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Date: Thu, 9 Jul 2020 13:03:45 +0200
From: Olivier Matz <olivier.matz@6wind.com>
To: Phil Yang <phil.yang@arm.com>
Cc: dev@dpdk.org, stephen@networkplumber.org, david.marchand@redhat.com,
drc@linux.vnet.ibm.com, Honnappa.Nagarahalli@arm.com,
Ruifeng.Wang@arm.com, nd@arm.com
Message-ID: <20200709110345.GR5869@platinum>
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<1594289442-18594-1-git-send-email-phil.yang@arm.com>
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Subject: Re: [dpdk-dev] [PATCH v3] mbuf: use C11 atomic built-ins for refcnt
operations
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Hi Phil,
On Thu, Jul 09, 2020 at 06:10:42PM +0800, Phil Yang wrote:
> Use C11 atomic built-ins 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>
> ---
> v3:
> 1.Fix ABI breakage.
> 2.Simplify data type cast.
>
> v2:
> Fix ABI issue: revert the rte_mbuf_ext_shared_info struct refcnt field
> to refcnt_atomic.
>
> lib/librte_mbuf/rte_mbuf.c | 1 -
> lib/librte_mbuf/rte_mbuf.h | 19 ++++++++++---------
> lib/librte_mbuf/rte_mbuf_core.h | 2 +-
> 3 files changed, 11 insertions(+), 11 deletions(-)
>
> diff --git a/lib/librte_mbuf/rte_mbuf.c b/lib/librte_mbuf/rte_mbuf.c
> index ae91ae2..8a456e5 100644
> --- a/lib/librte_mbuf/rte_mbuf.c
> +++ b/lib/librte_mbuf/rte_mbuf.c
> @@ -22,7 +22,6 @@
> #include <rte_eal.h>
> #include <rte_per_lcore.h>
> #include <rte_lcore.h>
> -#include <rte_atomic.h>
> #include <rte_branch_prediction.h>
> #include <rte_mempool.h>
> #include <rte_mbuf.h>
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index f8e492e..c1c0956 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -37,7 +37,6 @@
> #include <rte_config.h>
> #include <rte_mempool.h>
> #include <rte_memory.h>
> -#include <rte_atomic.h>
> #include <rte_prefetch.h>
> #include <rte_branch_prediction.h>
> #include <rte_byteorder.h>
> @@ -365,7 +364,7 @@ rte_pktmbuf_priv_flags(struct rte_mempool *mp)
> static inline uint16_t
> rte_mbuf_refcnt_read(const struct rte_mbuf *m)
> {
> - return (uint16_t)(rte_atomic16_read(&m->refcnt_atomic));
> + return __atomic_load_n(&m->refcnt, __ATOMIC_RELAXED);
> }
>
> /**
> @@ -378,14 +377,15 @@ rte_mbuf_refcnt_read(const struct rte_mbuf *m)
> static inline void
> rte_mbuf_refcnt_set(struct rte_mbuf *m, uint16_t new_value)
> {
> - rte_atomic16_set(&m->refcnt_atomic, (int16_t)new_value);
> + __atomic_store_n(&m->refcnt, new_value, __ATOMIC_RELAXED);
> }
>
> /* internal */
> static inline uint16_t
> __rte_mbuf_refcnt_update(struct rte_mbuf *m, int16_t value)
> {
> - return (uint16_t)(rte_atomic16_add_return(&m->refcnt_atomic, value));
> + return __atomic_add_fetch(&m->refcnt, (uint16_t)value,
> + __ATOMIC_ACQ_REL);
> }
>
> /**
> @@ -466,7 +466,7 @@ rte_mbuf_refcnt_set(struct rte_mbuf *m, uint16_t new_value)
> static inline uint16_t
> rte_mbuf_ext_refcnt_read(const struct rte_mbuf_ext_shared_info *shinfo)
> {
> - return (uint16_t)(rte_atomic16_read(&shinfo->refcnt_atomic));
> + return __atomic_load_n(&shinfo->refcnt_atomic, __ATOMIC_RELAXED);
> }
>
> /**
> @@ -481,7 +481,7 @@ static inline void
> rte_mbuf_ext_refcnt_set(struct rte_mbuf_ext_shared_info *shinfo,
> uint16_t new_value)
> {
> - rte_atomic16_set(&shinfo->refcnt_atomic, (int16_t)new_value);
> + __atomic_store_n(&shinfo->refcnt_atomic, new_value, __ATOMIC_RELAXED);
> }
>
> /**
> @@ -505,7 +505,8 @@ rte_mbuf_ext_refcnt_update(struct rte_mbuf_ext_shared_info *shinfo,
> return (uint16_t)value;
> }
>
> - return (uint16_t)rte_atomic16_add_return(&shinfo->refcnt_atomic, value);
> + return __atomic_add_fetch(&shinfo->refcnt_atomic, (uint16_t)value,
> + __ATOMIC_ACQ_REL);
> }
>
> /** Mbuf prefetch */
> @@ -1304,8 +1305,8 @@ static inline int __rte_pktmbuf_pinned_extbuf_decref(struct rte_mbuf *m)
> * Direct usage of add primitive to avoid
> * duplication of comparing with one.
> */
> - if (likely(rte_atomic16_add_return
> - (&shinfo->refcnt_atomic, -1)))
> + if (likely(__atomic_add_fetch(&shinfo->refcnt_atomic, (uint16_t)-1,
> + __ATOMIC_ACQ_REL)))
> return 1;
>
> /* Reinitialize counter before mbuf freeing. */
> diff --git a/lib/librte_mbuf/rte_mbuf_core.h b/lib/librte_mbuf/rte_mbuf_core.h
> index 16600f1..d65d1c8 100644
> --- a/lib/librte_mbuf/rte_mbuf_core.h
> +++ b/lib/librte_mbuf/rte_mbuf_core.h
> @@ -679,7 +679,7 @@ 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 */
> };
To avoid an API breakage (i.e. currently, an application that accesses
to refcnt_atomic expects that its type is rte_atomic16_t), I suggest to
do the same than in the mbuf struct:
union {
rte_atomic16_t refcnt_atomic;
uint16_t refcnt;
};
I hope the ABI checker won't complain.
It will also be better for 20.11 when the deprecated fields will be
renamed: the remaining one will be called 'refcnt' in both mbuf and
mbuf_ext_shared_info.
Olivier