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

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Message-ID: <20190904124333.GQ2332@hirez.programming.kicks-ass.net>
Date:   Wed, 4 Sep 2019 14:43:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
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 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?

> > +			if (t->mm == mm) {
> > +				atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED,
> > +					  &t->membarrier_state);
> > +			}
> 
> then you also need to change dup_task_struct(), it should clear
> ->membarrier_state unless CLONE_VM.

Or, as you suggest below.

> And probably unuse_mm() should clear current->membarrier_state too.

How about we hard exclude PF_KTHREAD and ignore {,un}use_mm() entirely?

> Hmm. And it can race with copy_process() anyway, tasklist_lock can't
> really help. So copy_process() needs to do
> 
> 	write_lock_irq(&tasklist_lock);
> 	...
> 
> 	if (clone_flags & CLONE_VM)
> 		p->membarrier_state = current->membarrier_state;
> 	else
> 		p->membarrier_state = 0;

Right you are.