Re: [Xen-devel] get_page put_page cmpxchg problem?

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 21 Jul 2005, at 21:59, King, Steven R wrote:

> Hi Folks,
> In -unstable (I haven't updated in a few days), I'm investigating a 
> problem in which revalidate_l1() fails because get_page() fails due to 
> a count_info of 0.  While that problem is still hanging over me, I did 
> notice that get_page() in include/asm-x86/mm.h does a 64-bit cmpxchg 
> while put_page() only does a 32-bit cmpxchg.  Seemed odd to me and 
> possibly related to my bug.    I'm running on a 32-bit machine without 
> shadow tables.
>  
> Can anyone offer an explanation?
In put_page() we can be sure the refcnt is non-zero and so the domain 
ptr will not change under our feet. Therefore there's no need to 
include it in the cmpxchg when we update the reference count.
 -- Keir


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