On Thu, 30 Oct 2008, Peter Zijlstra 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?
>


Ah, that sounds reasonable. We'll no longer require callback_mutex if we
accept races when current attaches to another cpuset here. We'll need
rcu_read_lock() to safely dereference task_cs(current) unless it's
top_cpuset, but that's much better than callback_mutex and spinning on
task_lock(current).

Thanks!
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