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From:   Wander Lairson Costa <wander@redhat.com>
Date:   Mon, 5 Jun 2023 08:24:13 -0300
Message-ID: <CAAq0SU=A5j2-GF80Thi2vm8W+_AUquj6t+QK7cnWLz1jKEA4zg@mail.gmail.com>
Subject: Re: [PATCH v9] kernel/fork: beware of __put_task_struct calling context
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>,
        Brian Cain <bcain@quicinc.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Stafford Horne <shorne@gmail.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Andrei Vagin <avagin@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Daniel Bristot de Oliveira <bristot@kernel.org>,
        Yu Zhao <yuzhao@google.com>,
        Alexey Gladkov <legion@kernel.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Yang Shi <shy828301@gmail.com>,
        open list <linux-kernel@vger.kernel.org>,
        Hu Chunyu <chuhu@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Luis Goncalves <lgoncalv@redhat.com>
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On Fri, Jun 2, 2023 at 2:34=E2=80=AFPM Oleg Nesterov <oleg@redhat.com> wrot=
e:
>
> On 06/01, Wander Lairson Costa wrote:
> >
> > On Thu, Jun 1, 2023 at 3:14=E2=80=AFPM Oleg Nesterov <oleg@redhat.com> =
wrote:
> > >
> > > > but only in the RT kernel
> > >
> > > this again suggests that your testing was wrong or I am totally confu=
sed (quite
> > > possible, I know nothing about RT). I did the testing without CONFIG_=
PREEMPT_RT.
> > >
> >
> > Hrm, could you please share your .config?
>
> Sure. I do not want to spam the list, I'll send you a private email.
>

Thanks. I found an unrelated earlier splat in the console code. That's
why I couldn't reproduce it in the stock kernel.

> Can you share your kernel module code?
>

*facepalm* I forgot to post the link: https://github.com/walac/test-prove-l=
ock/

> Did you verify that debug_locks !=3D 0 as I asked in my previous email ?
>
> > > > But running the reproducer for put_task_struct(), works fine.
> > >
> > > which reproducer ?
> > >
> >
> > Only now I noticed I didn't add the reproducer to the commit message:
> >
> > while true; do
> >     stress-ng --sched deadline --sched-period 1000000000
> > --sched-runtime 800000000 --sched-deadline 1000000000 --mmapfork 23 -t
> > 20
> > done
>
> Cough ;) I think we need a more simple one to enssure that
> refcount_sub_and_test(nr, &t->usage) returns true under raw_spin_lock()
> and then __put_task_struct() actually takes spin_lock().
>
> Oleg.
>