[PATCH AUTOSEL 5.6 65/68] btrfs: qgroup: ensure qgroup_rescan_running is only set when the worker is at least queued

Sasha Levin
2020-04-10T04:00:37Z

From: Qu Wenruo <wqu@suse.com>

[ Upstream commit d61acbbf54c612ea9bf67eed609494cda0857b3a ]

[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: 8d9eddad1946 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/btrfs/qgroup.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index ff1870ff3474a..afc9752e984c3 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -1030,6 +1030,7 @@ int btrfs_quota_enable(struct btrfs_fs_info *fs_info)
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);
+ }
}

/*
--
2.20.1