[PATCH] RCU documentation fixes (January 2006 update)

Updates to in-tree RCU documentation based on comments over the past few
months.

Signed-off-by: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

6 files changed, 69 insertions, 48 deletions

diff --git a/Documentation/RCU/RTFP.txt b/Documentation/RCU/RTFP.txt
index fcbcbc35b122..6221464d1a7e 100644
--- a/Documentation/RCU/RTFP.txt
+++ b/Documentation/RCU/RTFP.txt
@@ -90,16 +90,20 @@ at OLS.  The resulting abundance of RCU patches was presented the
 following year [McKenney02a], and use of RCU in dcache was first
 described that same year [Linder02a].
 
-Also in 2002, Michael [Michael02b,Michael02a] presented techniques
-that defer the destruction of data structures to simplify non-blocking
-synchronization (wait-free synchronization, lock-free synchronization,
-and obstruction-free synchronization are all examples of non-blocking
-synchronization).  In particular, this technique eliminates locking,
-reduces contention, reduces memory latency for readers, and parallelizes
-pipeline stalls and memory latency for writers.  However, these
-techniques still impose significant read-side overhead in the form of
-memory barriers.  Researchers at Sun worked along similar lines in the
-same timeframe [HerlihyLM02,HerlihyLMS03].
+Also in 2002, Michael [Michael02b,Michael02a] presented "hazard-pointer"
+techniques that defer the destruction of data structures to simplify
+non-blocking synchronization (wait-free synchronization, lock-free
+synchronization, and obstruction-free synchronization are all examples of
+non-blocking synchronization).  In particular, this technique eliminates
+locking, reduces contention, reduces memory latency for readers, and
+parallelizes pipeline stalls and memory latency for writers.  However,
+these techniques still impose significant read-side overhead in the
+form of memory barriers.  Researchers at Sun worked along similar lines
+in the same timeframe [HerlihyLM02,HerlihyLMS03].  These techniques
+can be thought of as inside-out reference counts, where the count is
+represented by the number of hazard pointers referencing a given data
+structure (rather than the more conventional counter field within the
+data structure itself).
 
 In 2003, the K42 group described how RCU could be used to create
 hot-pluggable implementations of operating-system functions.  Later that
@@ -113,7 +117,6 @@ number of operating-system kernels [PaulEdwardMcKenneyPhD], a paper
 describing how to make RCU safe for soft-realtime applications [Sarma04c],
 and a paper describing SELinux performance with RCU [JamesMorris04b].
 
-
 2005 has seen further adaptation of RCU to realtime use, permitting
 preemption of RCU realtime critical sections [PaulMcKenney05a,
 PaulMcKenney05b].
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index e118a7c1a092..49e27cc19385 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -177,3 +177,9 @@ over a rather long period of time, but improvements are always welcome!
 
         If you want to wait for some of these other things, you might
         instead need to use synchronize_irq() or synchronize_sched().
+
+12.     Any lock acquired by an RCU callback must be acquired elsewhere
+        with irq disabled, e.g., via spin_lock_irqsave().  Failing to
+        disable irq on a given acquisition of that lock will result in
+        deadlock as soon as the RCU callback happens to interrupt that
+        acquisition's critical section.
diff --git a/Documentation/RCU/listRCU.txt b/Documentation/RCU/listRCU.txt
index f8a54fa0d8ab..1fd175368a87 100644
--- a/Documentation/RCU/listRCU.txt
+++ b/Documentation/RCU/listRCU.txt
@@ -232,7 +232,7 @@ entry does not exist.  For this to be helpful, the search function must
 return holding the per-entry spinlock, as ipc_lock() does in fact do.
 
 Quick Quiz:  Why does the search function need to return holding the
-per-entry lock for this deleted-flag technique to be helpful?
+        per-entry lock for this deleted-flag technique to be helpful?
 
 If the system-call audit module were to ever need to reject stale data,
 one way to accomplish this would be to add a "deleted" flag and a "lock"
@@ -275,8 +275,8 @@ flag under the spinlock as follows:
         {
                 struct audit_entry  *e;
 
-                /* Do not use the _rcu iterator here, since this is the only
-                 * deletion routine. */
+                /* Do not need to use the _rcu iterator here, since this
+                 * is the only deletion routine. */
                 list_for_each_entry(e, list, list) {
                         if (!audit_compare_rule(rule, &e->rule)) {
                                 spin_lock(&e->lock);
@@ -304,9 +304,12 @@ function to reject newly deleted data.
 
 
 Answer to Quick Quiz
-
-If the search function drops the per-entry lock before returning, then
-the caller will be processing stale data in any case.  If it is really
-OK to be processing stale data, then you don't need a "deleted" flag.
-If processing stale data really is a problem, then you need to hold the
-per-entry lock across all of the code that uses the value looked up.
+        Why does the search function need to return holding the per-entry
+        lock for this deleted-flag technique to be helpful?
+
+        If the search function drops the per-entry lock before returning,
+        then the caller will be processing stale data in any case.  If it
+        is really OK to be processing stale data, then you don't need a
+        "deleted" flag.  If processing stale data really is a problem,
+        then you need to hold the per-entry lock across all of the code
+        that uses the value that was returned.
diff --git a/Documentation/RCU/rcu.txt b/Documentation/RCU/rcu.txt
index 6fa092251586..02e27bf1d365 100644
--- a/Documentation/RCU/rcu.txt
+++ b/Documentation/RCU/rcu.txt
@@ -111,6 +111,11 @@ o	What are all these files in this directory?
 
                 You are reading it!
 
+        rcuref.txt
+
+                Describes how to combine use of reference counts
+                with RCU.
+
         whatisRCU.txt
 
                 Overview of how the RCU implementation works.  Along
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index 3f60db41b2f0..451de2ad8329 100644
--- a/Documentation/RCU/rcuref.txt
+++ b/Documentation/RCU/rcuref.txt
@@ -1,7 +1,7 @@
-Refcounter design for elements of lists/arrays protected by RCU.
+Reference-count design for elements of lists/arrays protected by RCU.
 
-Refcounting on elements of  lists which are protected by traditional
-reader/writer spinlocks or semaphores are straight forward as in:
+Reference counting on elements of lists which are protected by traditional
+reader/writer spinlocks or semaphores are straightforward:
 
 1.                              2.
 add()                           search_and_reference()
@@ -28,12 +28,12 @@ release_referenced()			delete()
                                             ...
                                         }
 
-If this list/array is made lock free using rcu as in changing the
-write_lock in add() and delete() to spin_lock and changing read_lock
+If this list/array is made lock free using RCU as in changing the
+write_lock() in add() and delete() to spin_lock and changing read_lock
 in search_and_reference to rcu_read_lock(), the atomic_get in
 search_and_reference could potentially hold reference to an element which
-has already been deleted from the list/array.  atomic_inc_not_zero takes
-care of this scenario. search_and_reference should look as;
+has already been deleted from the list/array.  Use atomic_inc_not_zero()
+in this scenario as follows:
 
 1.                                      2.
 add()                                   search_and_reference()
@@ -51,17 +51,16 @@ add()					search_and_reference()
 release_referenced()                    delete()
 {                                       {
     ...                                     write_lock(&list_lock);
-    atomic_dec(&el->rc, relfunc)            ...
-    ...                                     delete_element
-}                                           write_unlock(&list_lock);
-                                            ...
+    if (atomic_dec_and_test(&el->rc))       ...
+        call_rcu(&el->head, el_free);       delete_element
+    ...                                     write_unlock(&list_lock);
+}                                           ...
                                             if (atomic_dec_and_test(&el->rc))
                                                 call_rcu(&el->head, el_free);
                                             ...
                                         }
 
-Sometimes, reference to the element need to be obtained in the
-update (write) stream.  In such cases, atomic_inc_not_zero might be an
-overkill since the spinlock serialising list updates are held. atomic_inc
-is to be used in such cases.
-
+Sometimes, a reference to the element needs to be obtained in the
+update (write) stream.  In such cases, atomic_inc_not_zero() might be
+overkill, since we hold the update-side spinlock.  One might instead
+use atomic_inc() in such cases.
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 15da16861fa3..5ed85af88789 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -200,10 +200,11 @@ rcu_assign_pointer()
         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().
+        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()
 
@@ -258,9 +259,11 @@ rcu_dereference()
         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
+        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
@@ -327,7 +330,7 @@ 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
+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 {
@@ -410,6 +413,8 @@ o	Use synchronize_rcu() -after- removing a data element from an
         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?
@@ -513,7 +518,7 @@ production-quality implementation, and see:
 
 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.
+more details on the current implementation as of early 2004.
 
 
 5A.  "TOY" IMPLEMENTATION #1: LOCKING
@@ -768,7 +773,6 @@ 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)
@@ -807,7 +811,8 @@ Quick Quiz #1:	Why is this argument naive?  How could a deadlock
 Answer:         Consider the following sequence of events:
 
                 1.      CPU 0 acquires some unrelated lock, call it
-                        "problematic_lock".
+                        "problematic_lock", disabling irq via
+                        spin_lock_irqsave().
 
                 2.      CPU 1 enters synchronize_rcu(), write-acquiring
                         rcu_gp_mutex.
@@ -894,7 +899,7 @@ Answer:		Just as PREEMPT_RT permits preemption of spinlock
 ACKNOWLEDGEMENTS
 
 My thanks to the people who helped make this human-readable, including
-Jon Walpole, Josh Triplett, Serge Hallyn, and Suzanne Wood.
+Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.
 
 
 For more information, see http://www.rdrop.com/users/paulmck/RCU.