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btrfs: qgroup: ensure qgroup_rescan_running is only set when the worker is at least queued 

[BUG]
There are some reports about btrfs wait forever to unmount itself, with
the following call trace:

  INFO: task umount:4631 blocked for more than 491 seconds.
        Tainted: G               X  5.3.8-2-default #1
  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  umount          D    0  4631   3337 0x00000000
  Call Trace:
  ([<00000000174adf7a>] __schedule+0x342/0x748)
   [<00000000174ae3ca>] schedule+0x4a/0xd8
   [<00000000174b1f08>] schedule_timeout+0x218/0x420
   [<00000000174af10c>] wait_for_common+0x104/0x1d8
   [<000003ff804d6994>] btrfs_qgroup_wait_for_completion+0x84/0xb0 [btrfs]
   [<000003ff8044a616>] close_ctree+0x4e/0x380 [btrfs]
   [<0000000016fa3136>] generic_shutdown_super+0x8e/0x158
   [<0000000016fa34d6>] kill_anon_super+0x26/0x40
   [<000003ff8041ba88>] btrfs_kill_super+0x28/0xc8 [btrfs]
   [<0000000016fa39f8>] deactivate_locked_super+0x68/0x98
   [<0000000016fcb198>] cleanup_mnt+0xc0/0x140
   [<0000000016d6a846>] task_work_run+0xc6/0x110
   [<0000000016d04f76>] do_notify_resume+0xae/0xb8
   [<00000000174b30ae>] system_call+0xe2/0x2c8

[CAUSE]
The problem happens when we have called qgroup_rescan_init(), but
not queued the worker. It can be caused mostly by error handling.

	Qgroup ioctl thread		|	Unmount thread
----------------------------------------+-----------------------------------
					|
btrfs_qgroup_rescan()			|
|- qgroup_rescan_init()			|
|  |- qgroup_rescan_running = true;	|
|					|
|- trans = btrfs_join_transaction()	|
|  Some error happened			|
|					|
|- btrfs_qgroup_rescan() returns error	|
   But qgroup_rescan_running == true;	|
					| close_ctree()
					| |- btrfs_qgroup_wait_for_completion()
					|    |- running == true;
					|    |- wait_for_completion();

btrfs_qgroup_rescan_worker is never queued, thus no one is going to wake
up close_ctree() and we get a deadlock.

All involved qgroup_rescan_init() callers are:

- btrfs_qgroup_rescan()
  The example above. It's possible to trigger the deadlock when error
  happened.

- btrfs_quota_enable()
  Not possible. Just after qgroup_rescan_init() we queue the work.

- btrfs_read_qgroup_config()
  It's possible to trigger the deadlock. It only init the work, the
  work queueing happens in btrfs_qgroup_rescan_resume().
  Thus if error happened in between, deadlock is possible.

We shouldn't set fs_info->qgroup_rescan_running just in
qgroup_rescan_init(), as at that stage we haven't yet queued qgroup
rescan worker to run.

[FIX]
Set qgroup_rescan_running before queueing the work, so that we ensure
the rescan work is queued when we wait for it.

Fixes: 8d9eddad19 ("Btrfs: fix qgroup rescan worker initialization")
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
[ Change subject and cause analyse, use a smaller fix ]
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Qu Wenruo 2020-02-07 13:38:20 +08:00 committed by David Sterba
parent 86eba9d34c
commit d61acbbf54
1 changed files with 9 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
fs/btrfs/qgroup.c
View File

@ -1030,6 +1030,7 @@ out_add_root:
ret = qgroup_rescan_init(fs_info, 0, 1);
if (!ret) {
qgroup_rescan_zero_tracking(fs_info);
fs_info->qgroup_rescan_running = true;
btrfs_queue_work(fs_info->qgroup_rescan_workers,
&fs_info->qgroup_rescan_work);
}
@ -3263,7 +3264,6 @@ qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
sizeof(fs_info->qgroup_rescan_progress));
fs_info->qgroup_rescan_progress.objectid = progress_objectid;
init_completion(&fs_info->qgroup_rescan_completion);
fs_info->qgroup_rescan_running = true;
spin_unlock(&fs_info->qgroup_lock);
mutex_unlock(&fs_info->qgroup_rescan_lock);
@ -3326,8 +3326,11 @@ btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info)
qgroup_rescan_zero_tracking(fs_info);
mutex_lock(&fs_info->qgroup_rescan_lock);
fs_info->qgroup_rescan_running = true;
btrfs_queue_work(fs_info->qgroup_rescan_workers,
&fs_info->qgroup_rescan_work);
mutex_unlock(&fs_info->qgroup_rescan_lock);
return 0;
}
@ -3363,9 +3366,13 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
void
btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
{
if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN) {
mutex_lock(&fs_info->qgroup_rescan_lock);
fs_info->qgroup_rescan_running = true;
btrfs_queue_work(fs_info->qgroup_rescan_workers,
&fs_info->qgroup_rescan_work);
mutex_unlock(&fs_info->qgroup_rescan_lock);
}
}
/*