atomic: Implement atomic_read_ctrl()

Provide atomic_read_ctrl() to mirror READ_ONCE_CTRL(), such that we can
more conveniently use atomics in control dependencies.

Since we can assume atomic_read() implies a READ_ONCE(), we must only
emit an extra smp_read_barrier_depends() in order to upgrade to
READ_ONCE_CTRL() semantics.

Requested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Cc: oleg@redhat.com
Link: http://lkml.kernel.org/r/20150918115637.GM3604@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 9 insertions, 8 deletions

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 2ba8461b0631..41ffd7e9cdcf 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -637,7 +637,8 @@ as follows:
 	b = p;  /* BUG: Compiler and CPU can both reorder!!! */
 
 Finally, the READ_ONCE_CTRL() includes an smp_read_barrier_depends()
-that DEC Alpha needs in order to respect control depedencies.
+that DEC Alpha needs in order to respect control depedencies. Alternatively
+use one of atomic{,64}_read_ctrl().
 
 So don't leave out the READ_ONCE_CTRL().
 
@@ -796,9 +797,9 @@ site: https://www.cl.cam.ac.uk/~pes20/ppcmem/index.html.
 
 In summary:
 
-  (*) Control dependencies must be headed by READ_ONCE_CTRL().
-      Or, as a much less preferable alternative, interpose
-      smp_read_barrier_depends() between a READ_ONCE() and the
+  (*) Control dependencies must be headed by READ_ONCE_CTRL(),
+      atomic{,64}_read_ctrl(). Or, as a much less preferable alternative,
+      interpose smp_read_barrier_depends() between a READ_ONCE() and the
       control-dependent write.
 
   (*) Control dependencies can order prior loads against later stores.
@@ -820,10 +821,10 @@ In summary:
       and WRITE_ONCE() can help to preserve the needed conditional.
 
   (*) Control dependencies require that the compiler avoid reordering the
-      dependency into nonexistence.  Careful use of READ_ONCE_CTRL()
-      or smp_read_barrier_depends() can help to preserve your control
-      dependency.  Please see the Compiler Barrier section for more
-      information.
+      dependency into nonexistence.  Careful use of READ_ONCE_CTRL(),
+      atomic{,64}_read_ctrl() or smp_read_barrier_depends() can help to
+      preserve your control dependency.  Please see the Compiler Barrier
+      section for more information.
 
   (*) Control dependencies pair normally with other types of barriers.