linux-kernel - Re: [RFC] coredump: Do not interrupt dump for TIF_NOTIFY

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Message-ID: <13250a8d-1a59-4b7b-92e4-1231d73cbdda@gmail.com>
Date:   Fri, 24 Dec 2021 10:37:31 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Olivier Langlois <olivier@...llion01.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, Oleg Nesterov <oleg@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC] coredump: Do not interrupt dump for TIF_NOTIFY_SIGNAL

On 12/24/21 01:34, Olivier Langlois wrote:
> On Fri, 2021-10-22 at 15:13 +0100, Pavel Begunkov wrote:
>> On 6/9/21 21:17, Eric W. Biederman wrote:
>> In short, a task creates an io_uring worker thread, then the worker
>> submits a task_work item to the creator task and won't die until
>> the item is executed/cancelled. And I found that the creator task is
>> sleeping in do_coredump() -> wait_for_completion()
>>
[...]
>> A hack executing tws there helps (see diff below).
>> Any chance anyone knows what this is and how to fix it?
>>
[...]
> Pavel,
> 
> I cannot comment on the merit of the proposed hack but my proposed
> patch to fix the coredump truncation issue when a process using
> io_uring core dumps that I submitted back in August is still
> unreviewed!

That's unfortunate. Not like I can help in any case, but I assumed
it was dealt with by

commit 06af8679449d4ed282df13191fc52d5ba28ec536
Author: Eric W. Biederman <ebiederm@...ssion.com>
Date:   Thu Jun 10 15:11:11 2021 -0500

     coredump: Limit what can interrupt coredumps
     
     Olivier Langlois has been struggling with coredumps being incompletely written in
     processes using io_uring.
     ...

> https://lore.kernel.org/lkml/1625bc89782bf83d9d8c7c63e8ffcb651ccb15fa.1629655338.git.olivier@trillion01.com/
> 
> I have been using it since then I must have generated many dozens of
> perfect core dump files with it and I have not seen a single truncated
> core dump files like I used to have prior to the patch.
> 
> I am bringing back my patch to your attention because one nice side
> effect of it is that it would have avoided totally the problem that you
> have encountered in coredump_wait() since it does cancel io_uring
> resources before calling coredump_wait()!

FWIW, I worked it around in io_uring back then by breaking the
dependency.

-- 
Pavel Begunkov