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Subject[PATCH 5.4 61/64] btrfs: qgroup: fix deadlock between rescan worker and remove qgroup
From: Sidong Yang <>

commit d4aef1e122d8bbdc15ce3bd0bc813d6b44a7d63a upstream.

The commit e804861bd4e6 ("btrfs: fix deadlock between quota disable and
qgroup rescan worker") by Kawasaki resolves deadlock between quota
disable and qgroup rescan worker. But also there is a deadlock case like
it. It's about enabling or disabling quota and creating or removing
qgroup. It can be reproduced in simple script below.

for i in {1..100}
btrfs quota enable /mnt &
btrfs qgroup create 1/0 /mnt &
btrfs qgroup destroy 1/0 /mnt &
btrfs quota disable /mnt &

Here's why the deadlock happens:

1) The quota rescan task is running.

2) Task A calls btrfs_quota_disable(), locks the qgroup_ioctl_lock
mutex, and then calls btrfs_qgroup_wait_for_completion(), to wait for
the quota rescan task to complete.

3) Task B calls btrfs_remove_qgroup() and it blocks when trying to lock
the qgroup_ioctl_lock mutex, because it's being held by task A. At that
point task B is holding a transaction handle for the current transaction.

4) The quota rescan task calls btrfs_commit_transaction(). This results
in it waiting for all other tasks to release their handles on the
transaction, but task B is blocked on the qgroup_ioctl_lock mutex
while holding a handle on the transaction, and that mutex is being held
by task A, which is waiting for the quota rescan task to complete,
resulting in a deadlock between these 3 tasks.

To resolve this issue, the thread disabling quota should unlock
qgroup_ioctl_lock before waiting rescan completion. Move
btrfs_qgroup_wait_for_completion() after unlock of qgroup_ioctl_lock.

Fixes: e804861bd4e6 ("btrfs: fix deadlock between quota disable and qgroup rescan worker")
CC: # 5.4+
Reviewed-by: Filipe Manana <>
Reviewed-by: Shin'ichiro Kawasaki <>
Signed-off-by: Sidong Yang <>
Reviewed-by: David Sterba <>
Signed-off-by: David Sterba <>
Signed-off-by: Greg Kroah-Hartman <>
fs/btrfs/qgroup.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -1117,13 +1117,20 @@ int btrfs_quota_disable(struct btrfs_fs_
goto out;

+ * Unlock the qgroup_ioctl_lock mutex before waiting for the rescan worker to
+ * complete. Otherwise we can deadlock because btrfs_remove_qgroup() needs
+ * to lock that mutex while holding a transaction handle and the rescan
+ * worker needs to commit a transaction.
+ */
+ mutex_unlock(&fs_info->qgroup_ioctl_lock);
+ /*
* Request qgroup rescan worker to complete and wait for it. This wait
* must be done before transaction start for quota disable since it may
* deadlock with transaction by the qgroup rescan worker.
clear_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags);
btrfs_qgroup_wait_for_completion(fs_info, false);
- mutex_unlock(&fs_info->qgroup_ioctl_lock);

* 1 For the root item

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