Message-ID: <1429063953.7039.88.camel@j-VirtualBox>
Subject: Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the
 scheduler
From: Jason Low <jason.low2@hp.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, Mel Gorman <mgorman@suse.de>,
        Preeti U Murthy <preeti@linux.vnet.ibm.com>, hideaki.kimura@hp.com,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        Scott J Norton <scott.norton@hp.com>, jason.low2@hp.com
Date: Tue, 14 Apr 2015 19:12:33 -0700
In-Reply-To: <20150414195906.3adc89d9@gandalf.local.home>
References: <1429052986-9420-1-git-send-email-jason.low2@hp.com>
	 <1429052986-9420-2-git-send-email-jason.low2@hp.com>
	 <20150414195906.3adc89d9@gandalf.local.home>
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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@hp.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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