Re: [PATCH for-5.15] io_uring: fix lacking of protection for compl_nr

[Jens Axboe <axboe@kernel.dk>]

On 8/20/21 4:41 PM, Pavel Begunkov wrote:
> On 8/20/21 11:30 PM, Jens Axboe wrote:
>> On 8/20/21 4:28 PM, Pavel Begunkov wrote:
>>> On 8/20/21 11:09 PM, Jens Axboe wrote:
>>>> 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:
>>>
>>> I'm used that locking may end up in spurious wakeups. May be
>>> different for eventfd, but considering that we do batch
>>> completions and so might be calling it only once per multiple
>>> CQEs, it shouldn't be.
>>
>> The wakeups are fine, it's the ev increment that's causing some issues.
> 
> If userspace doesn't expect that eventfd may get diverged from the
> number of posted CQEs, we need something like below. The weird part
> is that it looks nobody complained about this one, even though it
> should be happening pretty often. 

That wasn't the issue we ran into, it was more the fact that eventfd
would indicate that something had been posted, when nothing had.
We don't need eventfd notifications to be == number of posted events,
just if the eventfd notification is inremented, there should be new
events there.

-- 
Jens Axboe