Re: [External] Re: [PATCH v3 2/3] io_uring: avoid ring quiesce while registering/unregistering eventfd

[Pavel Begunkov]

On 2/3/22 19:54, Usama Arif wrote:
> On 03/02/2022 19:06, Jens Axboe wrote:
> > On 2/3/22 12:00 PM, Pavel Begunkov wrote:
> > > On 2/3/22 18:29, Jens Axboe wrote:
> > > > On 2/3/22 11:26 AM, Usama Arif wrote:
> > > > > Hmm, maybe i didn't understand you and Pavel correctly. Are you
> > > > > suggesting to do the below diff over patch 3? I dont think that would be
> > > > > correct, as it is possible that just after checking if ctx->io_ev_fd is
> > > > > present unregister can be called by another thread and set ctx->io_ev_fd
> > > > > to NULL that would cause a NULL pointer exception later? In the current
> > > > > patch, the check of whether ev_fd exists happens as the first thing
> > > > > after rcu_read_lock and the rcu_read_lock are extremely cheap i believe.
> > > >
> > > > They are cheap, but they are still noticeable at high requests/sec
> > > > rates. So would be best to avoid them.
> > > >
> > > > And yes it's obviously racy, there's the potential to miss an eventfd
> > > > notification if it races with registering an eventfd descriptor. But
> > > > that's not really a concern, as if you register with inflight IO
> > > > pending, then that always exists just depending on timing. The only
> > > > thing I care about here is that it's always _safe_. Hence something ala
> > > > what you did below is totally fine, as we're re-evaluating under rcu
> > > > protection.
> > >
> > > Indeed, the patch doesn't have any formal guarantees for propagation
> > > to already inflight requests, so this extra unsynchronised check
> > > doesn't change anything.
> > >
> > > I'm still more сurious why we need RCU and extra complexity when
> > > apparently there is no use case for that. If it's only about
> > > initial initialisation, then as I described there is a much
> > > simpler approach.
> >
> > Would be nice if we could get rid of the quiesce code in general, but I
> > haven't done a check to see what'd be missing after this...
>
> I had checked! I had posted below in in reply to v1 (https://lore.kernel.org/io-uring/02fb0bc3-fc38-b8f0-3067-edd2a525ef29@xxxxxxxxx/T/#m5ac7867ac61d86fe62c099be793ffe5a9a334976), but i think it got missed! Copy-pasting here for reference:
>
> "
> I see that if we remove ring quiesce from the the above 3 opcodes, then
> only IORING_REGISTER_ENABLE_RINGS and IORING_REGISTER_RESTRICTIONS is
> left for ring quiesce. I just had a quick look at those, and from what i
> see we might not need to enter ring quiesce in
> IORING_REGISTER_ENABLE_RINGS as the ring is already disabled at that point?
> And for IORING_REGISTER_RESTRICTIONS if we do a similar approach to

IORING_REGISTER_RESTRICTIONS and IORING_REGISTER_ENABLE_RINGS are simpler,
we can just remove quiesce (i.e. put them into io_register_op_must_quiesce())
without any extra changes.

TL;DR;
That's because IORING_SETUP_R_DISABLED prevents submitting requests
and so there will be no requests until IORING_REGISTER_ENABLE_RINGS is
called. And IORING_REGISTER_RESTRICTIONS works only before
IORING_REGISTER_ENABLE_RINGS was called.


--
Pavel Begunkov