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Subject[PATCH 3/3] alpha: atomics: Add smp_read_barrier_depends() to release/relaxed atomics
Date
As part of the fight against smp_read_barrier_depends(), we require
dependency ordering to be preserved when a dependency is headed by a load
performed using an atomic operation.

This patch adds smp_read_barrier_depends() to the _release and _relaxed
atomics on alpha, which otherwise lack anything to enforce dependency
ordering.

Signed-off-by: Will Deacon <will.deacon@arm.com>
---
arch/alpha/include/asm/atomic.h | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/arch/alpha/include/asm/atomic.h b/arch/alpha/include/asm/atomic.h
index 498933a7df97..16961a3f45ba 100644
--- a/arch/alpha/include/asm/atomic.h
+++ b/arch/alpha/include/asm/atomic.h
@@ -13,6 +13,15 @@
* than regular operations.
*/

+/*
+ * To ensure dependency ordering is preserved for the _relaxed and
+ * _release atomics, an smp_read_barrier_depends() is unconditionally
+ * inserted into the _relaxed variants, which are used to build the
+ * barriered versions. To avoid redundant back-to-back fences, we can
+ * define the _acquire and _fence versions explicitly.
+ */
+#define __atomic_op_acquire(op, args...) op##_relaxed(args)
+#define __atomic_op_fence __atomic_op_release

#define ATOMIC_INIT(i) { (i) }
#define ATOMIC64_INIT(i) { (i) }
@@ -60,6 +69,7 @@ static inline int atomic_##op##_return_relaxed(int i, atomic_t *v) \
".previous" \
:"=&r" (temp), "=m" (v->counter), "=&r" (result) \
:"Ir" (i), "m" (v->counter) : "memory"); \
+ smp_read_barrier_depends(); \
return result; \
}

@@ -77,6 +87,7 @@ static inline int atomic_fetch_##op##_relaxed(int i, atomic_t *v) \
".previous" \
:"=&r" (temp), "=m" (v->counter), "=&r" (result) \
:"Ir" (i), "m" (v->counter) : "memory"); \
+ smp_read_barrier_depends(); \
return result; \
}

@@ -111,6 +122,7 @@ static __inline__ long atomic64_##op##_return_relaxed(long i, atomic64_t * v) \
".previous" \
:"=&r" (temp), "=m" (v->counter), "=&r" (result) \
:"Ir" (i), "m" (v->counter) : "memory"); \
+ smp_read_barrier_depends(); \
return result; \
}

@@ -128,6 +140,7 @@ static __inline__ long atomic64_fetch_##op##_relaxed(long i, atomic64_t * v) \
".previous" \
:"=&r" (temp), "=m" (v->counter), "=&r" (result) \
:"Ir" (i), "m" (v->counter) : "memory"); \
+ smp_read_barrier_depends(); \
return result; \
}

--
2.1.4
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