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

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Date:	Wed, 15 Apr 2015 19:29:01 -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

On Tue, 2015-04-14 at 22:40 -0400, Steven Rostedt wrote:
> 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;

Just to confirm, is this a typo? Because there really is a numa_scan_seq
in the task_struct itself too :)

p->mm->numa_scan_seq is read in task_numa_placement() with
ACCESS_ONCE(), and so the benefit that I do see with it is that it makes
it consistent by doing the updates with ACCESS_ONCE too (for
documentation purposes).

If that's really the case:

    WRITE_ONCE(p->mm->numa_scan_seq, p->mm->numa_scan_seq + 1)

should be enough for that.

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