Misc locking fixes:

- Fix ww_mutex regression, which caused hangs/pauses in some DRM drivers
  - Fix rtmutex proxy-rollback bug
 
 Signed-off-by: Ingo Molnar <mingo@kernel.org>
 -----BEGIN PGP SIGNATURE-----
 
 iQJFBAABCgAvFiEEBpT5eoXrXCwVQwEKEnMQ0APhK1gFAmnrdJERHG1pbmdvQGtl
 cm5lbC5vcmcACgkQEnMQ0APhK1jnJw//bi0bzZnBYeU4ukrqnyVWUZS7lKCnl+3M
 Ys31Ls38nXTQPhs+qbgUlqLzybJ/96s7uattPIW8+0Jkr+PfQguCvT+QiXs0SFzC
 aEp6PcZAD1z+QQY1zj2c1TM414IdyWbkShRxsPULb1j8nYJ6mO50UzP4zFdl7QwH
 gwZRgPkriUhD3vfgtHX3U0rshrvFHu9Ed6uuMkLFxaEiHn0h/4rZa0q9vonxw+cU
 64MabrxtPyQA97Mjr/YVsJjodt5bzkBmKNDCu9YxfqFQ0LTXyi1DZG/zdFHmT1Kf
 WcRIXbCx27HcR+NOWRze4rby2SR27NnPAZC/rR7Y6MX0IHGg1m2tM9DIGnDw71Su
 3HkQ1W3KhQFO0uvbhkKCXk1SAfBrJwh0g3p91sgj/tZyaXXdEYRChhpie5jMy3eg
 3k49QfYHigQZCZa8IH0PjrxLzDHo8S2tB75X6dWlxC/UeN/6X1G+Z9a52G9kZrFJ
 ZhrxY5/WVF4LbeIYupFbjN393jVN4WPEJKN3Esouuc+bluqMjUjl65v12yTQZeFy
 7pObrM9nJ4FijWjt+OCHUUZLwt+k5zttdUjgEh0zonx94JtpDA0qjkpPtbYgCFak
 WQxwD+qs1r8yv+msqkQ65/ir5GT6GyAzQnQAQbAQyB4itC9iyDhoS6mzkFBsw3bk
 TxfGszSV5ng=
 =6wf3
 -----END PGP SIGNATURE-----

Merge tag 'locking-urgent-2026-04-24' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull locking fixes from Ingo Molnar:

 - Fix ww_mutex regression, which caused hangs/pauses in some DRM drivers

 - Fix rtmutex proxy-rollback bug

* tag 'locking-urgent-2026-04-24' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  locking/mutex: Fix ww_mutex wait_list operations
  rtmutex: Use waiter::task instead of current in remove_waiter()

kernel/locking/mutex.c

@@ -198,27 +198,43 @@ static inline void __mutex_clear_flag(struct mutex *lock, unsigned long flag)
 }
 
 /*
- * Add @waiter to a given location in the lock wait_list and set the
- * FLAG_WAITERS flag if it's the first waiter.
+ * Add @waiter to the @lock wait_list and set the FLAG_WAITERS flag if it's
+ * the first waiter.
+ *
+ * When @pos, @waiter is added before the waiter indicated by @pos. Otherwise
+ * @waiter will be added to the tail of the list.
  */
 static void
 __mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter,
-		   struct mutex_waiter *first)
+		   struct mutex_waiter *pos)
 	__must_hold(&lock->wait_lock)
 {
+	struct mutex_waiter *first = lock->first_waiter;
+
 	hung_task_set_blocker(lock, BLOCKER_TYPE_MUTEX);
 	debug_mutex_add_waiter(lock, waiter, current);
 
-	if (!first)
-		first = lock->first_waiter;
+	if (pos) {
+		/*
+		 * Insert @waiter before @pos.
+		 */
+		list_add_tail(&waiter->list, &pos->list);
+		/*
+		 * If @pos == @first, then @waiter will be the new first.
+		 */
+		if (pos == first)
+			lock->first_waiter = waiter;
+		return;
+	}
 
 	if (first) {
 		list_add_tail(&waiter->list, &first->list);
-	} else {
-		INIT_LIST_HEAD(&waiter->list);
-		lock->first_waiter = waiter;
-		__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
+		return;
 	}
+
+	INIT_LIST_HEAD(&waiter->list);
+	lock->first_waiter = waiter;
+	__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
 }
 
 static void
@@ -229,10 +245,8 @@ __mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter)
 		__mutex_clear_flag(lock, MUTEX_FLAGS);
 		lock->first_waiter = NULL;
 	} else {
-		if (lock->first_waiter == waiter) {
-			lock->first_waiter = list_first_entry(&waiter->list,
-							      struct mutex_waiter, list);
-		}
+		if (lock->first_waiter == waiter)
+			lock->first_waiter = list_next_entry(waiter, list);
 		list_del(&waiter->list);
 	}
 

kernel/locking/rtmutex.c

@@ -1544,6 +1544,8 @@ static bool rtmutex_spin_on_owner(struct rt_mutex_base *lock,
  *
  * Must be called with lock->wait_lock held and interrupts disabled. It must
  * have just failed to try_to_take_rt_mutex().
+ *
+ * When invoked from rt_mutex_start_proxy_lock() waiter::task != current !
  */
 static void __sched remove_waiter(struct rt_mutex_base *lock,
 				  struct rt_mutex_waiter *waiter)
@@ -1551,14 +1553,15 @@ static void __sched remove_waiter(struct rt_mutex_base *lock,
 {
 	bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
 	struct task_struct *owner = rt_mutex_owner(lock);
+	struct task_struct *waiter_task = waiter->task;
 	struct rt_mutex_base *next_lock;
 
 	lockdep_assert_held(&lock->wait_lock);
 
-	raw_spin_lock(&current->pi_lock);
-	rt_mutex_dequeue(lock, waiter);
-	current->pi_blocked_on = NULL;
-	raw_spin_unlock(&current->pi_lock);
+	scoped_guard(raw_spinlock, &waiter_task->pi_lock) {
+		rt_mutex_dequeue(lock, waiter);
+		waiter_task->pi_blocked_on = NULL;
+	}
 
 	/*
 	 * Only update priority if the waiter was the highest priority
@@ -1594,7 +1597,7 @@ static void __sched remove_waiter(struct rt_mutex_base *lock,
 	raw_spin_unlock_irq(&lock->wait_lock);
 
 	rt_mutex_adjust_prio_chain(owner, RT_MUTEX_MIN_CHAINWALK, lock,
-				   next_lock, NULL, current);
+				   next_lock, NULL, waiter_task);
 
 	raw_spin_lock_irq(&lock->wait_lock);
 }

kernel/locking/ww_mutex.h

@@ -6,6 +6,19 @@
 #define MUTEX_WAITER	mutex_waiter
 #define WAIT_LOCK	wait_lock
 
+/*
+ *           +--------+
+ *           | first  |
+ *           +--------+
+ *                |
+ *                v
+ *  +----+     +----+     +----+
+ *  | W3 | <-> | W1 | <-> | W2 |
+ *  +----+     +----+     +----+
+ *    ^                     ^
+ *    +---------------------+
+ */
+
 static inline struct mutex_waiter *
 __ww_waiter_first(struct mutex *lock)
 	__must_hold(&lock->wait_lock)
@@ -13,26 +26,43 @@ __ww_waiter_first(struct mutex *lock)
 	return lock->first_waiter;
 }
 
+/*
+ * for (cur = __ww_waiter_first(); cur; cur = __ww_waiter_next())
+ *
+ * Should iterate like: W1, W2, W3
+ */
 static inline struct mutex_waiter *
 __ww_waiter_next(struct mutex *lock, struct mutex_waiter *w)
 	__must_hold(&lock->wait_lock)
 {
 	w = list_next_entry(w, list);
+	/*
+	 * Terminate if the next entry is the first again, that has already
+	 * been observed.
+	 */
 	if (lock->first_waiter == w)
 		return NULL;
 
 	return w;
 }
 
+/*
+ * for (cur = __ww_waiter_last(); cur; cur = __ww_waiter_prev())
+ *
+ * Should iterate like: W3, W2, W1
+ */
 static inline struct mutex_waiter *
 __ww_waiter_prev(struct mutex *lock, struct mutex_waiter *w)
 	__must_hold(&lock->wait_lock)
 {
-	w = list_prev_entry(w, list);
+	/*
+	 * Terminate at the first entry, the previous entry of first is the
+	 * last and that has already been observed.
+	 */
 	if (lock->first_waiter == w)
 		return NULL;
 
-	return w;
+	return list_prev_entry(w, list);
 }
 
 static inline struct mutex_waiter *