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RE: [Xen-devel] [PATCH] "lock cmpxch8b" and split locks

To: "Puthiyaparambil, Aravindh" <aravindh.puthiyaparambil@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] [PATCH] "lock cmpxch8b" and split locks
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Wed, 5 Oct 2005 20:49:07 +0100
Cc: "Koren, Bradley J" <Bradley.Koren@xxxxxxxxxx>, "Subrahmanian, Raj" <raj.subrahmanian@xxxxxxxxxx>, "Vessey, Bruce A" <Bruce.Vessey@xxxxxxxxxx>
Delivery-date: Wed, 05 Oct 2005 19:46:35 +0000
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Thread-topic: [Xen-devel] [PATCH] "lock cmpxch8b" and split locks
> I have been looking at the following bug
> http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=252
> That investigation is still underway. 
> While looking at this problem I found that in "lock cmpxch8b 
> &page->count_info" occurring in "get_page( )", 
> "&page->count_info" is on a misaligned address. Furthermore 
> the address of "page" is not modulo 64. So the "lock 
> cmpxch8b" can happen across cache lines and lead to a split 
> lock. On systems which have multiple buses, the atomicity of 
> such a statement is not assured and can cause synchronization 
> problems. 
> I have attached a patch which solves this issue.

We can't just increase the size of pfn_info structure -- it has
implications for the amount of VM space that Xen uses. This will require
more thought.


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