Re: [patch] sched: fix set_task_cpu() and provide an unlocked runqueue variant

[Mike Galbraith <efault@gmx.de>]

On Wed, 2009-11-25 at 19:27 +0100, Peter Zijlstra wrote:
> On Sun, 2009-11-22 at 13:09 +0100, Mike Galbraith wrote:
> > sched: fix set_task_cpu() and provide an unlocked runqueue variant.
> > 
> > set_task_cpu() falsifies migration stats by unconditionally generating migration
> > stats whether a task's cpu actually changed or not.  As used in copy_process(),
> > the runqueue is unlocked, so we need to provide an unlocked variant which does
> > the locking to provide a write barrier.
> > 
> > Signed-off-by: Mike Galbraith <efault@gmx.de>
> > Cc: Ingo Molnar <mingo@elte.hu>
> > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > LKML-Reference: <new-submission>
> > 
> > ---
> 
> > +void set_task_cpu_unlocked(struct task_struct *p, unsigned int new_cpu)
> > +{
> > +	unsigned long flags;
> > +	struct rq *rq, *new_rq = cpu_rq(new_cpu);
> > +
> > +	smp_wmb();
> > +	rq = task_rq_lock(p, &flags);
> > +	update_rq_clock(rq);
> > +	if (rq != new_rq)
> > +		update_rq_clock(new_rq);
> > +	set_task_cpu(p, new_cpu);
> > +	task_rq_unlock(rq, &flags);
> > +}
> 
> I've got to ask, what's that barrier for?

It's a leftover from frustrated bug hunting.

	-Mike