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Date: Fri, 23 Jan 2015 21:11:03 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Jason Low <jason.low2@hp.com>
Cc: Ingo Molnar <mingo@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Scott J Norton <scott.norton@hp.com>, Chegu Vinod <chegu_vinod@hp.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] sched, timer: Use atomics for thread_group_cputimer
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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.