Re: [PATCH wq/for-6.9] workqueue: Fix pwq->nr_in_flight corruption in try_to_grab_pending()

[Chandan Babu R <chandanbabu@kernel.org>]

On Sun, Feb 04, 2024 at 11:14:21 AM -1000, Tejun Heo wrote:
> dd6c3c544126 ("workqueue: Move pwq_dec_nr_in_flight() to the end of work
> item handling") relocated pwq_dec_nr_in_flight() after
> set_work_pool_and_keep_pending(). However, the latter destroys information
> contained in work->data that's needed by pwq_dec_nr_in_flight() including
> the flush color. With flush color destroyed, flush_workqueue() can stall
> easily when mixed with cancel_work*() usages.
>
> This is easily triggered by running xfstests generic/001 test on xfs:
>
>      INFO: task umount:6305 blocked for more than 122 seconds.
>      ...
>      task:umount          state:D stack:13008 pid:6305  tgid:6305  ppid:6301   flags:0x00004000
>      Call Trace:
>       <TASK>
>       __schedule+0x2f6/0xa20
>       schedule+0x36/0xb0
>       schedule_timeout+0x20b/0x280
>       wait_for_completion+0x8a/0x140
>       __flush_workqueue+0x11a/0x3b0
>       xfs_inodegc_flush+0x24/0xf0
>       xfs_unmountfs+0x14/0x180
>       xfs_fs_put_super+0x3d/0x90
>       generic_shutdown_super+0x7c/0x160
>       kill_block_super+0x1b/0x40
>       xfs_kill_sb+0x12/0x30
>       deactivate_locked_super+0x35/0x90
>       deactivate_super+0x42/0x50
>       cleanup_mnt+0x109/0x170
>       __cleanup_mnt+0x12/0x20
>       task_work_run+0x60/0x90
>       syscall_exit_to_user_mode+0x146/0x150
>       do_syscall_64+0x5d/0x110
>       entry_SYSCALL_64_after_hwframe+0x6c/0x74
>
> Fix it by stashing work_data before calling set_work_pool_and_keep_pending()
> and using the stashed value for pwq_dec_nr_in_flight().
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reported-by: Chandan Babu R <chandanbabu@kernel.org>
> Link: http://lkml.kernel.org/r/87o7cxeehy.fsf@debian-BULLSEYE-live-builder-AMD64
> Fixes: dd6c3c544126 ("workqueue: Move pwq_dec_nr_in_flight() to the end of work item handling")
> ---
> Hello, Chandan.
>
> Thanks a lot for the report. I could reproduce the problem and verified that
> this patch fixes the issue. I'm applying this to wq/for-6.9 but would really
> appreciate if you could confirm the fix.

fstests executed without any regressions on a next-20240202 kernel with this
patch applied.

-- 
Chandan