Re: [PATCH 3/3] io_uring: move *queue_link_head() from common path

[Pavel Begunkov]

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@xxxxxxxxx>
>>>> ---
>>>>  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.

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.

-- 
Pavel Begunkov

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