Re: [RFC PATCH] sched, timer: Use atomics for thread_group_cputimer stats

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jan 23, 2015 at 10:07:31AM -0800, Jason Low wrote:
> On Fri, 2015-01-23 at 10:33 +0100, Peter Zijlstra wrote:
> > > +		.running = ATOMIC_INIT(0),                              \
> > > +	atomic_t running;
> > > +		atomic_set(&sig->cputimer.running, 1);
> > > @@ -174,7 +174,7 @@ static inline bool cputimer_running(struct task_struct *tsk)
> > > +	if (!atomic_read(&cputimer->running))
> > > +	if (!atomic_read(&cputimer->running)) {
> > > +		atomic_set(&cputimer->running, 1);
> > > +	if (atomic_read(&tsk->signal->cputimer.running))
> > > +	atomic_set(&cputimer->running, 0);
> > > +	if (atomic_read(&sig->cputimer.running)) {
> > > +	if (atomic_read(&tsk->signal->cputimer.running))
> > 
> > That doesn't really need an atomic_t.
> 
> Yeah, I was wondering about that, and made it atomic since we had:
> 
>     raw_spin_lock_irqsave(&cputimer->lock, flags);
>     cputimer->running = 0;
>     raw_spin_unlock_irqrestore(&cputimer->lock, flags);
> 
> in stop_process_timers().

Yeah, that could've been ACCESS_ONCE(cputimer->running) = 0. FWIW
atomic_set() seems to not actually include the needed volatile cast.