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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Olivier Langlois <olivier@trillion01.com>
Cc:     Jens Axboe <axboe@kernel.dk>,
        Pavel Begunkov <asml.silence@gmail.com>,
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        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:   Tue, 23 Aug 2022 13:22:53 -0500
In-Reply-To: <bb423622f97826f483100a1a7f20ce10a9090158.camel@trillion01.com>
        (Olivier Langlois's message of "Mon, 22 Aug 2022 23:35:37 -0400")
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Subject: Re: [PATCH 2/2] coredump: Allow coredumps to pipes to work with
 io_uring
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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.

Eric