Re: [PATCH 1/3] io_uring: move to using private ring references

[Pavel Begunkov <asml.silence@gmail.com>]

On 8/11/23 18:12, Jens Axboe wrote:
> io_uring currently uses percpu refcounts for the ring reference. This
> works fine, but exiting a ring requires an RCU grace period to lapse
> and this slows down ring exit quite a lot.
> 
> Add a basic per-cpu counter for our references instead, and use that.
> This is in preparation for doing a sync wait on on any request (notably
> file) references on ring exit. As we're going to be waiting on ctx refs
> going away as well with that, the RCU grace period wait becomes a
> noticeable slowdown.

How does it work?

- What prevents io_ring_ref_maybe_done() from miscalculating and either
1) firing while there are refs or
2) not triggering when we put down all refs?
E.g. percpu_ref relies on atomic counting after switching from
percpu mode.

- What contexts it can be used from? Task context only? I'll argue we
want to use it in [soft]irq for likes of *task_work_add().

-- 
Pavel Begunkov