linux-kernel - Re: [PATCH v4] kernel/fork: beware of __put_task

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Message-ID: <20230206152712.GA1487@redhat.com>
Date:   Mon, 6 Feb 2023 16:27:12 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Wander Lairson Costa <wander@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        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 v4] kernel/fork: beware of __put_task_struct calling
 context

On 02/06, Sebastian Andrzej Siewior wrote:
>
> On 2023-02-06 10:04:47 [-0300], Wander Lairson Costa wrote:
>
> > @@ -857,6 +857,29 @@ void __put_task_struct(struct task_struct *tsk)
> …
> > +void __put_task_struct(struct task_struct *tsk)
> > +{
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && (!preemptible() || !in_task()))
>
> Is it safe to use the rcu member in any case?

I thinks it is safe but deserves a comment. I guess Wander misunderstood
me when I asked him to do this...

__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.

> If so why not use it
> unconditionally?

performance ?

And... I still don't like the name of delayed_put_task_struct_rcu() to me
___put_task_struct_rcu() looks a bit less confusing, note that we already
have delayed_put_task_struct(). But this is minor.

Oleg.