linux-kernel - [PATCH 3.13 184/187] powerpc/thp: Use ACCESS

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Date:	Mon, 15 Sep 2014 15:09:54 -0700
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 3.13 184/187] powerpc/thp: Use ACCESS_ONCE when loading pmdp

3.13.11.7 -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: Kamal Mostafa <kamal@...onical.com>
---
 arch/powerpc/mm/hugepage-hash64.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/hugepage-hash64.c b/arch/powerpc/mm/hugepage-hash64.c
index e0da847..7d86c86 100644
--- 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, unsigned long access, unsigned long vsid,
 	 * 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;
-- 
1.9.1

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