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

[Pavel Begunkov]

On 17/12/2019 21:01, Jens Axboe wrote:
> 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@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.
> 
> 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?
> 

Sure

#define IOSQE_IO_LINK		(1U << 2)	/* links next sqe */
#define IOSQE_IO_HARDLINK	(1U << 3)	/* like LINK, but stronger */

That's 2 consequent bits, so 4 states:
0,0 -> not a link
1,0 -> common link
0,1 -> hard link
1,1 -> reserved, space for another link-quirk type

But that would require additional check, i.e.

if (flags&(LINK|HARDLINK) == (LINK|HARDLINK)) ...


-- 
Pavel Begunkov

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