On 7/18/19 9:41 AM, Jens Axboe wrote:
> On 7/18/19 6:44 AM, Zhengyuan Liu wrote:
>> There is a hang issue while using fio to do some basic test. The issue can
>> been easily reproduced using bellow scripts:
>>
>> while true
>> do
>> fio --ioengine=io_uring -rw=write -bs=4k -numjobs=1 \
>> -size=1G -iodepth=64 -name=uring --filename=/dev/zero
>> done
>>
>> After serveral minutes, maybe more, fio would block at
>> io_uring_enter->io_cqring_wait in order to waiting for previously committed
>> sqes to be completed and cann't return to user anymore until we send a SIGTERM
>> to fio. After got SIGTERM, fio turns to hang at io_ring_ctx_wait_and_kill with
>> a backtrace like this:
>>
>> [54133.243816] Call Trace:
>> [54133.243842] __schedule+0x3a0/0x790
>> [54133.243868] schedule+0x38/0xa0
>> [54133.243880] schedule_timeout+0x218/0x3b0
>> [54133.243891] ? sched_clock+0x9/0x10
>> [54133.243903] ? wait_for_completion+0xa3/0x130
>> [54133.243916] ? _raw_spin_unlock_irq+0x2c/0x40
>> [54133.243930] ? trace_hardirqs_on+0x3f/0xe0
>> [54133.243951] wait_for_completion+0xab/0x130
>> [54133.243962] ? wake_up_q+0x70/0x70
>> [54133.243984] io_ring_ctx_wait_and_kill+0xa0/0x1d0
>> [54133.243998] io_uring_release+0x20/0x30
>> [54133.244008] __fput+0xcf/0x270
>> [54133.244029] ____fput+0xe/0x10
>> [54133.244040] task_work_run+0x7f/0xa0
>> [54133.244056] do_exit+0x305/0xc40
>> [54133.244067] ? get_signal+0x13b/0xbd0
>> [54133.244088] do_group_exit+0x50/0xd0
>> [54133.244103] get_signal+0x18d/0xbd0
>> [54133.244112] ? _raw_spin_unlock_irqrestore+0x36/0x60
>> [54133.244142] do_signal+0x34/0x720
>> [54133.244171] ? exit_to_usermode_loop+0x7e/0x130
>> [54133.244190] exit_to_usermode_loop+0xc0/0x130
>> [54133.244209] do_syscall_64+0x16b/0x1d0
>> [54133.244221] entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> The reason is that we had added a req to ctx->pending_async at the very end, but
>> it got no chance to be processed anymore. How could this be happened?
>>
>> fio#cpu0 wq#cpu1
>>
>> io_add_to_prev_work io_sq_wq_submit_work
>>
>> atomic_read() <<< 1
>>
>> atomic_dec_return() << 1->0
>> list_empty(); <<< true;
>>
>> list_add_tail()
>> atomic_read() << 0 or 1?
>>
>> As was said in atomic_ops.rst, atomic_read does not guarantee that the runtime
>> initialization by any other thread is visible yet, so we must take care of that
>> with a proper implicit or explicit memory barrier;
>
> Thanks for looking at this and finding this issue, it does looks like a problem.
> But I'm not sure about the fix. Shouldn't we just need an smp_mb__after_atomic()
> on the atomic_dec_return() side of things? Like the below.
>
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 5ec06e5ba0be..3c2a6f88a6b0 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1881,6 +1881,7 @@ static void io_sq_wq_submit_work(struct work_struct *work)
> */
> if (async_list) {
> ret = atomic_dec_return(&async_list->cnt);
> + smp_mb__after_atomic();
> while (!ret && !list_empty(&async_list->list)) {
> spin_lock(&async_list->lock);
> atomic_inc(&async_list->cnt);
> @@ -1894,6 +1895,7 @@ static void io_sq_wq_submit_work(struct work_struct *work)
> goto restart;
> }
> ret = atomic_dec_return(&async_list->cnt);
> + smp_mb__after_atomic();
> }
> }
>
>
I don't think this is enough, I actually think your fix is the most
appropriate. I will apply it, thank you!
--
Jens Axboe
