Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752470Ab3J0IJz (ORCPT ); Sun, 27 Oct 2013 04:09:55 -0400 Received: from mail-ob0-f201.google.com ([209.85.214.201]:42749 "EHLO mail-ob0-f201.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751331Ab3J0IJt (ORCPT ); Sun, 27 Oct 2013 04:09:49 -0400 X-Greylist: delayed 1205 seconds by postgrey-1.27 at vger.kernel.org; Sun, 27 Oct 2013 04:09:49 EDT From: Greg Thelen To: Tejun Heo , Christoph Lameter , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Johannes Weiner , Michal Hocko , Balbir Singh , KAMEZAWA Hiroyuki , handai.szj@taobao.com, Andrew Morton Cc: x86@kernel.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org, Greg Thelen Subject: [PATCH 0/3] fix unsigned pcp adjustments Date: Sun, 27 Oct 2013 00:44:33 -0700 Message-Id: <1382859876-28196-1-git-send-email-gthelen@google.com> X-Mailer: git-send-email 1.8.4.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2125 Lines: 53 As of v3.11-9444-g3ea67d0 "memcg: add per cgroup writeback pages accounting" memcg use of __this_cpu_add(counter, -nr_pages) leads to incorrect statistic values because the negated nr_pages is not sign extended (counter is long, nr_pages is unsigned int). The memcg fix is __this_cpu_sub(counter, nr_pages). But that doesn't simply work because __this_cpu_sub(counter, nr_pages) was implemented as __this_cpu_add(counter, -nr_pages) which suffers the same problem. Example: unsigned int delta = 1 preempt_disable() this_cpu_write(long_counter, 0) this_cpu_sub(long_counter, delta) preempt_enable() Before this change long_counter on a 64 bit machine ends with value 0xffffffff, rather than 0xffffffffffffffff. This is because this_cpu_sub(pcp, delta) boils down to: long_counter = 0 + 0xffffffff Patch 1 creates a test module for percpu counters operations which demonstrates the __this_cpu_sub() problems. This patch is independent can be discarded if there is no interest. Patch 2 fixes __this_cpu_sub() to work with unsigned adjustments. Patch 3 uses __this_cpu_sub() in memcg. An alternative smaller solution is for memcg to use: __this_cpu_add(counter, -(int)nr_pages) admitting that __this_cpu_add/sub() doesn't work with unsigned adjustments. But I felt like fixing the core services to prevent this in the future. Greg Thelen (3): percpu counter: test module percpu counter: cast this_cpu_sub() adjustment memcg: use __this_cpu_sub to decrement stats arch/x86/include/asm/percpu.h | 3 +- include/linux/percpu.h | 8 +-- lib/Kconfig.debug | 9 +++ lib/Makefile | 2 + lib/percpu_test.c | 138 ++++++++++++++++++++++++++++++++++++++++++ mm/memcontrol.c | 2 +- 6 files changed, 156 insertions(+), 6 deletions(-) create mode 100644 lib/percpu_test.c -- 1.8.4.1 -- 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/