linux-kernel - [PATCH 3.19 175/175] x86/xen/p2m: Replace ACCESS_ONCE with READ

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Date:	Tue,  3 Mar 2015 22:15:53 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Juergen Gross <jgross@...e.com>,
	David Vrabel <david.vrabel@...rix.com>
Subject: [PATCH 3.19 175/175] x86/xen/p2m: Replace ACCESS_ONCE with READ_ONCE

3.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Christian Borntraeger <borntraeger@...ibm.com>

commit 1760f1eb7ec485197bd3a8a9c13e4160bb740275 upstream.

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>
Acked-by: David Vrabel <david.vrabel@...rix.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/xen/p2m.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 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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