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

[Jens Axboe]

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...

-- 
Jens Axboe