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

Linus Torvalds
2008-11-29T19:21:19Z

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.

Linus