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From: Xiao Guangrong <xiaoguangrong.eric@gmail.com>
To: Marcelo Tosatti <mtosatti@redhat.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
Date: Tue, 10 Dec 2013 14:58:12 +0800	[thread overview]
Message-ID: <52A6BB84.30706@gmail.com> (raw)
In-Reply-To: <20131206002244.GA29960@amt.cnet>

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!



  reply	other threads:[~2013-12-10  6:58 UTC|newest]

Thread overview: 69+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-10-23 13:29 [PATCH v3 00/15] KVM: MMU: locklessly write-protect Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 01/15] KVM: MMU: properly check last spte in fast_page_fault() Xiao Guangrong
2013-11-12  0:25   ` Marcelo Tosatti
2013-10-23 13:29 ` [PATCH v3 02/15] KVM: MMU: lazily drop large spte Xiao Guangrong
2013-11-12 22:44   ` Marcelo Tosatti
2013-10-23 13:29 ` [PATCH v3 03/15] KVM: MMU: flush tlb if the spte can be locklessly modified Xiao Guangrong
2013-11-13  0:10   ` Marcelo Tosatti
2013-10-23 13:29 ` [PATCH v3 04/15] KVM: MMU: flush tlb out of mmu lock when write-protect the sptes Xiao Guangrong
2013-11-14  0:36   ` Marcelo Tosatti
2013-11-14  5:15     ` Xiao Guangrong
2013-11-14 18:39       ` Marcelo Tosatti
2013-11-15  7:09         ` Xiao Guangrong
2013-11-19  0:19           ` Marcelo Tosatti
2013-10-23 13:29 ` [PATCH v3 05/15] KVM: MMU: update spte and add it into rmap before dirty log Xiao Guangrong
2013-11-15  0:08   ` Marcelo Tosatti
2013-10-23 13:29 ` [PATCH v3 06/15] KVM: MMU: redesign the algorithm of pte_list Xiao Guangrong
2013-11-19  0:48   ` Marcelo Tosatti
2013-10-23 13:29 ` [PATCH v3 07/15] KVM: MMU: introduce nulls desc Xiao Guangrong
2013-11-22 19:14   ` Marcelo Tosatti
2013-11-25  6:11     ` Xiao Guangrong
2013-11-25  6:29       ` Xiao Guangrong
2013-11-25 18:12         ` Marcelo Tosatti
2013-11-26  3:21           ` Xiao Guangrong
2013-11-26 10:12             ` Gleb Natapov
2013-11-26 19:31             ` Marcelo Tosatti
2013-11-28  8:53               ` Xiao Guangrong
2013-12-03  7:10                 ` Xiao Guangrong
2013-12-05 13:50                   ` Marcelo Tosatti
2013-12-05 15:30                     ` Xiao Guangrong
2013-12-06  0:15                       ` Marcelo Tosatti
2013-12-06  0:22                       ` Marcelo Tosatti
2013-12-10  6:58                         ` Xiao Guangrong [this message]
2013-11-25 10:19       ` Gleb Natapov
2013-11-25 10:25         ` Xiao Guangrong
2013-11-25 12:48       ` Avi Kivity
2013-11-25 14:23         ` Marcelo Tosatti
2013-11-25 14:29           ` Gleb Natapov
2013-11-25 18:06             ` Marcelo Tosatti
2013-11-26  3:10           ` Xiao Guangrong
2013-11-26 10:15             ` Gleb Natapov
2013-11-26 19:58             ` Marcelo Tosatti
2013-11-28  8:32               ` Xiao Guangrong
2013-11-25 14:08       ` Marcelo Tosatti
2013-11-26  3:02         ` Xiao Guangrong
2013-11-25  9:31     ` Peter Zijlstra
2013-11-25 10:59       ` Xiao Guangrong
2013-11-25 11:05         ` Peter Zijlstra
2013-11-25 11:29           ` Peter Zijlstra
2013-10-23 13:29 ` [PATCH v3 08/15] KVM: MMU: introduce pte-list lockless walker Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 09/15] KVM: MMU: initialize the pointers in pte_list_desc properly Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 10/15] KVM: MMU: allocate shadow pages from slab Xiao Guangrong
2013-10-24  9:19   ` Gleb Natapov
2013-10-24  9:29     ` Xiao Guangrong
2013-10-24  9:52       ` Gleb Natapov
2013-10-24 10:10         ` Xiao Guangrong
2013-10-24 10:39           ` Gleb Natapov
2013-10-24 11:01             ` Xiao Guangrong
2013-10-24 12:32               ` Gleb Natapov
2013-10-28  3:16                 ` Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 11/15] KVM: MMU: locklessly access shadow page under rcu protection Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 12/15] KVM: MMU: check last spte with unawareness of mapping level Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 13/15] KVM: MMU: locklessly write-protect the page Xiao Guangrong
2013-10-24  9:17   ` Gleb Natapov
2013-10-24  9:24     ` Xiao Guangrong
2013-10-24  9:32       ` Gleb Natapov
2013-10-23 13:29 ` [PATCH v3 14/15] KVM: MMU: clean up spte_write_protect Xiao Guangrong
2013-10-23 13:29 ` [PATCH v3 15/15] KVM: MMU: use rcu functions to access the pointer Xiao Guangrong
2013-11-03 12:29 ` [PATCH v3 00/15] KVM: MMU: locklessly write-protect Gleb Natapov
2013-11-11  5:33   ` Xiao Guangrong

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