Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752448AbbDQD0L (ORCPT ); Thu, 16 Apr 2015 23:26:11 -0400 Received: from g4t3427.houston.hp.com ([15.201.208.55]:47482 "EHLO g4t3427.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751276AbbDQD0D (ORCPT ); Thu, 16 Apr 2015 23:26:03 -0400 Message-ID: <1429241148.7039.187.camel@j-VirtualBox> Subject: Re: [PATCH 1/3] sched, timer: Remove usages of ACCESS_ONCE in the scheduler From: Jason Low To: Ingo Molnar Cc: Peter Zijlstra , Steven Rostedt , Rik van Riel , Linus Torvalds , Thomas Gleixner , linux-kernel@vger.kernel.org, "Paul E. McKenney" , Andrew Morton , Oleg Nesterov , Mike Galbraith , Frederic Weisbecker , Mel Gorman , Preeti U Murthy , hideaki.kimura@hp.com, Aswin Chandramouleeswaran , Scott J Norton , jason.low2@hp.com Date: Thu, 16 Apr 2015 20:25:48 -0700 In-Reply-To: <20150416182426.GA17852@gmail.com> 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> <1429063953.7039.88.camel@j-VirtualBox> <20150414224059.061ec5bf@grimm.local.home> <20150415074601.GC13449@gmail.com> <20150416165224.GD12676@worktop.ger.corp.intel.com> <20150416180227.GB17401@gmail.com> <20150416181535.GA23123@twins.programming.kicks-ass.net> <20150416182426.GA17852@gmail.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1626 Lines: 45 On Thu, 2015-04-16 at 20:24 +0200, Ingo Molnar wrote: > Would it make sense to add a few comments to the seq field definition > site(s), about how it's supposed to be accessed - or to the > READ_ONCE()/WRITE_ONCE() sites, to keep people from wondering? How about this: --- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 5a44371..63fa87f 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -1794,6 +1794,11 @@ static void task_numa_placement(struct task_struct *p) u64 runtime, period; spinlock_t *group_lock = NULL; + /* + * The p->mm->numa_scan_seq gets updated without + * exclusive access. Use READ_ONCE() here to ensure + * that the field is read in a single access. + */ seq = READ_ONCE(p->mm->numa_scan_seq); if (p->numa_scan_seq == seq) return; @@ -2107,6 +2112,13 @@ void task_numa_fault(int last_cpupid, int mem_node, int pages, int flags) static void reset_ptenuma_scan(struct task_struct *p) { + /* + * We only did a read acquisition of the mmap sem, so + * p->mm->numa_scan_seq is written to without exclusive access. + * That's not much of an issue though, since this is just used + * for statistical sampling. Use WRITE_ONCE and READ_ONCE, which + * are not expensive, to avoid load/store tearing. + */ WRITE_ONCE(p->mm->numa_scan_seq, READ_ONCE(p->mm->numa_scan_seq) + 1); p->mm->numa_scan_offset = 0; } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/