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

[Konstantin Ananyev]

> 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 at 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);

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