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

[Jens Axboe]

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.

-- 
Jens Axboe