ``__rcu`` can be treated as
 though a reference-count on that object has been temporarily increased.
 This prevents the object from changing type.  Exactly what this means
 will depend on normal expectations of objects of that type, but it
 typically includes that spinlocks can still be safely locked, normal
 reference counters can be safely manipulated, and ``__rcu`` pointers
 can be safely dereferenced.
 
 Some operations that one might expect to see on an object for
 which an RCU reference is held include:
 
  - Copying out data that is guaranteed to be stable by the object's type.
  - Using kref_get_unless_zero() or similar to get a longer-term
    reference.  This may fail of course.
  - Acquiring a spinlock in the object, and checking if the object still
    is the expected object and if so, manipulating it freely.
 
 The understanding that RCU provides a reference that only prevents a
 change of type is particularly visible with objects allocated from a
 slab cache marked ``SLAB_TYPESAFE_BY_RCU``.  RCU operations may yield a
 reference to an object from such a cache that has been concurrently
 freed and the memory reallocated to a completely different object,
 though of the same type.  In this case RCU doesn't even protect the
 identity of the object from changing, only its type.  So the object
 found may not be the one expected, but it will be one where it is safe
 to take a reference or spinlock and then confirm that the identity
 matches the expectations.
 
 With traditional reference counting -- such as that implemented by the
 kref library in Linux -- there is typically code that runs when the last
 reference to an object is dropped.  With kref, this is the function
 passed to kref_put().  When RCU is being used, such finalization code
 must not be run until all ``__rcu`` pointers referencing the object have
 been updated, and then a grace period has passed.  Every remaining
 globally visible pointer to the object must be considered to be a
 potential counted reference, and the finalization code is typically run
 using call_rcu() only after all those pointers have been changed.
 
 To see how to choose between these two analogies -- of RCU as a
 reader-writer lock and RCU as a reference counting system -- it is useful
 to reflect on the scale of the thing being protected.  The reader-writer
 lock analogy looks at larger multi-part objects such as a linked list
 and shows how RCU can facilitate concurrency while elements are added
 to, and removed from, the list.  The reference-count analogy looks at
 the individual objects and looks at how they can be accessed safely
 within whatever whole they are a part of.
 
 .. _8_whatisRCU:
 
 8.  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_entry_lockless
         list_first_entry_rcu
         list_next_rcu
         list_for_each_entry_rcu
         list_for_each_entry_continue_rcu
         list_for_each_entry_from_rcu
         list_first_or_null_rcu
         list_next_or_null_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_from_rcu
         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_add_tail_rcu
         hlist_del_rcu
         hlist_del_init_rcu
         hlist_replace_rcu
         list_splice_init_rcu
         list_splice_tail_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                rcu_barrier
         rcu_read_unlock_bh      synchronize_rcu
         [local_bh_disable]      synchronize_rcu_expedited
         [and friends]
         rcu_dereference_bh
         rcu_dereference_bh_check
         rcu_dereference_bh_protected
         rcu_read_lock_bh_held
 
 sched::
 
         Critical sections       Grace period            Barrier
 
         rcu_read_lock_sched     call_rcu                rcu_barrier
         rcu_read_unlock_sched   synchronize_rcu
         [preempt_disable]       synchronize_rcu_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          call_srcu               srcu_barrier
         srcu_read_unlock        synchronize_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 should disable softirq across your readers,
         for example, by using rcu_read_lock_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.
 
 .. _9_whatisRCU:
 
 9.  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().
 
 :ref:`Back to Quick Quiz #1 <quiz_1>`
 
 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_PREEMPTION
                 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.  ;-)
 
 :ref:`Back to Quick Quiz #2 <quiz_2>`
 
 Quick Quiz #3:
                 If it is illegal to block in an RCU read-side
                 critical section, what the heck do you do in
                 CONFIG_PREEMPT_RT, where normal spinlocks can block???
 
 Answer:
                 Just as CONFIG_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
                 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???
 
 :ref:`Back to Quick Quiz #3 <quiz_3>`
 
 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.

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