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Subject: Re: [PATCH 1/2] io_uring: avoid ring quiesce while
 registering/unregistering eventfd
To:     Usama Arif <usama.arif@bytedance.com>, 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>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <da09cb46-9d60-71a3-a758-46d082989bae@kernel.dk>
Date:   Thu, 3 Feb 2022 09:58:58 -0700
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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.

-- 
Jens Axboe