[PATCH 3.16 034/133] btrfs: resume qgroup rescan on rw remount

[Ben Hutchings <ben@decadent.org.uk>]

3.16.51-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Aleksa Sarai <asarai@suse.de>

commit 6c6b5a39c4bf3dbd8cf629c9f5450e983c19dbb9 upstream.

Several distributions mount the "proper root" as ro during initrd and
then remount it as rw before pivot_root(2). Thus, if a rescan had been
aborted by a previous shutdown, the rescan would never be resumed.

This issue would manifest itself as several btrfs ioctl(2)s causing the
entire machine to hang when btrfs_qgroup_wait_for_completion was hit
(due to the fs_info->qgroup_rescan_running flag being set but the rescan
itself not being resumed). Notably, Docker's btrfs storage driver makes
regular use of BTRFS_QUOTA_CTL_DISABLE and BTRFS_IOC_QUOTA_RESCAN_WAIT
(causing this problem to be manifested on boot for some machines).

Cc: Jeff Mahoney <jeffm@suse.com>
Fixes: b382a324b60f ("Btrfs: fix qgroup rescan resume on mount")
Signed-off-by: Aleksa Sarai <asarai@suse.de>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Tested-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
[bwh: Backported to 3.16: add #include "qgroup.h"]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -59,6 +59,7 @@
 #include "free-space-cache.h"
 #include "backref.h"
 #include "tests/btrfs-tests.h"
+#include "qgroup.h"
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/btrfs.h>
@@ -1495,6 +1496,8 @@ static int btrfs_remount(struct super_bl
 			goto restore;
 		}
 
+		btrfs_qgroup_rescan_resume(fs_info);
+
 		if (!fs_info->uuid_root) {
 			btrfs_info(fs_info, "creating UUID tree");
 			ret = btrfs_create_uuid_tree(fs_info);