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

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Message-ID: <Y+Ek98n1xUhTP+8a@linutronix.de>
Date:   Mon, 6 Feb 2023 17:04:07 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Oleg Nesterov <oleg@...hat.com>
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 2023-02-06 16:27:12 [+0100], Oleg Nesterov wrote:
> 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.

Sounds good.

> > If so why not use it
> > unconditionally?
> 
> performance ?

All the free() part is moved from the caller into rcu.

> 
> 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.

So if we do it unconditionally then we could get rid of
put_task_struct_rcu_user().
Otherwise we could use put_task_struct_rcu_user() in that timer
callback because it will lead to lockdep warnings once printk is fixed.

> Oleg.
Sebastian