linux-kernel - Re: [PATCH v3] kernel/fork: beware of __put_task

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Date:   Thu, 2 Feb 2023 21:22:46 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Wander Lairson Costa <wander@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Andrei Vagin <avagin@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        Hu Chunyu <chuhu@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Paul McKenney <paulmck@...nel.org>
Subject: Re: [PATCH v3] kernel/fork: beware of __put_task_struct calling
 context

On 02/02, Wander Lairson Costa wrote:
>
> > We already have delayed_put_task_struct() which differs very much.
> > Perhaps something like ___put_task_struct() will look less confusing.
> >
>
> ___put_task_struct()? I already added a function with this name below.

Ah, I meant ___put_task_struct_rcu() or something like this. Bug again
this is just cosmetic nit, please ignore

> > > +void __put_task_struct(struct task_struct *tsk)
> > > +{
> > > +     if (IS_ENABLED(CONFIG_PREEMPT_RT) && (!preemptible() || !in_task()))
> > > +             /*
> > > +              * under PREEMPT_RT, we can't call put_task_struct
> > > +              * in atomic context because it will indirectly
> > > +              * acquire sleeping locks.
> > > +              */
> > > +             call_rcu(&tsk->rcu, __delayed_put_task_struct);
> >
> > Perhaps this deserves additional note to explain why is it safe to use tsk->rcu
> > union. May be this is obvious, but I was confused when I looked at the previous
> > version ;)
> >
>
> Makes sense, I will add it in the next version.

Thanks ;)

Oleg.