tty: Replace smp_rmb/smp_wmb with smp_load_acquire/smp_store_release

Clarify flip buffer producer/consumer operation; the use of
smp_load_acquire() and smp_store_release() more clearly indicates
which memory access requires a barrier.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_buffer.c

@@ -291,12 +291,11 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size,
 			n->flags = flags;
 			buf->tail = n;
 			b->commit = b->used;
-			/* paired w/ barrier in flush_to_ldisc(); ensures the
+			/* paired w/ acquire in flush_to_ldisc(); ensures the
 			 * latest commit value can be read before the head is
 			 * advanced to the next buffer
 			 */
-			smp_wmb();
-			b->next = n;
+			smp_store_release(&b->next, n);
 		} else if (change)
 			size = 0;
 		else
@@ -488,12 +487,11 @@ static void flush_to_ldisc(struct work_struct *work)
 		if (atomic_read(&buf->priority))
 			break;
 
-		next = head->next;
-		/* paired w/ barrier in __tty_buffer_request_room();
+		/* paired w/ release in __tty_buffer_request_room();
 		 * ensures commit value read is not stale if the head
 		 * is advancing to the next buffer
 		 */
-		smp_rmb();
+		next = smp_load_acquire(&head->next);
 		count = head->commit - head->read;
 		if (!count) {
 			if (next == NULL) {