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: <654cb5de-a563-b812-a435-d9b435cee334@kernel.dk>
Date:   Tue, 23 Aug 2022 12:27:06 -0600
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:102.0) Gecko/20100101
 Thunderbird/102.1.2
Subject: Re: [PATCH 2/2] coredump: Allow coredumps to pipes to work with
 io_uring
Content-Language: en-US
To:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Olivier Langlois <olivier@trillion01.com>
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>
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>
 <bb423622f97826f483100a1a7f20ce10a9090158.camel@trillion01.com>
 <875yiisttu.fsf@email.froward.int.ebiederm.org>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <875yiisttu.fsf@email.froward.int.ebiederm.org>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 8/23/22 12:22 PM, Eric W. Biederman wrote:
> Olivier Langlois <olivier@trillion01.com> writes:
> 
>> 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...
> 
> I was under the apparently mistaken impression that the io_uring
> task_work_add only comes from the io_uring userspace helper threads.
> Those are definitely suppressed by my change.
> 
> Do you have any idea in the code where io_uring code is being called in
> an interrupt context?  I would really like to trace that code path so I
> have a better grasp on what is happening.
> 
> If task_work_add is being called from interrupt context then something
> additional from what I have proposed certainly needs to be done.

task_work may come from the helper threads, but generally it does not.
One example would be doing a read from a socket. There's no data there,
poll is armed to trigger a retry. When we get the poll notification that
there's now data to be read, then we kick that off with task_work. Since
it's from the poll handler, it can trigger from interrupt context. See
the path from io_uring/poll.c:io_poll_wake() -> __io_poll_execute() ->
io_req_task_work_add() -> task_work_add().

It can also happen for regular IRQ based reads from regular files, where
the completion is actually done via task_work added from the potentially
IRQ based completion path.

-- 
Jens Axboe