Re: [PATCH liburing 1/2] io_uring_enter: add timeout support

[Jiufei Xue <jiufei.xue@linux.alibaba.com>]

Hi Jens,

On 2020/7/31 上午11:57, Jens Axboe wrote:
> Then why not just make the sqe-less timeout path flush existing requests,
> if it needs to? Seems a lot simpler than adding odd x2 variants, which
> won't really be clear.
> 
Flushing the requests will access and modify the head of submit queue, that
may race with the submit thread. I think the reap thread should not touch
the submit queue when IORING_FEAT_GETEVENTS_TIMEOUT is supported.

> Chances are, if it's called with sq entries pending, the caller likely
> wants those submitted. Either the caller was aware and relying on that
> behavior, or the caller is simply buggy and has a case where it doesn't
> submit IO before waiting for completions.
>

That is not true when the SQ/CQ handling are split in two different threads.
The reaping thread is not aware of the submit queue. It should only wait for
completion of the requests, such as below:

submitting_thread:                   reaping_thread:

io_uring_get_sqe()
io_uring_prep_nop()     
                                 io_uring_wait_cqe_timeout2()
io_uring_submit()
                                 woken if requests are completed or timeout


And if the SQ/CQ handling are in the same thread, applications should use the
old API if they do not want to submit the request themselves.

io_uring_get_sqe
io_uring_prep_nop
io_uring_wait_cqe_timeout

> Hence I really don't think that's a big deal, and even arguably the right
> thing to do.