Re: [External] Re: [PATCH 1/2] io_uring: avoid ring quiesce while registering/unregistering eventfd

[Usama Arif]

On 03/02/2022 16:58, Jens Axboe wrote:
> On 2/3/22 9:49 AM, Usama Arif wrote:
> > > One thing that both mine and your version suffers from is if someone
> > > does an eventfd unregister, and then immediately does an eventfd
> > > register. If the rcu grace period hasn't passed, we'll get -EBUSY on
> > > trying to do that, when I think the right behavior there would be to
> > > wait for the grace period to pass.
> > >
> > > I do think we need to handle that gracefully, spurious -EBUSY is
> > > impossible for an application to deal with.
> >
> > I don't think my version would suffer from this as its protected by
> > locks? The mutex_unlock on ev_fd_lock in unregister happens only after
> > the call_rcu. And the mutex is locked in io_eventfd_register at the
> > start, so wouldnt get the -EBUSY if there is a register immediately
> > after unregister?
>
> The call_rcu() just registers it for getting the callback when the grace
> period has passed, it doesn't mean it's done by the time it returns.
> Hence there's definitely a window where you can enter
> io_uring_register() with the callback still being pending, and you'd get
> -EBUSY from that.

Does using synchronize_rcu make sense? I have sent v3 with how that 
would look. I have kept cq_ev_fd under io_ev_fd as it will be useful in 
patch 3 when eventfd_async is added.

Thanks,
Usama