Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Mike Christie <michael.christie@oracle.com>, linux@leemhuis.info,
        nicolas.dichtel@6wind.com, axboe@kernel.dk,
        torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, mst@redhat.com,
        sgarzare@redhat.com, jasowang@redhat.com, stefanha@redhat.com,
        brauner@kernel.org
References: <20230522025124.5863-1-michael.christie@oracle.com>
        <20230522025124.5863-4-michael.christie@oracle.com>
        <20230522123029.GA22159@redhat.com>
        <cfca7764-d210-6df9-e182-2c093101c6cf@oracle.com>
        <20230522174757.GC22159@redhat.com> <20230523121506.GA6562@redhat.com>
        <87bkib6nxr.fsf@email.froward.int.ebiederm.org>
        <20230524141022.GA19091@redhat.com>
Date:   Wed, 24 May 2023 09:44:29 -0500
In-Reply-To: <20230524141022.GA19091@redhat.com> (Oleg Nesterov's message of
        "Wed, 24 May 2023 16:10:23 +0200")
Message-ID: <87ttw1zt4i.fsf@email.froward.int.ebiederm.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps
 regression
Precedence: bulk

Oleg Nesterov <oleg@redhat.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