Messages in this thread | | | Date | Thu, 30 Oct 2008 03:02:45 -0700 (PDT) | From | David Rientjes <> | Subject | Re: [patch 6/7] cpusets: per cpuset dirty ratios |
| |
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!
| |