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

[Jens Axboe <axboe@kernel.dk>]

On 2/25/20 2:25 PM, Jens Axboe wrote:
> 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.

Because I'm a clown, the patch applied with offset. I meant to modify
the __io_queue_sqe() path, as mentioned in the commit message. Here's
the right one, dropped the check

The other one would not be correct without the nxt check, as the work
queue handler could pass it back. For the __io_queue_sqe() path, we
should do a 5.7 cleanup of the 'nxt passing, though. I don't want to
do that for 5.6.

commit 7df2fa5c9f6e92b2f80f4699425b463973d5242b
Author: Jens Axboe <axboe@kernel.dk>
Date:   Tue Feb 25 13:25:41 2020 -0700

    io_uring: pick up link work on submit reference drop
    
    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..c4ed8601e225 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -4749,7 +4749,7 @@ static void __io_queue_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 
 err:
 	/* drop submission reference */
-	io_put_req(req);
+	io_put_req_find_next(req, &nxt);
 
 	if (linked_timeout) {
 		if (!ret)


-- 
Jens Axboe