rbd: set up watch in rbd_dev_image_probe()

Move setting up the watch request for an image so it's done in
rbd_dev_image_probe() rather than rbd_dev_probe_finish().  Move
it all the way up to before doing the initial probe.  This avoids
a potential race condition, in which we get (and use) the initial
snapshot context for an image, and it gets changed between that
time and the time we get the watch set up.

This resolves:
    http://tracker.ceph.com/issues/3871

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -4721,11 +4721,6 @@ out_err:
 static int rbd_dev_probe_finish(struct rbd_device *rbd_dev)
 {
 	int ret;
-	int tmp;
-
-	ret = rbd_dev_header_watch_sync(rbd_dev, 1);
-	if (ret)
-		return ret;
 
 	ret = rbd_dev_mapping_set(rbd_dev);
 	if (ret)
@@ -4773,9 +4768,6 @@ err_out_blkdev:
 	unregister_blkdev(rbd_dev->major, rbd_dev->name);
 err_out_id:
 	rbd_dev_id_put(rbd_dev);
-	tmp = rbd_dev_header_watch_sync(rbd_dev, 0);
-	if (tmp)
-		rbd_warn(rbd_dev, "failed to cancel watch event (%d)\n", ret);
 	rbd_dev_mapping_clear(rbd_dev);
 
 	return ret;
@@ -4816,6 +4808,7 @@ static int rbd_dev_header_name(struct rbd_device *rbd_dev)
 static int rbd_dev_image_probe(struct rbd_device *rbd_dev)
 {
 	int ret;
+	int tmp;
 
 	/*
 	 * Get the id from the image id object.  If it's not a
@@ -4832,16 +4825,20 @@ static int rbd_dev_image_probe(struct rbd_device *rbd_dev)
 	if (ret)
 		goto err_out_format;
 
+	ret = rbd_dev_header_watch_sync(rbd_dev, 1);
+	if (ret)
+		goto out_header_name;
+
 	if (rbd_dev->image_format == 1)
 		ret = rbd_dev_v1_probe(rbd_dev);
 	else
 		ret = rbd_dev_v2_probe(rbd_dev);
 	if (ret)
-		goto out_header_name;
+		goto err_out_watch;
 
 	ret = rbd_dev_snaps_update(rbd_dev);
 	if (ret)
-		goto out_header_name;
+		goto err_out_watch;
 
 	ret = rbd_dev_spec_update(rbd_dev);
 	if (ret)
@@ -4861,6 +4858,10 @@ err_out_parent:
 	rbd_header_free(&rbd_dev->header);
 err_out_snaps:
 	rbd_remove_all_snaps(rbd_dev);
+err_out_watch:
+	tmp = rbd_dev_header_watch_sync(rbd_dev, 0);
+	if (tmp)
+		rbd_warn(rbd_dev, "unable to tear down watch request\n");
 out_header_name:
 	kfree(rbd_dev->header_name);
 	rbd_dev->header_name = NULL;