btrfs: qgroup: fix deadlock between rescan worker and remove qgroup
The commit e804861b ("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} do btrfs quota enable /mnt & btrfs qgroup create 1/0 /mnt & btrfs qgroup destroy 1/0 /mnt & btrfs quota disable /mnt & done 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: e804861b ("btrfs: fix deadlock between quota disable and qgroup rescan worker") CC: stable@vger.kernel.org # 5.4+ Reviewed-by:Filipe Manana <fdmanana@suse.com> Reviewed-by:
Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com> Signed-off-by:
Sidong Yang <realwakka@gmail.com> Reviewed-by:
David Sterba <dsterba@suse.com> Signed-off-by:
David Sterba <dsterba@suse.com>
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