linux-kernel - Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

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Date:   Wed, 24 May 2023 09:44:29 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Mike Christie <michael.christie@...cle.com>, linux@...mhuis.info,
        nicolas.dichtel@...nd.com, axboe@...nel.dk,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, mst@...hat.com,
        sgarzare@...hat.com, jasowang@...hat.com, stefanha@...hat.com,
        brauner@...nel.org
Subject: Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps
 regression

Oleg Nesterov <oleg@...hat.com> writes:

> On 05/23, Eric W. Biederman wrote:
>>
>> I want to point out that we need to consider not just SIGKILL, but
>> SIGABRT that causes a coredump, as well as the process peforming
>> an ordinary exit(2).  All of which will cause get_signal to return
>> SIGKILL in this context.
>
> Yes, but probably SIGABRT/exit doesn't really differ from SIGKILL wrt
> vhost_worker().

Actually I think it reveals that exiting with SIGABRT will cause
a deadlock.

coredump_wait will wait for all of the threads to reach
coredump_task_exit.  Meanwhile vhost_worker is waiting for
all of the other threads to reach exit_files to close their
file descriptors.

So it looks like the final pieces of work will actually need to be moved
into to vhost_xxx_flush or vhost_xxx_release to avoid the exiting
threads from waiting on each other, instead of depending upon the
vhost_worker to do the work.

Which gets back to most of your other questions.

>> It is probably not the worst thing in the world, but what this means
>> is now if you pass a copy of the vhost file descriptor to another
>> process the vhost_worker will persist, and thus the process will persist
>> until that copy of the file descriptor is closed.
>
> Hadn't thought about it.
>
> I am fighting with internal bugzillas today, will try to write another
> email tomorrow.
>
> But before that, I would like to have an answer to my "main" question in
> my previois email. Otherwise I am still not sure I understand what exactly
> we need to fix.

Let me repeat your "main" question just for clarity here.

If a signal comes in after the signal_pending check but before the
"work->fn(work)" call is "work->fn(work)" expected to run correctly
with signal_pending() or fatal_signal_pending returning true?

Eric