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Subject: Re: [External] Re: [PATCH 1/2] io_uring: avoid ring quiesce while
 registering/unregistering eventfd
To:     Jens Axboe <axboe@kernel.dk>, io-uring@vger.kernel.org,
        asml.silence@gmail.com, linux-kernel@vger.kernel.org
Cc:     fam.zheng@bytedance.com
References: <20220203151153.574032-1-usama.arif@bytedance.com>
 <20220203151153.574032-2-usama.arif@bytedance.com>
 <87fca94e-3378-edbb-a545-a6ed8319a118@kernel.dk>
 <62f59304-1a0e-1047-f474-94097cb8b13e@bytedance.com>
 <da09cb46-9d60-71a3-a758-46d082989bae@kernel.dk>
From:   Usama Arif <usama.arif@bytedance.com>
Message-ID: <b80ee8ae-2150-ac96-ccf5-0905f313751f@bytedance.com>
Date:   Thu, 3 Feb 2022 17:42:36 +0000
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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