Index: uspace/app/tester/meson.build
===================================================================
--- uspace/app/tester/meson.build	(revision 4e1221c8d8cc3a037950f3af0db2898aad17fb55)
+++ uspace/app/tester/meson.build	(revision 82ff0a11c276d85c5fe6d1634d9071c9e6b8f678)
@@ -1,4 +1,4 @@
 #
-# Copyright (c) 2023 Jiri Svoboda
+# Copyright (c) 2025 Jiri Svoboda
 # Copyright (c) 2005 Martin Decky
 # Copyright (c) 2007 Jakub Jermar
@@ -33,4 +33,5 @@
 	'tester.c',
 	'util.c',
+	'thread/deadlock.c',
 	'thread/thread1.c',
 	'thread/setjmp1.c',
Index: uspace/app/tester/tester.c
===================================================================
--- uspace/app/tester/tester.c	(revision 4e1221c8d8cc3a037950f3af0db2898aad17fb55)
+++ uspace/app/tester/tester.c	(revision 82ff0a11c276d85c5fe6d1634d9071c9e6b8f678)
@@ -1,3 +1,4 @@
 /*
+ * Copyright (c) 2025 Jiri Svoboda
  * Copyright (c) 2006 Ondrej Palkovsky
  * Copyright (c) 2007 Martin Decky
@@ -49,4 +50,5 @@
 
 test_t tests[] = {
+#include "thread/deadlock.def"
 #include "thread/thread1.def"
 #include "thread/setjmp1.def"
Index: uspace/app/tester/tester.h
===================================================================
--- uspace/app/tester/tester.h	(revision 4e1221c8d8cc3a037950f3af0db2898aad17fb55)
+++ uspace/app/tester/tester.h	(revision 82ff0a11c276d85c5fe6d1634d9071c9e6b8f678)
@@ -1,3 +1,4 @@
 /*
+ * Copyright (c) 2025 Jiri Svoboda
  * Copyright (c) 2007 Martin Decky
  * All rights reserved.
@@ -79,4 +80,5 @@
 } test_t;
 
+extern const char *test_deadlock(void);
 extern const char *test_thread1(void);
 extern const char *test_setjmp1(void);
Index: uspace/app/tester/thread/deadlock.c
===================================================================
--- uspace/app/tester/thread/deadlock.c	(revision 82ff0a11c276d85c5fe6d1634d9071c9e6b8f678)
+++ uspace/app/tester/thread/deadlock.c	(revision 82ff0a11c276d85c5fe6d1634d9071c9e6b8f678)
@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2025 Jiri Svoboda
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ *   notice, this list of conditions and the following disclaimer.
+ * - Redistributions in binary form must reproduce the above copyright
+ *   notice, this list of conditions and the following disclaimer in the
+ *   documentation and/or other materials provided with the distribution.
+ * - The name of the author may not be used to endorse or promote products
+ *   derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <errno.h>
+#include <fibril.h>
+#include <fibril_synch.h>
+//#include <stdio.h>
+//#include <stddef.h>
+#include "../tester.h"
+
+static fibril_mutex_t fm1, fm2;
+
+static errno_t fibril_fn(void *data)
+{
+	TPRINTF("F2: Lock M2\n");
+	fibril_mutex_lock(&fm2);
+	fibril_sleep(1);
+	TPRINTF("F2: Lock M1\n");
+	fibril_mutex_lock(&fm1);
+	TPRINTF("F2: Unlock M1, M2\n");
+	fibril_mutex_unlock(&fm1);
+	fibril_mutex_unlock(&fm2);
+	return EOK;
+}
+
+const char *test_deadlock(void)
+{
+	fid_t fid;
+
+	fibril_mutex_initialize(&fm1);
+	fibril_mutex_initialize(&fm2);
+
+	TPRINTF("Creating fibril\n");
+	fid = fibril_create(fibril_fn, NULL);
+	if (fid == 0) {
+		TPRINTF("\nCould not create fibril.\n");
+		goto error;
+	}
+
+	fibril_add_ready(fid);
+
+	TPRINTF("F1: Lock M1\n");
+	fibril_mutex_lock(&fm1);
+	fibril_sleep(1);
+	TPRINTF("F1: Lock M2\n");
+	fibril_mutex_lock(&fm2);
+
+	TPRINTF("F1: Unlock M2, M1\n");
+	fibril_mutex_unlock(&fm2);
+	fibril_mutex_unlock(&fm1);
+
+	return NULL;
+error:
+	return "Test failed";
+}
Index: uspace/app/tester/thread/deadlock.def
===================================================================
--- uspace/app/tester/thread/deadlock.def	(revision 82ff0a11c276d85c5fe6d1634d9071c9e6b8f678)
+++ uspace/app/tester/thread/deadlock.def	(revision 82ff0a11c276d85c5fe6d1634d9071c9e6b8f678)
@@ -0,0 +1,6 @@
+{
+	"deadlock",
+	"Fibril deadlock detection test",
+	&test_deadlock,
+	false
+},