Re: [PATCH 1/1] io_uring: fix infinite wait in khread_park() on io_finish_async()

[Jens Axboe <axboe@kernel.dk>]

On 5/13/19 12:20 PM, Roman Penyaev wrote:
> This fixes couple of races which lead to infinite wait of park completion
> with the following backtraces:
> 
>   [20801.303319] Call Trace:
>   [20801.303321]  ? __schedule+0x284/0x650
>   [20801.303323]  schedule+0x33/0xc0
>   [20801.303324]  schedule_timeout+0x1bc/0x210
>   [20801.303326]  ? schedule+0x3d/0xc0
>   [20801.303327]  ? schedule_timeout+0x1bc/0x210
>   [20801.303329]  ? preempt_count_add+0x79/0xb0
>   [20801.303330]  wait_for_completion+0xa5/0x120
>   [20801.303331]  ? wake_up_q+0x70/0x70
>   [20801.303333]  kthread_park+0x48/0x80
>   [20801.303335]  io_finish_async+0x2c/0x70
>   [20801.303336]  io_ring_ctx_wait_and_kill+0x95/0x180
>   [20801.303338]  io_uring_release+0x1c/0x20
>   [20801.303339]  __fput+0xad/0x210
>   [20801.303341]  task_work_run+0x8f/0xb0
>   [20801.303342]  exit_to_usermode_loop+0xa0/0xb0
>   [20801.303343]  do_syscall_64+0xe0/0x100
>   [20801.303349]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
>   [20801.303380] Call Trace:
>   [20801.303383]  ? __schedule+0x284/0x650
>   [20801.303384]  schedule+0x33/0xc0
>   [20801.303386]  io_sq_thread+0x38a/0x410
>   [20801.303388]  ? __switch_to_asm+0x40/0x70
>   [20801.303390]  ? wait_woken+0x80/0x80
>   [20801.303392]  ? _raw_spin_lock_irqsave+0x17/0x40
>   [20801.303394]  ? io_submit_sqes+0x120/0x120
>   [20801.303395]  kthread+0x112/0x130
>   [20801.303396]  ? kthread_create_on_node+0x60/0x60
>   [20801.303398]  ret_from_fork+0x35/0x40
> 
>  o kthread_park() waits for park completion, so io_sq_thread() loop
>    should check kthread_should_park() along with khread_should_stop(),
>    otherwise if kthread_park() is called before prepare_to_wait()
>    the following schedule() never returns.
> 
>  o if the flag ctx->sqo_stop is observed by the io_sq_thread() loop
>    it is quite possible, that kthread_should_park() check and the
>    following kthread_parkme() is never called, because kthread_park()
>    has not been yet called, but few moments later is is called and
>    waits there for park completion, which never happens, because
>    kthread has already exited.
> 
> It seems that parking here is not needed at all (thread is parked and
> then stopped and never unparked), so here in this patch I simply rely
> on kthread_should_stop() check and then exit the thread.

The park is a bit of a work-around, without it we get warning spews on
shutdown with SQPOLL set and affinity set to a single CPU.

Also see:

commit 06058632464845abb1af91521122fd04dd3daaec
Author: Jens Axboe <axboe@kernel.dk>
Date:   Sat Apr 13 09:26:03 2019 -0600

    io_uring: park SQPOLL thread if it's percpu

-- 
Jens Axboe