From: Roman Penyaev <rpenyaev@suse.de>
To: Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org
Subject: Re: [PATCH 1/1] io_uring: fix infinite wait in khread_park() on io_finish_async()
Date: Mon, 13 May 2019 20:25:35 +0200 [thread overview]
Message-ID: <f3aa208b4fbbd3d1afa8a486ce078b3b@suse.de> (raw)
In-Reply-To: <20190513182028.29912-1-rpenyaev@suse.de>
Hi Jens,
I forgot to mention that dead lock is quite well reproduced
if app is killed, when no IO is performed (i.e. polling thread
is scheduled out).
--
Roman
On 2019-05-13 20:20, 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.
>
> Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: linux-block@vger.kernel.org
> ---
> fs/io_uring.c | 9 +--------
> 1 file changed, 1 insertion(+), 8 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 452e35357865..449c652bb334 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -231,7 +231,6 @@ struct io_ring_ctx {
> struct task_struct *sqo_thread; /* if using sq thread polling */
> struct mm_struct *sqo_mm;
> wait_queue_head_t sqo_wait;
> - unsigned sqo_stop;
>
> struct {
> /* CQ ring */
> @@ -2028,7 +2027,7 @@ static int io_sq_thread(void *data)
> set_fs(USER_DS);
>
> timeout = inflight = 0;
> - while (!kthread_should_stop() && !ctx->sqo_stop) {
> + while (!kthread_should_stop()) {
> bool all_fixed, mm_fault = false;
> int i;
>
> @@ -2140,9 +2139,6 @@ static int io_sq_thread(void *data)
> mmput(cur_mm);
> }
>
> - if (kthread_should_park())
> - kthread_parkme();
> -
> return 0;
> }
>
> @@ -2273,9 +2269,6 @@ static int io_sqe_files_unregister(struct
> io_ring_ctx *ctx)
> static void io_sq_thread_stop(struct io_ring_ctx *ctx)
> {
> if (ctx->sqo_thread) {
> - ctx->sqo_stop = 1;
> - mb();
> - kthread_park(ctx->sqo_thread);
> kthread_stop(ctx->sqo_thread);
> ctx->sqo_thread = NULL;
> }
next prev parent reply other threads:[~2019-05-13 18:25 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-13 18:20 [PATCH 1/1] io_uring: fix infinite wait in khread_park() on io_finish_async() Roman Penyaev
2019-05-13 18:25 ` Roman Penyaev [this message]
2019-05-13 18:52 ` Jens Axboe
2019-05-14 9:17 ` Roman Penyaev
2019-05-14 9:27 ` Roman Penyaev
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