Re: [PATCH] io_uring: pick up link work on submit reference drop

[Jens Axboe <axboe@kernel.dk>]

On 2/25/20 2:22 PM, Pavel Begunkov wrote:
> On 25/02/2020 23:27, Jens Axboe wrote:
>> If work completes inline, then we should pick up a dependent link item
>> in __io_queue_sqe() as well. If we don't do so, we're forced to go async
>> with that item, which is suboptimal.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>
>> ---
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index ffd9bfa84d86..160cf1b0f478 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -4531,8 +4531,15 @@ static void io_wq_submit_work(struct io_wq_work **workptr)
>>  		} while (1);
>>  	}
>>  
>> -	/* drop submission reference */
>> -	io_put_req(req);
>> +	/*
>> +	 * Drop submission reference. In case the handler already dropped the
>> +	 * completion reference, then it didn't pick up any potential link
>> +	 * work. If 'nxt' isn't set, try and do that here.
>> +	 */
>> +	if (nxt)
> 
> It can't even get here, because of the submission ref, isn't it? would the
> following do?
> 
> -	io_put_req(req);
> +	io_put_req_find_next(req, &nxt);

I don't think it can, let me make that change. And test.

> BTW, as I mentioned before, it appears to me, we don't even need completion ref
> as it always pinned by the submission ref. I'll resurrect the patches doing
> that, but after your poll work will land.

We absolutely do need two references, unfortunately. Otherwise we could complete
the io_kiocb deep down the stack through the callback.

-- 
Jens Axboe