From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751050Ab0CKFEj (ORCPT ); Thu, 11 Mar 2010 00:04:39 -0500 Received: from cn.fujitsu.com ([222.73.24.84]:63418 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1750721Ab0CKFEi (ORCPT ); Thu, 11 Mar 2010 00:04:38 -0500 Message-ID: <4B9879E1.6000606@cn.fujitsu.com> Date: Thu, 11 Mar 2010 13:04:33 +0800 From: Miao Xie Reply-To: miaox@cn.fujitsu.com User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; zh-CN; rv:1.9.1.8) Gecko/20100227 Thunderbird/3.0.3 MIME-Version: 1.0 To: Paul Menage CC: David Rientjes , Lee Schermerhorn , Nick Piggin , Linux-Kernel , Linux-MM Subject: Re: [PATCH 4/4] cpuset,mm: use rwlock to protect task->mempolicy and mems_allowed References: <4B8E3F77.6070201@cn.fujitsu.com> <6599ad831003050403v2e988723k1b6bf38d48707ab1@mail.gmail.com> <4B931068.70900@cn.fujitsu.com> <6599ad831003091142t38c9ffc9rea7d351742ecbd98@mail.gmail.com> In-Reply-To: <6599ad831003091142t38c9ffc9rea7d351742ecbd98@mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org on 2010-3-10 3:42, Paul Menage wrote: > On Sat, Mar 6, 2010 at 6:33 PM, Miao Xie wrote: >> >> Before applying this patch, cpuset updates task->mems_allowed just like >> what you said. But the allocator is still likely to see an empty nodemask. >> This problem have been pointed out by Nick Piggin. >> >> The problem is following: >> The size of nodemask_t is greater than the size of long integer, so loading >> and storing of nodemask_t are not atomic operations. If task->mems_allowed >> don't intersect with new_mask, such as the first word of the mask is empty >> and only the first word of new_mask is not empty. When the allocator >> loads a word of the mask before >> >> current->mems_allowed |= new_mask; >> >> and then loads another word of the mask after >> >> current->mems_allowed = new_mask; >> >> the allocator gets an empty nodemask. > > Couldn't that be solved by having the reader read the nodemask twice > and compare them? In the normal case there's no race, so the second > read is straight from L1 cache and is very cheap. In the unlikely case > of a race, the reader would keep trying until it got two consistent > values in a row. I think this method can't fix the problem because we can guarantee the second read is after the update of mask completes. Thanks! Miao > > Paul > > >