locking/atomics/alpha: Add smp_read_barrier_depends() to _release()/_relaxed() atomics

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>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1508840570-22169-6-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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;							\
 }