dm snapshot: lock snapshot while supplying status

commit 4c6fff44 upstream.

This patch locks the snapshot when returning status.  It fixes a race
when it could return an invalid number of free chunks if someone
was simultaneously modifying it.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/md/dm-snap.c

@@ -1147,6 +1147,8 @@ static int snapshot_status(struct dm_target *ti, status_type_t type,
 	unsigned sz = 0;
 	struct dm_snapshot *snap = ti->private;
 
+	down_write(&snap->lock);
+
 	switch (type) {
 	case STATUSTYPE_INFO:
 		if (!snap->valid)
@@ -1178,6 +1180,8 @@ static int snapshot_status(struct dm_target *ti, status_type_t type,
 		break;
 	}
 
+	up_write(&snap->lock);
+
 	return 0;
 }