Re: Waiting for requests completions from multiple threads

[Jens Axboe <axboe@kernel.dk>]

On 1/21/20 8:09 PM, Dmitry Sychov wrote:
> Thank you for quick reply! Yes I understand that I need a sort of
> serializable-level isolation
> when accessing the rings - I hope this could be done with a simple
> atomic cmp-add after optimistic write ring update.

That's not a bad idea, that could definitely work, and would be more
efficient than just grabbing a lock.

Could also be made to work quite nicely with restartable sequences. I'd
love to see liburing grow support for smarter sharing of a ring, that's
really where that belongs.

> Correct me if I'am wrong, but from my understanding the kernel can
> start to pick up newly written Uring jobs
> without waiting for the "io_uring_enter" user level call and that's
> why we need a write barrier(so that
> the ring state is always valid for the kernel), else "io_uring_enter"
> could serve as a write barrier itself as well...

By uring jobs, you mean SQEs, or submission queue entries? The kernel
only picks up what you ask it to, it won't randomly just grab entries
from the SQ ring unless you do an io_uring_enter() and tell it to
consume N entries. The exception is if you setup the ring with
IORING_SETUP_SQPOLL, in which case the kernel will maintain a submission
thread. For that case, yes, the kernel can pickup an entry as soon as
the SQ tail is updated by the application.

-- 
Jens Axboe