From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail138.messagelabs.com (mail138.messagelabs.com [216.82.249.35]) by kanga.kvack.org (Postfix) with SMTP id A6D306B00B0 for ; Thu, 11 Mar 2010 02:57:28 -0500 (EST) Message-ID: <4B98A263.8030903@cn.fujitsu.com> Date: Thu, 11 Mar 2010 15:57:23 +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> <4B9879E1.6000606@cn.fujitsu.com> <20100311053059.GG5812@laptop> In-Reply-To: <20100311053059.GG5812@laptop> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Nick Piggin Cc: Paul Menage , David Rientjes , Lee Schermerhorn , Linux-Kernel , Linux-MM List-ID: on 2010-3-11 13:30, Nick Piggin wrote: >>>> 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. > > Any problem with using a seqlock? > > The other thing you could do is store a pointer to the nodemask, and > allocate a new nodemask when changing it, issue a smp_wmb(), and then > store the new pointer. Read side only needs a smp_read_barrier_depends() Comparing with my second version patch, I think both of these methods will cause worse performance and the changing of code is more. Thanks Miao -- 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