vppinfra: fix corner-cases in bihash lookup

In a case where one pounds on a single kvp in a KVP_AT_BUCKET_LEVEL
table, the code would sporadically return a transitional value (junk)
from a half-deleted kvp. At most, 64-bits worth of the kvp will be
written atomically, so using memset(...) to smear 0xFF's across a kvp
to free it left a lot to be desired.

Performance impact: very mild positive, thanks to FC for doing a
multi-thread host stack perf/scale test.

Added an ASSERT to catch attempts to add a (key,value) pair which
contains the magic "free kvp" value.

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I6a1aa8a2c30bc70bec4b696ce7b17c2839927065

1 files changed, 7 insertions, 2 deletions

diff --git a/src/vppinfra/bihash_12_4.h b/src/vppinfra/bihash_12_4.h
index fe050e15924..3fdf1847861 100644
--- a/src/vppinfra/bihash_12_4.h
+++ b/src/vppinfra/bihash_12_4.h
@@ -36,11 +36,16 @@ typedef union
   u64 as_u64[2];
 } clib_bihash_kv_12_4_t;
 
+static inline void
+clib_bihash_mark_free_12_4 (clib_bihash_kv_12_4_t *v)
+{
+  v->value = 0xFEEDFACE;
+}
+
 static inline int
 clib_bihash_is_free_12_4 (const clib_bihash_kv_12_4_t *v)
 {
-  /* Free values are clib_memset to 0xff, check a bit... */
-  if (v->as_u64[0] == ~0ULL && v->value == ~0)
+  if (v->value == 0xFEEDFACE)
     return 1;
   return 0;
 }