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

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Date:   Fri, 10 Feb 2023 17:48:39 +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 17:27:58 [+0100], Oleg Nesterov wrote:
> 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,

That callback does mostly free() and it is batched with other free()
invocations. This also is moved away from the caller which _might_
benefit.

> > > 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() ?

Oh, this is usage vs rcu_users. Okay, mixed that up.

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

Just noticed ;)

> 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...

That is true. So you are saying that we don't what to use RCU for
put_task_struct() unconditionally?

> Oleg.

Sebastian