Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state racy load

[Oleg Nesterov <oleg@redhat.com>]

On 09/04, Peter Zijlstra wrote:
>
> On Wed, Sep 04, 2019 at 02:03:37PM +0200, Oleg Nesterov wrote:
> > On 09/04, Peter Zijlstra wrote:
> > >
> > > +		struct task_struct *g, *t;
> > > +
> > > +		read_lock(&tasklist_lock);
> > > +		do_each_thread(g, t) {
> > 
> > for_each_process_thread() looks better
> 
> Argh, I always get confused. Why do we have multiple version of this
> again?

Because I am lazy ;)

but actually for_each_process_thread() is suboptimal, I think you need

	for_each_process(p) {
		if (p->flags & PF_KTHREAD)
			continue;

		if (p->mm != mm)
			continue;

		for_each_thread(p, t)
			atomic_or(t->membarrier_state, ...);
	}

to avoid unnecessary each-thread when group leader has another ->mm.

Unfortunately a zombie leader has ->mm == NULL, so perhaps something like

	for_each_process(p) {
		if (p->flags & PF_KTHREAD)
			continue;

		for_each_thread(p, t) {
			if (unlikely(!t->mm))
				continue;
			if (t->mm != mm)
				break;
			atomic_or(t->membarrier_state, ...);
		}
	}

and to we really need the new atomic_t member? can we use t->atomic_flags
and add PFA_MEMBARRIER_EXPEDITED ?

Oleg.