tdb: Use atomic operations for tdb_[increment|get]_seqnum

With locking.tdb now based on g_lock.c code, we change locking.tdb a
lot more often. I have a customer case where LDX tortures smbd very
hard with 800+ concurrent connections, which now completely falls over
where 4.12 still worked fine. Some debugging showed a thundering herd
on fcntl locking.tdb index 48 (TDB_SEQNUM_OFS). We still use fcntl for
the seqnum, back when we converted the chainlocks to mutexes we did
not consider it to be a problem. Now it is, but all we need to do with
the SEQNUM is to increment it, so an __atomic_add_fetch() of one is
sufficient.

I've taken a look at the C11 standard atomics, but I could not figure
out how to use them properly, to me they seem more general to be
initialized first etc. All we need is a X86 "lock incl 48(%rax)" to be
emitted, and the gcc __atomic_add_fetch seems to do this.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

1 files changed, 24 insertions, 0 deletions

diff --git a/lib/tdb/common/tdb.c b/lib/tdb/common/tdb.c
index c56b37be5ca..de829bb48c4 100644
--- a/lib/tdb/common/tdb.c
+++ b/lib/tdb/common/tdb.c
@@ -64,6 +64,15 @@ static void tdb_increment_seqnum(struct tdb_context *tdb)
 		return;
 	}
 
+#if defined(HAVE___ATOMIC_ADD_FETCH) && defined(HAVE___ATOMIC_ADD_LOAD)
+	if (tdb->map_ptr != NULL) {
+		uint32_t *pseqnum = (uint32_t *)(
+			TDB_SEQNUM_OFS + (char *)tdb->map_ptr);
+		__atomic_add_fetch(pseqnum, 1, __ATOMIC_SEQ_CST);
+		return;
+	}
+#endif
+
 	if (tdb_nest_lock(tdb, TDB_SEQNUM_OFS, F_WRLCK,
 			  TDB_LOCK_WAIT|TDB_LOCK_PROBE) != 0) {
 		return;
@@ -838,6 +847,21 @@ _PUBLIC_ int tdb_get_seqnum(struct tdb_context *tdb)
 {
 	tdb_off_t seqnum=0;
 
+	if (tdb->transaction != NULL) {
+		tdb_ofs_read(tdb, TDB_SEQNUM_OFS, &seqnum);
+		return seqnum;
+	}
+
+#if defined(HAVE___ATOMIC_ADD_FETCH) && defined(HAVE___ATOMIC_ADD_LOAD)
+	if (tdb->map_ptr != NULL) {
+		uint32_t *pseqnum = (uint32_t *)(
+			TDB_SEQNUM_OFS + (char *)tdb->map_ptr);
+		uint32_t ret;
+		__atomic_load(pseqnum, &ret,__ATOMIC_SEQ_CST);
+		return ret;
+	}
+#endif
+
 	tdb_ofs_read(tdb, TDB_SEQNUM_OFS, &seqnum);
 	return seqnum;
 }