[RFC 13/32] ring: replace SMP read barrier with C11 acquire fence

[Stephen Hemminger <stephen@networkplumber.org>]

rte_smp_rmb() is deprecated. The acquire fence generates the same
code everywhere the gcc implementation is used: a compiler barrier
on x86 and dmb ishld on ThunderX.

The gcc implementation itself is kept: unlike the lock-free stack,
the in-tree comment records a 10% x86 performance drop with the
C11 version, which needs re-measuring with current compilers
before RTE_USE_C11_MEM_MODEL can be removed.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 lib/ring/rte_ring_gcc_pvt.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lib/ring/rte_ring_gcc_pvt.h b/lib/ring/rte_ring_gcc_pvt.h
index 340ece28c7..9c2c58824d 100644
--- a/lib/ring/rte_ring_gcc_pvt.h
+++ b/lib/ring/rte_ring_gcc_pvt.h
@@ -55,10 +55,10 @@ __rte_ring_headtail_move_head_st(struct rte_ring_headtail *d,
 
 	*old_head = d->head;
 
-	/* add rmb barrier to avoid load/load reorder in weak
-	 * memory model. It is noop on x86
+	/* Acquire fence to avoid load/load reorder in weak
+	 * memory model. It is noop on x86.
 	 */
-	rte_smp_rmb();
+	rte_atomic_thread_fence(rte_memory_order_acquire);
 
 	/*
 	 *  The subtraction is done between two unsigned 32bits value
-- 
2.53.0