Re: Question about patch "io_uring: add submission polling"

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

hi,

> On 10/16/20 2:55 AM, Xiaoguang Wang wrote:
>> hi，
>>
>> I have questions about below code comments, which was included in patch
>> "io_uring: add submission polling",
>> ------------------------------------------------------------
>>       /*
>>        * Drop cur_mm before scheduling, we can't hold it for
>>        * long periods (or over schedule()). Do this before
>>        * adding ourselves to the waitqueue, as the unuse/drop
>>        * may sleep.
>>        */
>>       if (cur_mm) {
>>               unuse_mm(cur_mm);
>>               mmput(cur_mm);
>>               cur_mm = NULL;
>>       }
>>
>>       prepare_to_wait(&ctx->sqo_wait, &wait, TASK_INTERRUPTIBLE);
>> -------------------------------------------------------------
>> Stefano submited a patch "io_uring: prevent sq_thread from spinning when it should stop",
>> I understand what issue Stefano fixed, but don't understand below comments.
>>
>> Can anyone help to explain why we need to rop cur_mm before scheduling, or
>> why we can't hold it for long periods (or over schedule()), and if we
>> unuse/drop mm after adding ourselves to the waitqueue, what issue will
>> happen when unuse/drop sleeps, thanks.
> 
> The not holding it too long it just trying to be nice. But we can't drop
> it after we've done prepare_to_wait(), as that sets our task runstate to
> a non-running state. This conflicts with with mmput(), which might
> sleep.
I see, thanks.
Now I'm trying to improve io_sq_thread a bit and will send patches soon.

Regards,
Xiaoguang Wang
>