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

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Message-ID: <alpine.DEB.2.00.1002230041240.12015@chino.kir.corp.google.com>
Date:	Tue, 23 Feb 2010 00:44:02 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Miao Xie <miaox@...fujitsu.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 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.
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