linux-kernel - Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier

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Date:   Wed, 4 Sep 2019 15:17:35 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        paulmck <paulmck@...ux.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Russell King, ARM Linux" <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Chris Lameter <cl@...ux.com>, Kirill Tkhai <tkhai@...dex.ru>,
        Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state
 racy load

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.