comment on userlocks synchronization

goatherd.go

@@ -259,14 +259,17 @@ func handle_conn(db *sql.DB, conn net.Conn) {
     }
     log.Printf("%v: %v", name, result)
 
-    // name exists, get or create its lock
+    // name exists, so we can get or create its lock, double-checked locking style
+    // NOTE: the read lock is important to synchronize with the mutex initiali-
+    // zation, not just the map itself (i.e. a reader might observe a non-
+    // zeroed mutex if we used a concurrent map without a temporary variable
+    // and further memory barriers)
     faildelay.RLock()
     delay, exists := faildelay.userlocks[name]
     faildelay.RUnlock()
     if !exists {
         debugf("[%v] not yet in faildelay.userlocks", remote)
 
-        // no atomic upgrade with sync.RWMutex, so we have to do the lookup again
         faildelay.Lock()
         delay, exists = faildelay.userlocks[name]
         if !exists {