linux-kernel - Re: [PATCH 2/3] sched, timer: Use atomics for thread_group

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Date:	Wed, 15 Apr 2015 12:09:21 -0700
From:	Jason Low <jason.low2@...com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mel Gorman <mgorman@...e.de>,
	Steven Rostedt <rostedt@...dmis.org>, hideaki.kimura@...com,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>, jason.low2@...com
Subject: Re: [PATCH 2/3] sched, timer: Use atomics for thread_group_cputimer
 to improve scalability

On Wed, 2015-04-15 at 16:07 +0530, Preeti U Murthy wrote:
> On 04/15/2015 04:39 AM, Jason Low wrote:
> >  /*
> > @@ -885,11 +890,8 @@ static void check_thread_timers(struct task_struct *tsk,
> >  static void stop_process_timers(struct signal_struct *sig)
> >  {
> >  	struct thread_group_cputimer *cputimer = &sig->cputimer;
> > -	unsigned long flags;
> > 
> > -	raw_spin_lock_irqsave(&cputimer->lock, flags);
> > -	cputimer->running = 0;
> > -	raw_spin_unlock_irqrestore(&cputimer->lock, flags);
> > +	WRITE_ONCE(cputimer->running, 0);
> 
> Why do a WRITE_ONCE() here ?

Perhaps Peter can confirm/elaborate, but since we're now updating the
running field without the lock, we use WRITE_ONCE to guarantee that this
doesn't get optimized in any way. This can also serve as "documentation"
that we're writing to a shared variable without a lock.

>  Maybe you should explicitly mention this
> through a comment like Steven pointed out about all
> WRITE/READ/ACCESS_ONCE() usage.

Yeah, we should add a comment here.

Thanks,
Jason

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