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

[Jens Axboe <axboe@kernel.dk>]

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@gmail.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?

-- 
Jens Axboe