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Date:	Tue, 29 Dec 2009 15:24:41 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>,
	cl@...ux-foundation.org
Subject: Re: [RFC PATCH] asynchronous page fault.

* Peter Zijlstra <peterz@...radead.org> [2009-12-28 09:32:53]:

> On Mon, 2009-12-28 at 06:27 +0530, Balbir Singh wrote:
> > * Peter Zijlstra <peterz@...radead.org> [2009-12-27 12:19:56]:
> > 
> > > Your changelog states as much.
> > > 
> > > "Even if RB-tree rotation occurs while we walk tree for look-up, we just
> > > miss vma without oops."
> > > 
> > > However, since this is the case, do we still need the
> > > rcu_assign_pointer() conversion your patch does? All I can see it do is
> > > slow down all RB-tree users, without any gain.
> > 
> > Don't we need the rcu_assign_pointer() on the read side primarily to
> > make sure the pointer is still valid and assignments (writes) are not
> > re-ordered? Are you suggesting that the pointer assignment paths be
> > completely atomic?
> 
> rcu_assign_pointer() is the write side, but if you need a barrier, you
> can make do with a single smp_wmb() after doing the rb-tree op. There is
> no need to add multiple in the tree-ops themselves.
>

Yes, that makes sense.
 
> You cannot make the assignment paths atomic (without locks) that's the
> whole problem.
>

True, but pre-emption can be nasty in some cases. But I am no expert
in the atomicity of operations like assignments across architectures.
I assume all word, long assignments are.

-- 
	Balbir
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