linux-kernel - Re: [regression] cpuset,mm: update tasks' mems

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Message-ID: <4B84F645.6030404@cn.fujitsu.com>
Date:	Wed, 24 Feb 2010 17:49:57 +0800
From:	Miao Xie <miaox@...fujitsu.com>
To:	David Rientjes <rientjes@...gle.com>
CC:	Nick Piggin <npiggin@...e.de>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Lee Schermerhorn <lee.schermerhorn@...com>
Subject: Re: [regression] cpuset,mm: update tasks' mems_allowed in time (58568d2)

on 2010-2-23 16:44, David Rientjes wrote:
> On Tue, 23 Feb 2010, Miao Xie wrote:
> 
>> Sorry, Could you explain what you advised?
>> I think it is hard to fix this problem by adding a variant, because it is
>> hard to avoid loading a word of the mask before
>>
>> 	nodes_or(tsk->mems_allowed, tsk->mems_allowed, *newmems);
>>
>> and then loading another word of the mask after
>>
>> 	tsk->mems_allowed = *newmems;
>>
>> unless we use lock.
>>
>> Maybe we need a rw-lock to protect task->mems_allowed.
>>
> 
> I meant that we need to define synchronization only for configurations 
> that do not do atomic nodemask_t stores, it's otherwise unnecessary.  
> We'll need to load and store tsk->mems_allowed via a helper function that 
> is defined to take the rwlock for such configs and only read/write the 
> nodemask for others.
> 

By investigating, we found that it is hard to guarantee the consistent between
mempolicy and mems_allowed because mempolicy was designed as a self-update function.
it just can be changed by one's self. Maybe we must change the implement of mempolicy.

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