linux-kernel - Re: [PATCH 3/8] x86/xen/p2m: Replace ACCESS_ONCE with READ

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Date:	Thu, 15 Jan 2015 10:26:47 +0100
From:	Jürgen Groß <jgross@...e.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>,
	linux-kernel@...r.kernel.org
CC:	linux-arch@...r.kernel.org, kvm@...r.kernel.org,
	kvm-ppc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	x86@...nel.org, xen-devel@...ts.xenproject.org, linux-mm@...ck.org
Subject: Re: [PATCH 3/8] x86/xen/p2m: Replace ACCESS_ONCE with READ_ONCE

On 01/15/2015 09:58 AM, Christian Borntraeger wrote:
> ACCESS_ONCE does not work reliably on non-scalar types. For
> example gcc 4.6 and 4.7 might remove the volatile tag for such
> accesses during the SRA (scalar replacement of aggregates) step
> (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145)
>
> Change the p2m code to replace ACCESS_ONCE with READ_ONCE.
>
> Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>

Reviewed-by: Juergen Gross <jgross@...e.com>

> ---
>   arch/x86/xen/p2m.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index edbc7a6..cb71016 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -554,7 +554,7 @@ static bool alloc_p2m(unsigned long pfn)
>   		mid_mfn = NULL;
>   	}
>
> -	p2m_pfn = pte_pfn(ACCESS_ONCE(*ptep));
> +	p2m_pfn = pte_pfn(READ_ONCE(*ptep));
>   	if (p2m_pfn == PFN_DOWN(__pa(p2m_identity)) ||
>   	    p2m_pfn == PFN_DOWN(__pa(p2m_missing))) {
>   		/* p2m leaf page is missing */
>

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