Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <bb423622f97826f483100a1a7f20ce10a9090158.camel@trillion01.com>
Subject: Re: [PATCH 2/2] coredump: Allow coredumps to pipes to work with
 io_uring
From:   Olivier Langlois <olivier@trillion01.com>
To:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Jens Axboe <axboe@kernel.dk>
Cc:     Pavel Begunkov <asml.silence@gmail.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        io-uring@vger.kernel.org, Alexander Viro <viro@zeniv.linux.org.uk>,
        Oleg Nesterov <oleg@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 22 Aug 2022 23:35:37 -0400
In-Reply-To: <61abfb5a517e0ee253b0dc7ba9cd32ebd558bcb0.camel@trillion01.com>
References: <192c9697e379bf084636a8213108be6c3b948d0b.camel@trillion01.com>
         <9692dbb420eef43a9775f425cb8f6f33c9ba2db9.camel@trillion01.com>
         <87h7i694ij.fsf_-_@disp2133>
         <1b519092-2ebf-3800-306d-c354c24a9ad1@gmail.com>
         <b3e43e07c68696b83a5bf25664a3fa912ba747e2.camel@trillion01.com>
         <13250a8d-1a59-4b7b-92e4-1231d73cbdda@gmail.com>
         <878rw9u6fb.fsf@email.froward.int.ebiederm.org>
         <303f7772-eb31-5beb-2bd0-4278566591b0@gmail.com>
         <87ilsg13yz.fsf@email.froward.int.ebiederm.org>
         <8218f1a245d054c940e25142fd00a5f17238d078.camel@trillion01.com>
         <a29a1649-5e50-4221-9f44-66a35fbdff80@kernel.dk>
         <87y1wnrap0.fsf_-_@email.froward.int.ebiederm.org>
         <87mtd3rals.fsf_-_@email.froward.int.ebiederm.org>
         <61abfb5a517e0ee253b0dc7ba9cd32ebd558bcb0.camel@trillion01.com>
Organization: Trillion01 Inc
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 8BIT
User-Agent: Evolution 3.44.3 
MIME-Version: 1.0
Precedence: bulk

On Mon, 2022-08-22 at 17:16 -0400, Olivier Langlois wrote:
> 
> What is stopping the task calling do_coredump() to be interrupted and
> call task_work_add() from the interrupt context?
> 
> This is precisely what I was experiencing last summer when I did work
> on this issue.
> 
> My understanding of how async I/O works with io_uring is that the
> task
> is added to a wait queue without being put to sleep and when the
> io_uring callback is called from the interrupt context,
> task_work_add()
> is called so that the next time io_uring syscall is invoked, pending
> work is processed to complete the I/O.
> 
> So if:
> 
> 1. io_uring request is initiated AND the task is in a wait queue
> 2. do_coredump() is called before the I/O is completed
> 
> IMHO, this is how you end up having task_work_add() called while the
> coredump is generated.
> 
I forgot to add that I have experienced the issue with TCP/IP I/O.

I suspect that with a TCP socket, the race condition window is much
larger than if it was disk I/O and this might make it easier to
reproduce the issue this way...