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

Xiao Guangrong
2013-12-10T06:58:28Z

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!