[Xen-devel] Re: [PATCH 04/11] xen/mmu: BUG_ON when racing to swap middle leaf.

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 01/31/2011 02:44 PM, Konrad Rzeszutek Wilk wrote:
> The initial bootup code uses set_phys_to_machine quite a lot, and after
> bootup it would be used by the balloon driver. The balloon driver does have
> mutex lock so this should not be necessary - but just in case, add
> a BUG_ON if we do hit this scenario.
>
> [v2: Change from WARN to BUG_ON]
> [v3: Rebased on top of xen->p2m code split]
> Reviewed-by: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> ---
>  arch/x86/xen/p2m.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 19b0a65..fbbd2ab 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -478,7 +478,8 @@ bool __set_phys_to_machine(unsigned long pfn, unsigned 
> long mfn)
>  
>               /* Swap over from MISSING to IDENTITY if needed. */
>               if (p2m_top[topidx][mididx] == p2m_missing) {
> -                     p2m_top[topidx][mididx] = p2m_identity;
> +                     BUG_ON(cmpxchg(&p2m_top[topidx][mididx], p2m_missing,
> +                             p2m_identity) != p2m_missing);

Don't put side-effects in BUG_ONs.  Why is it a bug anyway?

    J

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