Re: [PATCH v8] kernel/fork: beware of __put_task_struct calling context

[Oleg Nesterov <oleg@redhat.com>]

On 05/15, Wander Lairson Costa wrote:
> On Mon, May 15, 2023 at 1:43 PM Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > Certainly I have missed something...
> >
> > but,
> >
> > On 05/15, Wander Lairson Costa wrote:
> > >
> > > -extern void __put_task_struct(struct task_struct *t);
> > > +extern void ___put_task_struct(struct task_struct *t);
> > > +extern void __put_task_struct_rcu_cb(struct rcu_head *rhp);
> >
> > I don't understand these renames, why can't you simply put this fix
> > into put_task_struct() ?
> >
>
> No particular reason, it was just a matter of style and keep the parts simple.

Well, to me a single/simple change in put_task_struct() makes more
sense, but I won't argue.

	static inline void put_task_struct(struct task_struct *t)
	{
		if (!refcount_dec_and_test(...))
			return;

		if (IS_ENABLED(PREEMPT_RT) && ...)
			return call_rcu(...);

		...
		__put_task_struct();
		...
	}

> > > +static inline void __put_task_struct(struct task_struct *tsk)
> > > +{
> > ...
> > > +     if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
> > > +             call_rcu(&tsk->rcu, __put_task_struct_rcu_cb);
> > > +     else
> > > +             ___put_task_struct(tsk);
> > > +}
> >
> > did you see the emails from Peter? In particular, this one:
> >
> >         https://lore.kernel.org/lkml/20230505133902.GC38236@hirez.programming.kicks-ass.net/
> >
>
> I didn't notice the lock_acquire/lock_release part. However, I tested
> the patch with CONFIG_PROVE_RAW_LOCK_NESTING and there was no warning.

Hmm. I tend to trust the Sebastian's analysis in

	https://lore.kernel.org/all/Y+zFNrCjBn53%2F+Q2@linutronix.de/

I'll try to look at it later, although I hope Sebastian or Peter
can explain this before I try ;)

Oleg.