[dpdk-dev] [PATCH v4 1/2] mbuf: use C11 atomic built-ins for refcnt operations

[Phil Yang]

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.y...@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.w...@arm.com>
---
v4:
1. Add union for refcnt_atomic and refcnt in rte_mbuf_ext_shared_info
to avoid ABI breakage. (Olivier)
2. Add notice of refcnt_atomic deprecation. (Honnappa)

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 |  6 +++++-
 3 files changed, 15 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..7259575 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_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, 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, (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, (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..8cd7137 100644
--- a/lib/librte_mbuf/rte_mbuf_core.h
+++ b/lib/librte_mbuf/rte_mbuf_core.h
@@ -679,7 +679,11 @@ 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 */
+       RTE_STD_C11
+       union {
+               rte_atomic16_t refcnt_atomic; /**< Atomically accessed refcnt */
+               uint16_t refcnt;
+       };
 };
 
 /**< Maximum number of nb_segs allowed. */
-- 
2.7.4