From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with SMTP id 64A6F6B00A0 for ; Thu, 11 Mar 2010 00:04:40 -0500 (EST) 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 MIME-Version: 1.0 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: owner-linux-mm@kvack.org To: Paul Menage Cc: David Rientjes , Lee Schermerhorn , Nick Piggin , Linux-Kernel , Linux-MM List-ID: 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 > > > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org