dm snapshot: lock snapshot while supplying status

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.

Cc: stable@kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 9cb392b3e920..9bc814aa2bbd 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -1152,6 +1152,8 @@ 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)
@@ -1183,6 +1185,8 @@ static int snapshot_status(struct dm_target *ti, status_type_t type,
                 break;
         }
 
+        up_write(&snap->lock);
+
         return 0;
 }