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

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Message-ID: <4D487C6B.9000003@goop.org>
Date:	Tue, 01 Feb 2011 13:34:35 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	linux-kernel@...r.kernel.org, Xen-devel@...ts.xensource.com,
	konrad@...nel.org, hpa@...or.com, stefano.stabellini@...citrix.com,
	Ian.Campbell@...citrix.com
Subject: Re: [PATCH 04/11] xen/mmu: BUG_ON when racing to swap middle leaf.

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@...citrix.com>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
>  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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