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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>,
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>,
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Date: Tue, 23 Aug 2022 13:22:53 -0500
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(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