Re: [PATCH v2 6/6] stop_machine: kill stop_cpus_lock and lg_double_lock/unlock()

[Oleg Nesterov <oleg@redhat.com>]

Hi Peter,

Thanks for looking. I'll try to reply on Monday, just one note...

On 07/30, Peter Zijlstra wrote:
>
> On Tue, Jul 21, 2015 at 09:22:47PM +0200, Oleg Nesterov wrote:
>
> > +static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1,
> > +				    int cpu2, struct cpu_stop_work *work2)
> > +{
> > +	struct cpu_stopper *stopper1 = per_cpu_ptr(&cpu_stopper, cpu1);
> > +	struct cpu_stopper *stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);
> > +	int err;
> > +retry:
> > +	spin_lock_irq(&stopper1->lock);
> > +	spin_lock_nested(&stopper2->lock, SINGLE_DEPTH_NESTING);
> > +	/*
> > +	 * If we observe both CPUs active we know _cpu_down() cannot yet have
> > +	 * queued its stop_machine works and therefore ours will get executed
> > +	 * first. Or its not either one of our CPUs that's getting unplugged,
> > +	 * in which case we don't care.
> > +	 */
> > +	err = -ENOENT;
> > +	if (!cpu_active(cpu1) || !cpu_active(cpu2))
> > +		goto unlock;
> > +
> > +	WARN_ON(!stopper1->enabled || !stopper2->enabled);
> > +	/*
> > +	 * Ensure that if we race with stop_cpus() the stoppers won't
> > +	 * get queued up in reverse order, leading to system deadlock.
> > +	 */
> > +	err = -EDEADLK;
> > +	if (stop_work_pending(stopper1) != stop_work_pending(stopper2))
> > +		goto unlock;
>
> You could DoS/false positive this by running stop_one_cpu() in a loop,
> and thereby 'always' having work pending on one but not the other.

IIRC no. I am pretty sure stop_one_cpu() doesn't use stopper->stop_work,
only stop_machine() does.

Oleg.