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SubjectRe: [PATCH 3/3] alpha: atomics: Add smp_read_barrier_depends() to release/relaxed atomics
On Thu, Oct 12, 2017 at 03:26:17PM +0100, Will Deacon wrote:
> 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>

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.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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