rbd: make exists flag atomic

The rbd_device->exists field can be updated asynchronously, changing
from set to clear if a mapped snapshot disappears from the base
image's snapshot context.

Currently, value of the "exists" flag is only read and modified
under protection of the header semaphore, but that will change with
the next patch.  Making it atomic ensures this won't be a problem
because the a the non-existence of device will be immediately known.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -229,7 +229,7 @@ struct rbd_device {
 	spinlock_t		lock;		/* queue lock */
 
 	struct rbd_image_header	header;
-	bool                    exists;
+	atomic_t		exists;
 	struct rbd_spec		*spec;
 
 	char			*header_name;
@@ -751,7 +751,7 @@ static int rbd_dev_set_mapping(struct rbd_device *rbd_dev)
 			goto done;
 		rbd_dev->mapping.read_only = true;
 	}
-	rbd_dev->exists = true;
+	atomic_set(&rbd_dev->exists, 1);
 done:
 	return ret;
 }
@@ -1671,7 +1671,7 @@ static void rbd_rq_fn(struct request_queue *q)
 		/* Grab a reference to the snapshot context */
 
 		down_read(&rbd_dev->header_rwsem);
-		if (rbd_dev->exists) {
+		if (atomic_read(&rbd_dev->exists)) {
 			snapc = ceph_get_snap_context(rbd_dev->header.snapc);
 			rbd_assert(snapc != NULL);
 		}
@@ -2294,6 +2294,7 @@ struct rbd_device *rbd_dev_create(struct rbd_client *rbdc,
 		return NULL;
 
 	spin_lock_init(&rbd_dev->lock);
+	atomic_set(&rbd_dev->exists, 0);
 	INIT_LIST_HEAD(&rbd_dev->node);
 	INIT_LIST_HEAD(&rbd_dev->snaps);
 	init_rwsem(&rbd_dev->header_rwsem);
@@ -2918,7 +2919,7 @@ static int rbd_dev_snaps_update(struct rbd_device *rbd_dev)
 			/* Existing snapshot not in the new snap context */
 
 			if (rbd_dev->spec->snap_id == snap->id)
-				rbd_dev->exists = false;
+				atomic_set(&rbd_dev->exists, 0);
 			rbd_remove_snap_dev(snap);
 			dout("%ssnap id %llu has been removed\n",
 				rbd_dev->spec->snap_id == snap->id ?