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

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Message-ID: <20130829093141.GC22899@redhat.com>
Date:	Thu, 29 Aug 2013 12:31:41 +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 Thu, Aug 29, 2013 at 02:50:51PM +0800, Xiao Guangrong wrote:
> After more thinking, I still think rcu_assign_pointer() is unneeded when a entry
> is removed. The remove-API does not care the order between unlink the entry and
> the changes to its fields. It is the caller's responsibility:
> - in the case of rcuhlist, the caller uses call_rcu()/synchronize_rcu(), etc to
>   enforce all lookups exit and the later change on that entry is invisible to the
>   lookups.
> 
> - In the case of rculist_nulls, it seems refcounter is used to guarantee the order
>   (see the example from Documentation/RCU/rculist_nulls.txt).
> 
> - In our case, we allow the lookup to see the deleted desc even if it is in slab cache
>   or its is initialized or it is re-added.
> 
BTW is it a good idea? We can access deleted desc while it is allocated
and initialized to zero by kmem_cache_zalloc(), are we sure we cannot
see partially initialized desc->sptes[] entry? On related note what about
32 bit systems, they do not have atomic access to desc->sptes[].

--
			Gleb.
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