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Date: Thu, 5 Dec 2013 22:22:44 -0200
From: Marcelo Tosatti <mtosatti@redhat.com>
To: Xiao Guangrong <xiaoguangrong.eric@gmail.com>
Cc: Gleb Natapov <gleb@redhat.com>, avi.kivity@gmail.com,
        "pbonzini@redhat.com Bonzini" <pbonzini@redhat.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Eric Dumazet <dada1@cosmosbay.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc
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On Thu, Dec 05, 2013 at 11:30:27PM +0800, Xiao Guangrong wrote:
> In some cases, the lockless walker will do endless-walking on desc and
> without rewalk, consider this case:
> 
> there are two descs: desc1 and desc2 who is pointed by desc1->next：
> desc1->next = desc2.
> 
> CPU 0                                                                            CPU 1
> 
> lockless walk on desc1
>                                                                  deleting desc1 from the rmap
> lockless walk on desc2 (desc1->next)
>                                                                 delete desc2 from the rmap
>                                                                 add desc1
>                                                                 add desc2, then desc2->next = desc1
> 
> lockless walk on desc1
>                                                                delete desc2
>                                                                delete desc1
>                                                                add desc2
>                                                                add desc1; the desc1->next = desc2
> lockless walk on desc2
> 
> ……
> 
> Then, the walker is endlessly walking on desc1 and desc2 without any rewalk.

The counter can be local to the walker. If its more than maximum rmap
size, break.

(but still, please consider carefully whether lockless list walking is
really necessary or can be avoided).