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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <b80ee8ae-2150-ac96-ccf5-0905f313751f@bytedance.com>
Date:   Thu, 3 Feb 2022 17:42:36 +0000
From:   Usama Arif <usama.arif@...edance.com>
To:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        asml.silence@...il.com, linux-kernel@...r.kernel.org
Cc:     fam.zheng@...edance.com
Subject: Re: [External] Re: [PATCH 1/2] io_uring: avoid ring quiesce while
 registering/unregistering eventfd



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