linux-kernel - Re: [PATCH 4/4] cpuset,mm: use rwlock to protect task->mempolicy and mems

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4B9879E1.6000606@cn.fujitsu.com>
Date:	Thu, 11 Mar 2010 13:04:33 +0800
From:	Miao Xie <miaox@...fujitsu.com>
To:	Paul Menage <menage@...gle.com>
CC:	David Rientjes <rientjes@...gle.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 4/4] cpuset,mm: use rwlock to protect task->mempolicy
 and 	mems_allowed

on 2010-3-10 3:42, Paul Menage wrote:
> On Sat, Mar 6, 2010 at 6:33 PM, Miao Xie <miaox@...fujitsu.com> 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 from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/