Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1031154AbbD1UAv (ORCPT ); Tue, 28 Apr 2015 16:00:51 -0400 Received: from g4t3427.houston.hp.com ([15.201.208.55]:42307 "EHLO g4t3427.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030826AbbD1UAr (ORCPT ); Tue, 28 Apr 2015 16:00:47 -0400 From: Jason Low To: Peter Zijlstra , Ingo Molnar , Thomas Gleixner Cc: linux-kernel@vger.kernel.org, "Paul E. McKenney" , Andrew Morton , Oleg Nesterov , Frederic Weisbecker , Mel Gorman , Rik van Riel , Steven Rostedt , Preeti U Murthy , Mike Galbraith , Davidlohr Bueso , Waiman Long , Aswin Chandramouleeswaran , Scott J Norton , Jason Low Subject: [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan_seq Date: Tue, 28 Apr 2015 13:00:21 -0700 Message-Id: <1430251224-5764-3-git-send-email-jason.low2@hp.com> X-Mailer: git-send-email 1.7.9.5 In-Reply-To: <1430251224-5764-1-git-send-email-jason.low2@hp.com> References: <1430251224-5764-1-git-send-email-jason.low2@hp.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1699 Lines: 47 The p->mm->numa_scan_seq is accessed using READ_ONCE/WRITE_ONCE and modified without exclusive access. It is not clear why it is accessed this way. This patch provides some documentation on that. Signed-off-by: Jason Low --- kernel/sched/fair.c | 12 ++++++++++++ 1 files changed, 12 insertions(+), 0 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 5a44371..794f7d7 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; } -- 1.7.2.5 -- 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/