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

[Usama Arif <usama.arif@bytedance.com>]

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