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

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> 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.

Ian

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