Re: [PATCH 1/1] sched: prevent divide by zero error incpu_avg_load_per_task

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Wed, 26 Nov 2008, Steven Rostedt wrote:
> >  {
> >  	struct rq *rq = cpu_rq(cpu);
> > +	unsigned long nr_running = rq->nr_running;
> >  
> > -	if (rq->nr_running)
> > -		rq->avg_load_per_task = rq->load.weight / rq->nr_running;
> > +	if (nr_running)
> > +		rq->avg_load_per_task = rq->load.weight / nr_running;
> >  	else
> >  		rq->avg_load_per_task = 0;
> 
> I don't think this necessarily fixes it.
> 
> There's nothing that keeps gcc from deciding not to reload 
> rq->nr_running.
> 
> Of course, in _practice_, I don't think gcc ever will (if it decides 
> that it will spill, gcc is likely going to decide that it will 
> literally spill the local variable to the stack rather than decide to 
> reload off the pointer), but it's a valid compiler optimization, and it 
> even has a name (rematerialization).
> 
> So I suspect that your patch does fix the bug, but it still leaves the 
> fairly unlikely _potential_ for it to re-appear at some point.
> 
> We have ACCESS_ONCE() as a macro to guarantee that the compiler doesn't 
> rematerialize a pointer access. That also would clarify the fact that 
> we access something unsafe outside a lock.

Okay - i've queued up the fix below, to be on the safe side.

	Ingo

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