On 7/18/19 6:54 PM, Zhengyuan Liu wrote:
>
> On 7/19/19 12:43 AM, Jens Axboe wrote:
>> 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!
>>
>
> Hi, Jens.
> I have tested you fix and the issue still existed. Actually the
> implementation of atomic_dec_return has been implicitly surrounded
> already by mb() and as I know, smp_mb__after/before_atomic are not
> suitable for atomic_t operation which does not return a value.
We aren't guaranteed to see the atomic_dec_return() update if it happens
at the same time. So we can either force ordering with the smp_mb(), or
we can do something ala:
if (!atomic_sub_return(0, &list->cnt)) {
...
io_add_to_prev_work() to achieve the same sort of effect. That should
work as well.
--
Jens Axboe
