linux-kernel - Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc

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Date:	Tue, 10 Dec 2013 14:58:12 +0800
From:	Xiao Guangrong <xiaoguangrong.eric@...il.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Gleb Natapov <gleb@...hat.com>, avi.kivity@...il.com,
	"pbonzini@...hat.com Bonzini" <pbonzini@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Eric Dumazet <dada1@...mosbay.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 07/15] KVM: MMU: introduce nulls desc

On 12/06/2013 08:22 AM, Marcelo Tosatti wrote:
> 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).

Yep, Marcelo, you're right.

After thinking more, i do not have any idea to simplify this. Your approach
(lockless on the first level) seems a better solution. Will do it based on that
ways.

Thanks!


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