linux-kernel - Re: [PATCH 09/12] KVM: MMU: introduce pte-list lockless walker

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Date:	Wed, 28 Aug 2013 13:49:38 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	avi.kivity@...il.com, mtosatti@...hat.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 09/12] KVM: MMU: introduce pte-list lockless walker

On Wed, Aug 28, 2013 at 06:13:43PM +0800, Xiao Guangrong wrote:
> On 08/28/2013 05:46 PM, Gleb Natapov wrote:
> > On Wed, Aug 28, 2013 at 05:33:49PM +0800, Xiao Guangrong wrote:
> >>> Or what if desc is moved to another rmap, but then it
> >>> is moved back to initial rmap (but another place in the desc list) so
> >>> the check here will not catch that we need to restart walking?
> >>
> >> It is okay. We always add the new desc to the head, then we will walk
> >> all the entires under this case.
> >>
> > Which races another question: What if desc is added in front of the list
> > behind the point where lockless walker currently is?
> 
> That case is new spte is being added into the rmap. We need not to care the
> new sptes since it will set the dirty-bitmap then they can be write-protected
> next time.
> 
OK.

> > 
> >> Right?
> > Not sure. While lockless walker works on a desc rmap can be completely
> > destroyed and recreated again. It can be any order.
> 
> I think the thing is very similar as include/linux/rculist_nulls.h
include/linux/rculist_nulls.h is for implementing hash tables, so they
may not care about add/del/lookup race for instance, but may be we are
(you are saying above that we are not), so similarity does not prove
correctness for our case. BTW I do not see
rcu_assign_pointer()/rcu_dereference() in your patches which hints on
incorrect usage of RCU. I think any access to slab pointers will need to
use those.

--
			Gleb.
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