[PATCH/RFC 4/7] x86: Replace ACCESS_ONCE in gup with a barrier

[Christian Borntraeger]

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)

Let's use a barrier instead of ACCESS_ONCE to avoid that issue.

Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
---
 arch/x86/mm/gup.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
index 207d9aef..35991f6 100644
--- a/arch/x86/mm/gup.c
+++ b/arch/x86/mm/gup.c
@@ -14,8 +14,12 @@
 
 static inline pte_t gup_get_pte(pte_t *ptep)
 {
+	pte_t pte;
+
 #ifndef CONFIG_X86_PAE
-	return ACCESS_ONCE(*ptep);
+	pte = *ptep;
+	barrier();
+	return pte;
 #else
 	/*
 	 * With get_user_pages_fast, we walk down the pagetables without taking
@@ -49,7 +53,6 @@ static inline pte_t gup_get_pte(pte_t *ptep)
 	 * very careful -- it does not atomically load the pte or anything that
 	 * is likely to be useful for you.
 	 */
-	pte_t pte;
 
 retry:
 	pte.pte_low = ptep->pte_low;
-- 
1.9.3

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