On 11/6/2019 12:06 PM, Pavel Begunkov wrote:
> On 11/6/2019 11:36 AM, Bob Liu wrote:
>> On 11/6/19 5:22 AM, Pavel Begunkov wrote:
>>> After a call to io_submit_sqe(), it's already known whether it needs
>>> to queue a link or not. Do it there, as it's simplier and doesn't keep
>>> an extra variable across the loop.
>>>
>>> Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
>>> ---
>>> fs/io_uring.c | 22 ++++++++++------------
>>> 1 file changed, 10 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index ebe2a4edd644..82c2da99cb5c 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -2687,7 +2687,6 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
>>> struct io_submit_state state, *statep = NULL;
>>> struct io_kiocb *link = NULL;
>>> struct io_kiocb *shadow_req = NULL;
>>> - bool prev_was_link = false;
>>> int i, submitted = 0;
>>> bool mm_fault = false;
>>>
>>> @@ -2710,17 +2709,6 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
>>> }
>>> }
>>>
>>> - /*
>>> - * If previous wasn't linked and we have a linked command,
>>> - * that's the end of the chain. Submit the previous link.
>>> - */
>>> - if (!prev_was_link && link) {
>>> - io_queue_link_head(ctx, link, &link->submit, shadow_req);
>>> - link = NULL;
>>> - shadow_req = NULL;
>>> - }
>>> - prev_was_link = (s.sqe->flags & IOSQE_IO_LINK) != 0;
>>> -
>>> if (link && (s.sqe->flags & IOSQE_IO_DRAIN)) {
>>> if (!shadow_req) {
>>> shadow_req = io_get_req(ctx, NULL);
>>> @@ -2741,6 +2729,16 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
>>> trace_io_uring_submit_sqe(ctx, s.sqe->user_data, true, async);
>>> io_submit_sqe(ctx, &s, statep, &link);
>>> submitted++;
>>> +
>>> + /*
>>> + * If previous wasn't linked and we have a linked command,
>>> + * that's the end of the chain. Submit the previous link.
>>> + */
>>> + if (!(s.sqe->flags & IOSQE_IO_LINK) && link)
>> The behavior changed to 'current seq' instead of previous after dropping prev_was_link?
>>
> The old behaviour was to remember @prev_was_link for current sqe, and
> use at the beginning of the next iteration, where it becomes
> "previous/last sqe". See, prev_was_link was set after io_queue_link_head.
>
> If @i is iteration idx, then timeline was:
> i: sqe[i-1].is_link -> io_queue_link_head() # if (prev_was_link)
> i: sqe[i].is_link = prev_was_link = (sqe[i].flags & LINK)
> i+1: sqe[i].is_link -> io_queue_link_head() # if (prev_was_link)
> i+1: sqe[i+1].is_link = ...
>
>
> After the change, it's done at the same loop iteration by swapping order
> of checking @prev_was_link and io_queue_link_head().
>
> i: sqe[i].is_link = ...
> i: sqe[i].is_link -> io_queue_link_head()
> i+1: sqe[i+1].is_link = ...
> i+1: sqe[i+1].is_link -> io_queue_link_head()
>
> Shouldn't change the behavior, if I'm not missing something.
>
And the same goes for ordering with io_submit_sqe(), which assembles a link.
i: prev_was_link = ... # for sqe[i]
i: io_submit_sqe() # for sqe[i]
i+1: prev_was_link -> io_queue_link_head # for sqe[i]
after:
i: io_submit_sqe() # for sqe[i]
i: is_link = ... # for sqe[i]
i: is_link -> io_queue_link_head # for sqe[i]
>
>>> + io_queue_link_head(ctx, link, &link->submit, shadow_req);
>>> + link = NULL;
>>> + shadow_req = NULL;
>>> + }
>>> }
>>>
>>> if (link)
>>>
>>
