[PATCH/RFC 6/7] arm64: Replace ACCESS_ONCE for spinlock code with barriers

[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)

Change the spinlock code to access the lock with a barrier.

Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
---
 arch/arm64/include/asm/spinlock.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h
index c45b7b1..f72dc64 100644
--- a/arch/arm64/include/asm/spinlock.h
+++ b/arch/arm64/include/asm/spinlock.h
@@ -99,12 +99,15 @@ static inline int arch_spin_value_unlocked(arch_spinlock_t lock)
 
 static inline int arch_spin_is_locked(arch_spinlock_t *lock)
 {
-	return !arch_spin_value_unlocked(ACCESS_ONCE(*lock));
+	arch_spinlock_t lockval = *lock;
+	barrier();
+	return !arch_spin_value_unlocked(lockval);
 }
 
 static inline int arch_spin_is_contended(arch_spinlock_t *lock)
 {
-	arch_spinlock_t lockval = ACCESS_ONCE(*lock);
+	arch_spinlock_t lockval = *lock;
+	barrier();
 	return (lockval.next - lockval.owner) > 1;
 }
 #define arch_spin_is_contended	arch_spin_is_contended
-- 
1.9.3

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