> 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 <[email protected]>
> ---
> 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);
Probably safer to use 'acq_release' order here?
We do want that all previous loads to be completed at that point.
> + 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
