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From:   Wander Lairson Costa <wander@redhat.com>
Date:   Mon, 15 May 2023 15:58:09 -0300
Message-ID: <CAAq0SUkrF12gQpb6Qq-P5r4eobcajF=c_oGGTeOfwVNY8rppww@mail.gmail.com>
Subject: Re: [PATCH v8] kernel/fork: beware of __put_task_struct calling context
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     "Christian Brauner (Microsoft)" <brauner@kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Mike Christie <michael.christie@oracle.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Guo Ren <guoren@kernel.org>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Andrei Vagin <avagin@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>,
        Paul McKenney <paulmck@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>
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On Mon, May 15, 2023 at 3:09=E2=80=AFPM Oleg Nesterov <oleg@redhat.com> wro=
te:
>
> On 05/15, Wander Lairson Costa wrote:
> > On Mon, May 15, 2023 at 1:43=E2=80=AFPM Oleg Nesterov <oleg@redhat.com>=
 wrote:
> > >
> > > Certainly I have missed something...
> > >
> > > but,
> > >
> > > On 05/15, Wander Lairson Costa wrote:
> > > >
> > > > -extern void __put_task_struct(struct task_struct *t);
> > > > +extern void ___put_task_struct(struct task_struct *t);
> > > > +extern void __put_task_struct_rcu_cb(struct rcu_head *rhp);
> > >
> > > I don't understand these renames, why can't you simply put this fix
> > > into put_task_struct() ?
> > >
> >
> > No particular reason, it was just a matter of style and keep the parts =
simple.
>
> Well, to me a single/simple change in put_task_struct() makes more
> sense, but I won't argue.
>

My initial thought was to break the code in smaller functions, but
maybe just changing put_task_struct() could be better.

>         static inline void put_task_struct(struct task_struct *t)
>         {
>                 if (!refcount_dec_and_test(...))
>                         return;
>
>                 if (IS_ENABLED(PREEMPT_RT) && ...)
>                         return call_rcu(...);
>
>                 ...
>                 __put_task_struct();
>                 ...
>         }
>
> > > > +static inline void __put_task_struct(struct task_struct *tsk)
> > > > +{
> > > ...
> > > > +     if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
> > > > +             call_rcu(&tsk->rcu, __put_task_struct_rcu_cb);
> > > > +     else
> > > > +             ___put_task_struct(tsk);
> > > > +}
> > >
> > > did you see the emails from Peter? In particular, this one:
> > >
> > >         https://lore.kernel.org/lkml/20230505133902.GC38236@hirez.pro=
gramming.kicks-ass.net/
> > >
> >
> > I didn't notice the lock_acquire/lock_release part. However, I tested
> > the patch with CONFIG_PROVE_RAW_LOCK_NESTING and there was no warning.
>
> Hmm. I tend to trust the Sebastian's analysis in
>
>         https://lore.kernel.org/all/Y+zFNrCjBn53%2F+Q2@linutronix.de/
>
> I'll try to look at it later, although I hope Sebastian or Peter
> can explain this before I try ;)
>

The inability to see and reproduce the potential issue is part of my
confusion about addressing this property.

> Oleg.
>