linux-kernel - Re: [PATCH 3/3] io_uring: move *queue_link

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Date:   Tue, 17 Dec 2019 11:01:18 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] io_uring: move *queue_link_head() from common path

On 12/17/19 10:52 AM, Pavel Begunkov wrote:
> On 17/12/2019 20:37, Jens Axboe wrote:
>> On 12/17/19 9:45 AM, Jens Axboe wrote:
>>> On 12/16/19 4:38 PM, Pavel Begunkov wrote:
>>>> On 17/12/2019 02:22, Pavel Begunkov wrote:
>>>>> Move io_queue_link_head() to links handling code in io_submit_sqe(),
>>>>> so it wouldn't need extra checks and would have better data locality.
>>>>>
>>>>> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
>>>>> ---
>>>>>  fs/io_uring.c | 32 ++++++++++++++------------------
>>>>>  1 file changed, 14 insertions(+), 18 deletions(-)
>>>>>
>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>>> index bac9e711e38d..a880ed1409cb 100644
>>>>> --- a/fs/io_uring.c
>>>>> +++ b/fs/io_uring.c
>>>>> @@ -3373,13 +3373,15 @@ static bool io_submit_sqe(struct io_kiocb *req, struct io_submit_state *state,
>>>>>  			  struct io_kiocb **link)
>>>>>  {
>>>>>  	struct io_ring_ctx *ctx = req->ctx;
>>>>> +	unsigned int sqe_flags;
>>>>>  	int ret;
>>>>>  
>>>>> +	sqe_flags = READ_ONCE(req->sqe->flags);
>>>>>  	req->user_data = READ_ONCE(req->sqe->user_data);
>>>>>  	trace_io_uring_submit_sqe(ctx, req->user_data, true, req->in_async);
>>>>>  
>>>>>  	/* enforce forwards compatibility on users */
>>>>> -	if (unlikely(req->sqe->flags & ~SQE_VALID_FLAGS)) {
>>>>> +	if (unlikely(sqe_flags & ~SQE_VALID_FLAGS)) {
>>>>>  		ret = -EINVAL;
>>>>>  		goto err_req;
>>>>>  	}
>>>>> @@ -3402,10 +3404,10 @@ static bool io_submit_sqe(struct io_kiocb *req, struct io_submit_state *state,
>>>>>  	if (*link) {
>>>>>  		struct io_kiocb *head = *link;
>>>>>  
>>>>> -		if (req->sqe->flags & IOSQE_IO_DRAIN)
>>>>> +		if (sqe_flags & IOSQE_IO_DRAIN)
>>>>>  			head->flags |= REQ_F_DRAIN_LINK | REQ_F_IO_DRAIN;
>>>>>  
>>>>> -		if (req->sqe->flags & IOSQE_IO_HARDLINK)
>>>>> +		if (sqe_flags & IOSQE_IO_HARDLINK)
>>>>>  			req->flags |= REQ_F_HARDLINK;
>>>>>  
>>>>>  		if (io_alloc_async_ctx(req)) {
>>>>> @@ -3421,9 +3423,15 @@ static bool io_submit_sqe(struct io_kiocb *req, struct io_submit_state *state,
>>>>>  		}
>>>>>  		trace_io_uring_link(ctx, req, head);
>>>>>  		list_add_tail(&req->link_list, &head->link_list);
>>>>> -	} else if (req->sqe->flags & (IOSQE_IO_LINK|IOSQE_IO_HARDLINK)) {
>>>>> +
>>>>> +		/* last request of a link, enqueue the link */
>>>>> +		if (!(sqe_flags & IOSQE_IO_LINK)) {
>>>>
>>>> This looks suspicious (as well as in the current revision). Returning back
>>>> to my questions a few days ago can sqe->flags have IOSQE_IO_HARDLINK, but not
>>>> IOSQE_IO_LINK? I don't find any check.
>>>>
>>>> In other words, should it be as follows?
>>>> !(sqe_flags & (IOSQE_IO_LINK|IOSQE_IO_HARDLINK))
>>>
>>> Yeah, I think that should check for both. I'm fine with either approach
>>> in general:
>>>
>>> - IOSQE_IO_HARDLINK must have IOSQE_IO_LINK set
>>>
>>> or
>>>
>>> - IOSQE_IO_HARDLINK implies IOSQE_IO_LINK
>>>
>>> Seems like the former is easier to verify in terms of functionality,
>>> since we can rest easy if we check this early and -EINVAL if that isn't
>>> the case.
>>>
>>> What do you think?
>>
>> If you agree, want to send in a patch for that for 5.5? Then I can respin
>> for-5.6/io_uring on top of that, and we can apply your cleanups there.
>>
> Yes, that's the idea. Already got a patch, if you haven't done it yet.

I haven't.

> Just was thinking, whether to add a check for not setting both flags
> at the same moment in the "imply" case. Would give us 1 state in 2 bits
> for future use.

Not sure I follow what you're saying here, can you elaborate?


-- 
Jens Axboe