rcu: Update docs for rcu_access_pointer and rcu_dereference_protected

Update examples and lists of APIs to include these new
primitives.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
Cc: eric.dumazet@gmail.com
LKML-Reference: <1270852752-25278-3-git-send-email-paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

4 files changed, 58 insertions, 22 deletions

diff --git a/Documentation/RCU/NMI-RCU.txt b/Documentation/RCU/NMI-RCU.txt
index a6d32e65d222..a8536cb88091 100644
--- a/Documentation/RCU/NMI-RCU.txt
+++ b/Documentation/RCU/NMI-RCU.txt
@@ -34,7 +34,7 @@ NMI handler.
                 cpu = smp_processor_id();
                 ++nmi_count(cpu);
 
-                if (!rcu_dereference(nmi_callback)(regs, cpu))
+                if (!rcu_dereference_sched(nmi_callback)(regs, cpu))
                         default_do_nmi(regs);
 
                 nmi_exit();
@@ -47,12 +47,13 @@ function pointer.  If this handler returns zero, do_nmi() invokes the
 default_do_nmi() function to handle a machine-specific NMI.  Finally,
 preemption is restored.
 
-Strictly speaking, rcu_dereference() is not needed, since this code runs
-only on i386, which does not need rcu_dereference() anyway.  However,
-it is a good documentation aid, particularly for anyone attempting to
-do something similar on Alpha.
+In theory, rcu_dereference_sched() is not needed, since this code runs
+only on i386, which in theory does not need rcu_dereference_sched()
+anyway.  However, in practice it is a good documentation aid, particularly
+for anyone attempting to do something similar on Alpha or on systems
+with aggressive optimizing compilers.
 
-Quick Quiz:  Why might the rcu_dereference() be necessary on Alpha,
+Quick Quiz:  Why might the rcu_dereference_sched() be necessary on Alpha,
              given that the code referenced by the pointer is read-only?
 
 
@@ -99,17 +100,21 @@ invoke irq_enter() and irq_exit() on NMI entry and exit, respectively.
 
 Answer to Quick Quiz
 
-        Why might the rcu_dereference() be necessary on Alpha, given
+        Why might the rcu_dereference_sched() be necessary on Alpha, given
         that the code referenced by the pointer is read-only?
 
         Answer: The caller to set_nmi_callback() might well have
-                initialized some data that is to be used by the
-                new NMI handler.  In this case, the rcu_dereference()
-                would be needed, because otherwise a CPU that received
-                an NMI just after the new handler was set might see
-                the pointer to the new NMI handler, but the old
-                pre-initialized version of the handler's data.
-
-                More important, the rcu_dereference() makes it clear
-                to someone reading the code that the pointer is being
-                protected by RCU.
+                initialized some data that is to be used by the new NMI
+                handler.  In this case, the rcu_dereference_sched() would
+                be needed, because otherwise a CPU that received an NMI
+                just after the new handler was set might see the pointer
+                to the new NMI handler, but the old pre-initialized
+                version of the handler's data.
+
+                This same sad story can happen on other CPUs when using
+                a compiler with aggressive pointer-value speculation
+                optimizations.
+
+                More important, the rcu_dereference_sched() makes it
+                clear to someone reading the code that the pointer is
+                being protected by RCU-sched.
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index cbc180f90194..790d1a812376 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -260,7 +260,8 @@ over a rather long period of time, but improvements are always welcome!
         The reason that it is permissible to use RCU list-traversal
         primitives when the update-side lock is held is that doing so
         can be quite helpful in reducing code bloat when common code is
-        shared between readers and updaters.
+        shared between readers and updaters.  Additional primitives
+        are provided for this case, as discussed in lockdep.txt.
 
 10.     Conversely, if you are in an RCU read-side critical section,
         and you don't hold the appropriate update-side lock, you -must-
@@ -344,8 +345,8 @@ over a rather long period of time, but improvements are always welcome!
         requiring SRCU's read-side deadlock immunity or low read-side
         realtime latency.
 
-        Note that, rcu_assign_pointer() and rcu_dereference() relate to
-        SRCU just as they do to other forms of RCU.
+        Note that, rcu_assign_pointer() relates to SRCU just as they do
+        to other forms of RCU.
 
 15.     The whole point of call_rcu(), synchronize_rcu(), and friends
         is to wait until all pre-existing readers have finished before
diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.txt
index fe24b58627bd..d7a49b2f6994 100644
--- a/Documentation/RCU/lockdep.txt
+++ b/Documentation/RCU/lockdep.txt
@@ -32,9 +32,20 @@ checking of rcu_dereference() primitives:
         srcu_dereference(p, sp):
                 Check for SRCU read-side critical section.
         rcu_dereference_check(p, c):
-                Use explicit check expression "c".
+                Use explicit check expression "c".  This is useful in
+                code that is invoked by both readers and updaters.
         rcu_dereference_raw(p)
                 Don't check.  (Use sparingly, if at all.)
+        rcu_dereference_protected(p, c):
+                Use explicit check expression "c", and omit all barriers
+                and compiler constraints.  This is useful when the data
+                structure cannot change, for example, in code that is
+                invoked only by updaters.
+        rcu_access_pointer(p):
+                Return the value of the pointer and omit all barriers,
+                but retain the compiler constraints that prevent duplicating
+                or coalescsing.  This is useful when when testing the
+                value of the pointer itself, for example, against NULL.
 
 The rcu_dereference_check() check expression can be any boolean
 expression, but would normally include one of the rcu_read_lock_held()
@@ -59,7 +70,20 @@ In case (1), the pointer is picked up in an RCU-safe manner for vanilla
 RCU read-side critical sections, in case (2) the ->file_lock prevents
 any change from taking place, and finally, in case (3) the current task
 is the only task accessing the file_struct, again preventing any change
-from taking place.
+from taking place.  If the above statement was invoked only from updater
+code, it could instead be written as follows:
+
+        file = rcu_dereference_protected(fdt->fd[fd],
+                                         lockdep_is_held(&files->file_lock) ||
+                                         atomic_read(&files->count) == 1);
+
+This would verify cases #2 and #3 above, and furthermore lockdep would
+complain if this was used in an RCU read-side critical section unless one
+of these two cases held.  Because rcu_dereference_protected() omits all
+barriers and compiler constraints, it generates better code than do the
+other flavors of rcu_dereference().  On the other hand, it is illegal
+to use rcu_dereference_protected() if either the RCU-protected pointer
+or the RCU-protected data that it points to can change concurrently.
 
 There are currently only "universal" versions of the rcu_assign_pointer()
 and RCU list-/tree-traversal primitives, which do not (yet) check for
diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt
index 1dc00ee97163..cfaac34c4557 100644
--- a/Documentation/RCU/whatisRCU.txt
+++ b/Documentation/RCU/whatisRCU.txt
@@ -840,6 +840,12 @@ SRCU:	Initialization/cleanup
         init_srcu_struct
         cleanup_srcu_struct
 
+All:  lockdep-checked RCU-protected pointer access
+
+        rcu_dereference_check
+        rcu_dereference_protected
+        rcu_access_pointer
+
 See the comment headers in the source code (or the docbook generated
 from them) for more information.