Re: [dpdk-dev] [PATCH v2] mbuf: use C11 atomics for refcnt operations

[Phil Yang <Phil.Yang@arm.com>]

> -----Original Message-----
> From: dev <dev-bounces@dpdk.org> On Behalf Of Phil Yang
> Sent: Tuesday, July 7, 2020 6:11 PM
> To: david.marchand@redhat.com; dev@dpdk.org
> Cc: drc@linux.vnet.ibm.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; olivier.matz@6wind.com; Ruifeng Wang
> <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> Subject: [dpdk-dev] [PATCH v2] mbuf: use C11 atomics for refcnt operations
> 
> Use C11 atomics with explicit ordering instead of rte_atomic ops which
> enforce unnecessary barriers on aarch64.
> 
> Signed-off-by: Phil Yang <phil.yang@arm.com>
> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> ---
> v2:
> Fix ABI issue: revert the rte_mbuf_ext_shared_info struct refcnt field
> to refcnt_atomic.
> 

<snip>

> diff --git a/lib/librte_mbuf/rte_mbuf_core.h
> b/lib/librte_mbuf/rte_mbuf_core.h
> index 16600f1..806313a 100644
> --- a/lib/librte_mbuf/rte_mbuf_core.h
> +++ b/lib/librte_mbuf/rte_mbuf_core.h
> @@ -18,7 +18,6 @@
> 
>  #include <stdint.h>
>  #include <rte_compat.h>
> -#include <generic/rte_atomic.h>
> 
>  #ifdef __cplusplus
>  extern "C" {
> @@ -495,12 +494,8 @@ struct rte_mbuf {
>  	 * or non-atomic) is controlled by the
> CONFIG_RTE_MBUF_REFCNT_ATOMIC
>  	 * config option.
>  	 */
> -	RTE_STD_C11
> -	union {
> -		rte_atomic16_t refcnt_atomic; /**< Atomically accessed
> refcnt */
> -		/** Non-atomically accessed refcnt */
> -		uint16_t refcnt;
> -	};
> +	uint16_t refcnt;
> +
>  	uint16_t nb_segs;         /**< Number of segments. */
> 
>  	/** Input port (16 bits to support more than 256 virtual ports).
> @@ -679,7 +674,7 @@ typedef void
> (*rte_mbuf_extbuf_free_callback_t)(void *addr, void *opaque);
>  struct rte_mbuf_ext_shared_info {
>  	rte_mbuf_extbuf_free_callback_t free_cb; /**< Free callback
> function */
>  	void *fcb_opaque;                        /**< Free callback argument */
> -	rte_atomic16_t refcnt_atomic;        /**< Atomically accessed refcnt */
> +	uint16_t refcnt_atomic;              /**< Atomically accessed refcnt */

It still causes an ABI check failure in Travis CI on this type change.
I think we need an exception in libabigail.abignore for this. 

Thanks,
Phil
>  };
> 
>  /**< Maximum number of nb_segs allowed. */
> --
> 2.7.4