Re: [patch 6/7] cpusets: per cpuset dirty ratios

Peter Zijlstra
2008-10-30T09:34:07Z

On Thu, 2008-10-30 at 02:03 -0700, David Rientjes wrote:
> On Thu, 30 Oct 2008, Peter Zijlstra wrote:
>
> > On Tue, 2008-10-28 at 09:08 -0700, David Rientjes wrote:
> >
> > > +/*
> > > + * Determine the dirty ratios for the currently active cpuset
> > > + */
> > > +void cpuset_get_current_dirty_ratios(int *background, int *throttle)
> > > +{
> > > + mutex_lock(&callback_mutex);
> > > + task_lock(current);
> > > + *background = task_cs(current)->dirty_background_ratio;
> > > + *throttle = task_cs(current)->cpuset_dirty_ratio;
> > > + task_unlock(current);
> > > + mutex_unlock(&callback_mutex);
> > > +
> > > + if (*background == -1)
> > > + *background = dirty_background_ratio;
> > > + if (*throttle == -1)
> > > + *throttle = vm_dirty_ratio;
> > > +}
> >
> > That's rather an awful lot of locking to read just two integers.
> >
>
> As far as I know, task_lock(current) is required to dereference
> task_cs(current) and callback_mutex is required to ensure its the same
> cpuset.

Since we read these things for every evaluation, getting it wrong isn't
too harmful.

So I would suggest just enough locking to ensure we don't reference any
NULL pointers and such.

IIRC the cpuset stuff is RCU freed, so some racy read should be
possible, no?