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

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Date:	Tue, 14 Apr 2015 19:12:33 -0700
From:	Jason Low <jason.low2@...com>
To:	Steven Rostedt <rostedt@...dmis.org>
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>, jason.low2@...com
Subject: Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the
 scheduler

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.

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