trx_commit_for_mysql(): Avoid acquiring and releasing kernel_mutex when

trx->sess or trx_dummy_sess is non-NULL.

trx/trx0trx.c

@@ -1570,19 +1570,21 @@ trx_commit_for_mysql(
 	the transaction object does not have an InnoDB session object, and we
 	set the dummy session that we use for all MySQL transactions. */
 
-	mutex_enter(&kernel_mutex);
-
 	if (trx->sess == NULL) {
 		/* Open a dummy session */
 
 		if (!trx_dummy_sess) {
-			trx_dummy_sess = sess_open();
+			mutex_enter(&kernel_mutex);
+
+			if (!trx_dummy_sess) {
+				trx_dummy_sess = sess_open();
+			}
+
+			mutex_exit(&kernel_mutex);
 		}
 
 		trx->sess = trx_dummy_sess;
 	}
-	
-	mutex_exit(&kernel_mutex);
 
 	trx_start_if_not_started(trx);