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

Mathieu Desnoyers
2010-10-26T21:38:19Z

* Rafael J. Wysocki (rjw@sisk.pl) wrote:
> On Tuesday, October 26, 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)
>
> No, it isn't.
>
> > kref should certainly be used there.
>
> No, it shouldn't.
>
> Please try to understand the code you're commenting on first.

Please see my reply to Alan Stern:

http://www.spinics.net/lists/linux-omap/msg39382.html

Thanks,

Mathieu

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.