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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Olivier Langlois <olivier@trillion01.com>,
        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>
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Date:   Wed, 24 Aug 2022 10:11:23 -0500
In-Reply-To: <654cb5de-a563-b812-a435-d9b435cee334@kernel.dk> (Jens Axboe's
        message of "Tue, 23 Aug 2022 12:27:06 -0600")
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Subject: Re: [PATCH 2/2] coredump: Allow coredumps to pipes to work with
 io_uring
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Jens Axboe <axboe@kernel.dk> writes:

> 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().

But that is a task_work to the helper thread correct?

> 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.

I can see that.

Which leaves me with the question do these task_work's directly wake up
the thread that submitted the I/O request?   Or is there likely to be
something for an I/O thread to do before an ordinary program thread is
notified.

I am asking because it is only the case of notifying ordinary program
threads that is interesting in the case of a coredump.  As I understand
it a data to read notification would typically be handled by the I/O
uring worker thread to trigger reading the data before letting userspace
know everything it asked to be done is complete.

Eric