On 8/20/21 3:32 PM, Pavel Begunkov wrote:
> On 8/20/21 9:39 PM, Hao Xu wrote:
>> 在 2021/8/21 上午2:59, Pavel Begunkov 写道:
>>> On 8/20/21 7:40 PM, Hao Xu wrote:
>>>> coml_nr in ctx_flush_and_put() is not protected by uring_lock, this
>>>> may cause problems when accessing it parallelly.
>>>
>>> Did you hit any problem? It sounds like it should be fine as is:
>>>
>>> The trick is that it's only responsible to flush requests added
>>> during execution of current call to tctx_task_work(), and those
>>> naturally synchronised with the current task. All other potentially
>>> enqueued requests will be of someone else's responsibility.
>>>
>>> So, if nobody flushed requests, we're finely in-sync. If we see
>>> 0 there, but actually enqueued a request, it means someone
>>> actually flushed it after the request had been added.
>>>
>>> Probably, needs a more formal explanation with happens-before
>>> and so.
>> I should put more detail in the commit message, the thing is:
>> say coml_nr > 0
>>
>> ctx_flush_and put other context
>> if (compl_nr) get mutex
>> coml_nr > 0
>> do flush
>> coml_nr = 0
>> release mutex
>> get mutex
>> do flush (*)
>> release mutex
>>
>> in (*) place, we do a bunch of unnecessary works, moreover, we
>
> I wouldn't care about overhead, that shouldn't be much
>
>> call io_cqring_ev_posted() which I think we shouldn't.
>
> IMHO, users should expect spurious io_cqring_ev_posted(),
> though there were some eventfd users complaining before, so
> for them we can do
It does sometimes cause issues, see:
commit b18032bb0a883cd7edd22a7fe6c57e1059b81ed0
Author: Jens Axboe <axboe@xxxxxxxxx>
Date: Sun Jan 24 16:58:56 2021 -0700
io_uring: only call io_cqring_ev_posted() if events were posted
I would tend to agree with Hao here, and the usual optimization idiom
looks like:
if (struct->nr) {
mutex_lock(&struct->lock);
if (struct->nr)
do_something();
mutex_unlock(&struct->lock);
}
like you posted, which would be fine and avoid this whole discussion :-)
Hao, care to spin a patch that does that?
--
Jens Axboe
