md/raid5: STRIPE_ACTIVE has lock semantics, add barriers

All updates that occur under STRIPE_ACTIVE should be globally visible
when STRIPE_ACTIVE clears.  test_and_set_bit() implies a barrier, but
clear_bit() does not.

This is suitable for 3.1-stable.
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Cc: stable@kernel.org

drivers/md/raid5.c

@@ -3110,7 +3110,7 @@ static void handle_stripe(struct stripe_head *sh)
 	struct r5dev *pdev, *qdev;
 
 	clear_bit(STRIPE_HANDLE, &sh->state);
-	if (test_and_set_bit(STRIPE_ACTIVE, &sh->state)) {
+	if (test_and_set_bit_lock(STRIPE_ACTIVE, &sh->state)) {
 		/* already being handled, ensure it gets handled
 		 * again when current action finishes */
 		set_bit(STRIPE_HANDLE, &sh->state);
@@ -3375,7 +3375,7 @@ static void handle_stripe(struct stripe_head *sh)
 
 	return_io(s.return_bi);
 
-	clear_bit(STRIPE_ACTIVE, &sh->state);
+	clear_bit_unlock(STRIPE_ACTIVE, &sh->state);
 }
 
 static void raid5_activate_delayed(struct r5conf *conf)