snapshot: add bdrv_drain_all() to bdrv_snapshot_delete() to avoid concurrency problem

If there are still pending i/o while deleting snapshot,
because deleting snapshot is done in non-coroutine context, and
the pending i/o read/write (bdrv_co_do_rw) is done in coroutine context,
so it's possible to cause concurrency problem between above two operations.
Add bdrv_drain_all() to bdrv_snapshot_delete() to avoid this problem.
Signed-off-by: NZhang Haoyu <zhanghy@sangfor.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-id: 201410211637596311287@sangfor.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 3432a192)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

block/snapshot.c

@@ -236,6 +236,10 @@ int bdrv_snapshot_delete(BlockDriverState *bs,
         error_setg(errp, "snapshot_id and name are both NULL");
         return -EINVAL;
     }
+
+    /* drain all pending i/o before deleting snapshot */
+    bdrv_drain_all();
+
     if (drv->bdrv_snapshot_delete) {
         return drv->bdrv_snapshot_delete(bs, snapshot_id, name, errp);
     }