linux-kernel - [PATCH 3.16 045/158] powerpc/thp: Use ACCESS

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Date:	Mon, 15 Sep 2014 12:24:44 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: [PATCH 3.16 045/158] powerpc/thp: Use ACCESS_ONCE when loading pmdp

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

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

From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>

commit 7e467245bf5226db34c4b12d3cbacfa2f7a15a8b upstream.

We would get wrong results in compiler recomputed old_pmd. Avoid
that by using ACCESS_ONCE

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/powerpc/mm/hugepage-hash64.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/powerpc/mm/hugepage-hash64.c
+++ b/arch/powerpc/mm/hugepage-hash64.c
@@ -84,7 +84,9 @@ int __hash_page_thp(unsigned long ea, un
 	 * atomically mark the linux large page PMD busy and dirty
 	 */
 	do {
-		old_pmd = pmd_val(*pmdp);
+		pmd_t pmd = ACCESS_ONCE(*pmdp);
+
+		old_pmd = pmd_val(pmd);
 		/* If PMD busy, retry the access */
 		if (unlikely(old_pmd & _PAGE_BUSY))
 			return 0;


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