[RFC 01/32] bpf: replace deprecated SMP barriers with C11 fences

[Stephen Hemminger <stephen@networkplumber.org>]

The use counter handshake is a store-load pattern, so rte_smp_mb()
becomes a seq_cst thread fence; the read barrier in
bpf_eth_cbi_unuse() becomes an acquire fence. Same code generated
on x86 and arm64.

Use relaxed loads and stores for the counter itself. With
enable_stdatomic, the plain increment of an RTE_ATOMIC() field
compiled to a seq_cst add, i.e. two locked operations per burst.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 lib/bpf/bpf_pkt.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/lib/bpf/bpf_pkt.c b/lib/bpf/bpf_pkt.c
index f072fdaaed..a831b5ad86 100644
--- a/lib/bpf/bpf_pkt.c
+++ b/lib/bpf/bpf_pkt.c
@@ -80,9 +80,11 @@ static struct bpf_eth_cbh tx_cbh = {
 static __rte_always_inline void
 bpf_eth_cbi_inuse(struct bpf_eth_cbi *cbi)
 {
-	cbi->use++;
+	rte_atomic_store_explicit(&cbi->use,
+		rte_atomic_load_explicit(&cbi->use, rte_memory_order_relaxed) + 1,
+		rte_memory_order_relaxed);
 	/* make sure no store/load reordering could happen */
-	rte_smp_mb();
+	rte_atomic_thread_fence(rte_memory_order_seq_cst);
 }
 
 /*
@@ -92,8 +94,10 @@ static __rte_always_inline void
 bpf_eth_cbi_unuse(struct bpf_eth_cbi *cbi)
 {
 	/* make sure all previous loads are completed */
-	rte_smp_rmb();
-	cbi->use++;
+	rte_atomic_thread_fence(rte_memory_order_acquire);
+	rte_atomic_store_explicit(&cbi->use,
+		rte_atomic_load_explicit(&cbi->use, rte_memory_order_relaxed) + 1,
+		rte_memory_order_relaxed);
 }
 
 /*
@@ -105,9 +109,9 @@ bpf_eth_cbi_wait(const struct bpf_eth_cbi *cbi)
 	uint32_t puse;
 
 	/* make sure all previous loads and stores are completed */
-	rte_smp_mb();
+	rte_atomic_thread_fence(rte_memory_order_seq_cst);
 
-	puse = cbi->use;
+	puse = rte_atomic_load_explicit(&cbi->use, rte_memory_order_relaxed);
 
 	/* in use, busy wait till current RX/TX iteration is finished */
 	if ((puse & BPF_ETH_CBI_INUSE) != 0) {
@@ -439,7 +443,7 @@ bpf_eth_cbi_unload(struct bpf_eth_cbi *bc)
 {
 	/* mark this cbi as empty */
 	bc->cb = NULL;
-	rte_smp_mb();
+	rte_atomic_thread_fence(rte_memory_order_seq_cst);
 
 	/* make sure datapath doesn't use bpf anymore, then destroy bpf */
 	bpf_eth_cbi_wait(bc);
-- 
2.53.0