linux-kernel - Re: [PATCH v9] kernel/fork: beware of __put_task

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Date:   Wed, 17 May 2023 13:57:09 -0300
From:   Wander Lairson Costa <wander@...hat.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
        Brian Cain <bcain@...cinc.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Stafford Horne <shorne@...il.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrei Vagin <avagin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...nel.org>,
        Yu Zhao <yuzhao@...gle.com>,
        Alexey Gladkov <legion@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Yang Shi <shy828301@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        Hu Chunyu <chuhu@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luis Goncalves <lgoncalv@...hat.com>
Subject: Re: [PATCH v9] kernel/fork: beware of __put_task_struct calling context

On Wed, May 17, 2023 at 12:26 PM Oleg Nesterov <oleg@...hat.com> wrote:
>
> On 05/16, Wander Lairson Costa wrote:
> >
> >  static inline void put_task_struct(struct task_struct *t)
> >  {
> > -     if (refcount_dec_and_test(&t->usage))
> > +     if (!refcount_dec_and_test(&t->usage))
> > +             return;
> > +
> > +     /*
> > +      * under PREEMPT_RT, we can't call put_task_struct
> > +      * in atomic context because it will indirectly
> > +      * acquire sleeping locks.
> > +      *
> > +      * call_rcu() will schedule delayed_put_task_struct_rcu()
> > +      * to be called in process context.
> > +      *
> > +      * __put_task_struct() is called when
> > +      * refcount_dec_and_test(&t->usage) succeeds.
> > +      *
> > +      * This means that it can't "conflict" with
> > +      * put_task_struct_rcu_user() which abuses ->rcu the same
> > +      * way; rcu_users has a reference so task->usage can't be
> > +      * zero after rcu_users 1 -> 0 transition.
> > +      *
> > +      * delayed_free_task() also uses ->rcu, but it is only called
> > +      * when it fails to fork a process. Therefore, there is no
> > +      * way it can conflict with put_task_struct().
> > +      */
> > +     if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
> > +             call_rcu(&t->rcu, __put_task_struct_rcu_cb);
> > +     else
> >               __put_task_struct(t);
> >  }
>
> LGTM but we still need to understand the possible problems with CONFIG_PROVE_RAW_LOCK_NESTING ...
>
> Again, I'll try to investigate when I have time although I am not sure I can really help.
>
> Perhaps you too can try to do this ? ;)
>

FWIW, I tested this patch with CONFIG_PROVE_LOCK_NESTING in RT and
stock kernels. No splat happened.

> Oleg.
>