[PATCH] btrfs: resume qgroup rescan on rw remount

From: Aleksa Sarai
Date: Tue Jul 04 2017 - 07:49:26 EST

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
(causing this problem to be manifested on boot for some machines).

Cc: <stable@xxxxxxxxxxxxxxx> # v3.11+
Cc: Jeff Mahoney <jeffm@xxxxxxxx>
Fixes: b382a324b60f ("Btrfs: fix qgroup rescan resume on mount")
Signed-off-by: Aleksa Sarai <asarai@xxxxxxx>
fs/btrfs/super.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 6346876c97ea..ff6690389343 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1821,6 +1821,8 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
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);