locking/Documentation: Mention smp_cond_acquire()

... do this next to smp_load_acquire() when first mentioning
ACQUIRE. While this call is briefly explained and control
dependencies are mentioned later, it does not hurt the reader.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bobby.prani@gmail.com
Cc: dhowells@redhat.com
Cc: dipankar@in.ibm.com
Cc: dvhart@linux.intel.com
Cc: edumazet@google.com
Cc: fweisbec@gmail.com
Cc: jiangshanlai@gmail.com
Cc: josh@joshtriplett.org
Cc: mathieu.desnoyers@efficios.com
Cc: oleg@redhat.com
Cc: rostedt@goodmis.org
Link: http://lkml.kernel.org/r/1460476375-27803-7-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Documentation/memory-barriers.txt

@@ -431,8 +431,9 @@ And a couple of implicit varieties:
      This acts as a one-way permeable barrier.  It guarantees that all memory
      operations after the ACQUIRE operation will appear to happen after the
      ACQUIRE operation with respect to the other components of the system.
-     ACQUIRE operations include LOCK operations and smp_load_acquire()
-     operations.
+     ACQUIRE operations include LOCK operations and both smp_load_acquire()
+     and smp_cond_acquire() operations. The later builds the necessary ACQUIRE
+     semantics from relying on a control dependency and smp_rmb().
 
      Memory operations that occur before an ACQUIRE operation may appear to
      happen after it completes.