Index: kernel/generic/include/synch/mutex.h
===================================================================
--- kernel/generic/include/synch/mutex.h	(revision c626117d454226d577af1687d21ab9f141251eea)
+++ kernel/generic/include/synch/mutex.h	(revision 9f2f5eef7293f0126e93c9ba1905f6fc156afa9d)
@@ -44,5 +44,4 @@
 	MUTEX_PASSIVE,
 	MUTEX_RECURSIVE,
-	MUTEX_ACTIVE
 } mutex_type_t;
 
@@ -51,14 +50,14 @@
 typedef struct {
 	mutex_type_t type;
+	int nesting;
 	semaphore_t sem;
-	struct thread *owner;
-	unsigned nesting;
+	_Atomic(struct thread *) owner;
 } mutex_t;
 
 #define MUTEX_INITIALIZER(name, mtype) (mutex_t) { \
 	.type = (mtype), \
+	.nesting = 0, \
 	.sem = SEMAPHORE_INITIALIZER((name).sem, 1), \
 	.owner = NULL, \
-	.nesting = 0, \
 }
 
Index: kernel/generic/src/synch/mutex.c
===================================================================
--- kernel/generic/src/synch/mutex.c	(revision c626117d454226d577af1687d21ab9f141251eea)
+++ kernel/generic/src/synch/mutex.c	(revision 9f2f5eef7293f0126e93c9ba1905f6fc156afa9d)
@@ -1,5 +1,5 @@
 /*
  * Copyright (c) 2001-2004 Jakub Jermar
- * Copyright (c) 2023 Jiří Zárevúcky
+ * Copyright (c) 2025 Jiří Zárevúcky
  * All rights reserved.
  *
@@ -39,10 +39,8 @@
 #include <assert.h>
 #include <errno.h>
+#include <proc/thread.h>
+#include <stdatomic.h>
 #include <synch/mutex.h>
 #include <synch/semaphore.h>
-#include <arch.h>
-#include <stacktrace.h>
-#include <cpu.h>
-#include <proc/thread.h>
 
 /** Initialize mutex.
@@ -56,4 +54,21 @@
 }
 
+/** A race in mtx->owner access is unavoidable, so we have to make
+ * access to it formally atomic. These are convenience functions to
+ * read/write the variable without memory barriers, since we don't need
+ * them and C11 atomics default to the strongest possible memory ordering
+ * by default, which is utterly ridiculous.
+ */
+static inline thread_t *_get_owner(mutex_t *mtx)
+{
+	return atomic_load_explicit(&mtx->owner, memory_order_relaxed);
+}
+
+/** Counterpart to _get_owner(). */
+static inline void _set_owner(mutex_t *mtx, thread_t *owner)
+{
+	atomic_store_explicit(&mtx->owner, owner, memory_order_relaxed);
+}
+
 /** Find out whether the mutex is currently locked.
  *
@@ -64,30 +79,8 @@
 bool mutex_locked(mutex_t *mtx)
 {
-	errno_t rc = semaphore_trydown(&mtx->sem);
-	if (rc == EOK) {
-		semaphore_up(&mtx->sem);
-	}
-	return rc != EOK;
-}
+	if (!THREAD)
+		return mtx->nesting > 0;
 
-static void mutex_lock_active(mutex_t *mtx)
-{
-	assert((mtx->type == MUTEX_ACTIVE) || !THREAD);
-
-	const unsigned deadlock_treshold = 100000000;
-	unsigned int cnt = 0;
-	bool deadlock_reported = false;
-
-	while (semaphore_trydown(&mtx->sem) != EOK) {
-		if (cnt++ > deadlock_treshold) {
-			printf("cpu%u: looping on active mutex %p\n", CPU->id, mtx);
-			stack_trace();
-			cnt = 0;
-			deadlock_reported = true;
-		}
-	}
-
-	if (deadlock_reported)
-		printf("cpu%u: not deadlocked\n", CPU->id);
+	return _get_owner(mtx) == THREAD;
 }
 
@@ -98,17 +91,22 @@
 void mutex_lock(mutex_t *mtx)
 {
-	if (mtx->type == MUTEX_RECURSIVE && mtx->owner == THREAD) {
-		assert(THREAD);
+	if (!THREAD) {
+		assert(mtx->type == MUTEX_RECURSIVE || mtx->nesting == 0);
 		mtx->nesting++;
 		return;
 	}
 
-	if (mtx->type == MUTEX_ACTIVE || !THREAD) {
-		mutex_lock_active(mtx);
+	if (_get_owner(mtx) == THREAD) {
+		/* This will also detect nested locks on a non-recursive mutex. */
+		assert(mtx->type == MUTEX_RECURSIVE);
+		assert(mtx->nesting > 0);
+		mtx->nesting++;
 		return;
 	}
 
 	semaphore_down(&mtx->sem);
-	mtx->owner = THREAD;
+
+	_set_owner(mtx, THREAD);
+	assert(mtx->nesting == 0);
 	mtx->nesting = 1;
 }
@@ -123,11 +121,13 @@
 errno_t mutex_lock_timeout(mutex_t *mtx, uint32_t usec)
 {
-	if (usec != 0) {
-		assert(mtx->type != MUTEX_ACTIVE);
-		assert(THREAD);
+	if (!THREAD) {
+		assert(mtx->type == MUTEX_RECURSIVE || mtx->nesting == 0);
+		mtx->nesting++;
+		return EOK;
 	}
 
-	if (mtx->type == MUTEX_RECURSIVE && mtx->owner == THREAD) {
-		assert(THREAD);
+	if (_get_owner(mtx) == THREAD) {
+		assert(mtx->type == MUTEX_RECURSIVE);
+		assert(mtx->nesting > 0);
 		mtx->nesting++;
 		return EOK;
@@ -135,9 +135,11 @@
 
 	errno_t rc = semaphore_down_timeout(&mtx->sem, usec);
-	if (rc == EOK) {
-		mtx->owner = THREAD;
-		mtx->nesting = 1;
-	}
-	return rc;
+	if (rc != EOK)
+		return rc;
+
+	_set_owner(mtx, THREAD);
+	assert(mtx->nesting == 0);
+	mtx->nesting = 1;
+	return EOK;
 }
 
@@ -157,10 +159,17 @@
 void mutex_unlock(mutex_t *mtx)
 {
-	if (mtx->type == MUTEX_RECURSIVE) {
-		assert(mtx->owner == THREAD);
-		if (--mtx->nesting > 0)
-			return;
-		mtx->owner = NULL;
+	if (--mtx->nesting > 0) {
+		assert(mtx->type == MUTEX_RECURSIVE);
+		return;
 	}
+
+	assert(mtx->nesting == 0);
+
+	if (!THREAD)
+		return;
+
+	assert(_get_owner(mtx) == THREAD);
+	_set_owner(mtx, NULL);
+
 	semaphore_up(&mtx->sem);
 }