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

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Date:   Mon, 6 Feb 2023 17:27:58 +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 16:27:12 [+0100], Oleg Nesterov wrote:
>
> > > If so why not use it
> > > unconditionally?
> >
> > performance ?
>
> All the free() part is moved from the caller into rcu.

sorry, I don't understand,

>
> >
> > 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().

Yes. But the whole purpose of rcu_users is that we want to avoid the unconditional
rcu grace period before free_task() ?

Just in case... please note that delayed_put_task_struct() delays
refcount_sub(t->usage), not free_task().

Why do we need this? Consider

	rcu_read_lock();

	task = find-task-in-rcu-protected-list;

	// Safe, task->usage can't be zero
	get_task_struct(task);

	rcu_read_unlock();


> Otherwise we could use put_task_struct_rcu_user() in that timer
> callback because it will lead to lockdep warnings once printk is fixed.

IIUC there are more in-atomic callers of put_task_struct(). But perhaps
I misunderstood you...

Oleg.