linux-kernel - Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS

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Date:	Tue, 14 Apr 2015 22:40:59 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jason Low <jason.low2@...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>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	hideaki.kimura@...com, Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the
 scheduler

On Tue, 14 Apr 2015 19:12:33 -0700
Jason Low <jason.low2@...com> wrote:

> Hi Steven,
> 
> On Tue, 2015-04-14 at 19:59 -0400, Steven Rostedt wrote:
> > On Tue, 14 Apr 2015 16:09:44 -0700
> > Jason Low <jason.low2@...com> wrote:
> > 
> > 
> > > @@ -2088,7 +2088,7 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags)
> > >  
> > >  static void reset_ptenuma_scan(struct task_struct *p)
> > >  {
> > > -	ACCESS_ONCE(p->mm->numa_scan_seq)++;
> > > +	WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1);
> > 
> > Is the READ_ONCE() inside the WRITE_ONCE() really necessary?
> 
> Yeah, I think so to be safe, otherwise, the access of
> p->mm->numa_scan_seq in the 2nd parameter doesn't have the volatile
> cast.

You are correct. Now I'm thinking that the WRITE_ONCE() is not needed,
and just a:

	p->mm->numa_scan_seq = READ_ONCE(p->numa_scan_seq) + 1;

Can be done. But I'm still trying to wrap my head around why this is
needed here. Comments would have been really helpful. We should make
all READ_ONCE() WRITE_ONCE and obsolete ACCESS_ONCE() have mandatory
comments just like we do with memory barriers.

-- Steve

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