linux-kernel - Re: [PATCH v2 1/5] sched, timer: Remove usages of ACCESS

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Date:	Wed, 29 Apr 2015 13:05:55 -0400
From:	Waiman Long <waiman.long@...com>
To:	Jason Low <jason.low2@...com>
CC:	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>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH v2 1/5] sched, timer: Remove usages of ACCESS_ONCE in
 the scheduler

On 04/28/2015 04:00 PM, Jason Low wrote:
> ACCESS_ONCE doesn't work reliably on non-scalar types. This patch removes
> the rest of the existing usages of ACCESS_ONCE in the scheduler, and use
> the new READ_ONCE and WRITE_ONCE APIs.
>
> Signed-off-by: Jason Low<jason.low2@...com>
> ---
>   include/linux/sched.h          |    4 ++--
>   kernel/fork.c                  |    2 +-
>   kernel/sched/auto_group.c      |    2 +-
>   kernel/sched/auto_group.h      |    2 +-
>   kernel/sched/core.c            |    4 ++--
>   kernel/sched/cputime.c         |    2 +-
>   kernel/sched/deadline.c        |    2 +-
>   kernel/sched/fair.c            |   14 +++++++-------
>   kernel/sched/proc.c            |    4 ++--
>   kernel/sched/rt.c              |    2 +-
>   kernel/sched/sched.h           |    2 +-
>   kernel/sched/wait.c            |    4 ++--
>   kernel/time/posix-cpu-timers.c |    8 ++++----
>   13 files changed, 26 insertions(+), 26 deletions(-)
>
> ...
>   		goto no_join;
> @@ -2107,7 +2107,7 @@ 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);
>   	p->mm->numa_scan_offset = 0;
>   }
>
>

Generally, I am for replacing ACCESS_ONCE() with the more descriptive 
READ_ONCE() and WRITE_ONCE() except the above case where it makes the 
code harder to read without any real advantage.

Other than that,

Acked-by: Waiman Long <Waiman.Long@...com>

Cheers,
Longman
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