Re: [linux-pm] [PATCH] PERF(kernel): Cleanup power events V2

Alan Stern
2010-10-26T18:50:06Z

On Tue, 26 Oct 2010, Mathieu Desnoyers wrote:

> * Peter Zijlstra (peterz@infradead.org) wrote:
> > On Tue, 2010-10-26 at 11:56 -0500, Pierre Tardy wrote:
> > >
> > > + trace_runtime_pm_usage(dev, atomic_read(&dev->power.usage_count)+1);
> > > atomic_inc(&dev->power.usage_count);
> >
> > That's terribly racy..
>
> Looking at the original code, it looks racy even without considering the
> tracepoint:
>
> int __pm_runtime_get(struct device *dev, bool sync)
> {
> int retval;
>
> + trace_runtime_pm_usage(dev, atomic_read(&dev->power.usage_count)+1);
> atomic_inc(&dev->power.usage_count);
> retval = sync ? pm_runtime_resume(dev) : pm_request_resume(dev);
>
> There is no implied memory barrier after "atomic_inc". So either all these
> inc/dec are protected with mutexes or spinlocks, in which case one might wonder
> why atomic operations are used at all, or it's a racy mess. (I vote for the
> second option)

I don't understand. What's the problem? The inc/dec are atomic
because they are not protected by spinlocks, but everything else is
(aside from the tracepoint, which is new).

> kref should certainly be used there.

What for?

Alan Stern