Index: uspace/lib/c/generic/fibril_synch.c
===================================================================
--- uspace/lib/c/generic/fibril_synch.c	(revision 22027b6e7981eafebc4220c771cc114cfcc00c1f)
+++ uspace/lib/c/generic/fibril_synch.c	(revision 80bffdb0707ffcda3727a4cbeb4ec52003630551)
@@ -104,4 +104,7 @@
 	fibril_t *f = (fibril_t *) fibril_get_id();
 
+	if (fibril_get_sercount() != 0)
+		core();
+
 	futex_down(&async_futex);
 	if (fm->counter-- <= 0) {
@@ -194,4 +197,7 @@
 	fibril_t *f = (fibril_t *) fibril_get_id();
 	
+	if (fibril_get_sercount() != 0)
+		core();
+
 	futex_down(&async_futex);
 	if (frw->writers) {
@@ -219,4 +225,7 @@
 	fibril_t *f = (fibril_t *) fibril_get_id();
 	
+	if (fibril_get_sercount() != 0)
+		core();
+
 	futex_down(&async_futex);
 	if (frw->writers || frw->readers) {