Re: [PATCH v4 1/2] futex: Provide rt_mutex_.*_schedule() equivalents for futex scheduling

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-09-01 15:54:51 [+0200], To linux-kernel@vger.kernel.org wrote:
> By mistake futex_wait_requeue_pi() -> rt_mutex_wait_proxy_lock() did not
> get the rt_mutex_{pre|post}_schedule() annotation. This was not noticed
> because in this callchain the lock is (usually) not contended and so
> rt_mutex_slowlock_block() does not schedule, triggering the assert.

The following makes it trigger more reliably.

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 4728631ae7194..15037c35c684f 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1633,13 +1633,14 @@ static int __sched rt_mutex_slowlock_block(struct rt_mutex_base *lock,
 	struct rt_mutex *rtm = container_of(lock, struct rt_mutex, rtmutex);
 	struct task_struct *owner;
 	int ret = 0;
+	int first_loop = 1;
 
 	__assume_ctx_lock(&rtm->rtmutex.wait_lock);
 
 	lockevent_inc(rtmutex_slow_block);
 	for (;;) {
 		/* Try to acquire the lock: */
-		if (try_to_take_rt_mutex(lock, current, waiter)) {
+		if (!first_loop && try_to_take_rt_mutex(lock, current, waiter)) {
 			lockevent_inc(rtmutex_slow_acq3);
 			break;
 		}
@@ -1665,7 +1666,11 @@ static int __sched rt_mutex_slowlock_block(struct rt_mutex_base *lock,
 			owner = NULL;
 		raw_spin_unlock_irq_wake(&lock->wait_lock, wake_q);
 
-		if (!owner || !rtmutex_spin_on_owner(lock, waiter, owner)) {
+		if (first_loop || !owner || !rtmutex_spin_on_owner(lock, waiter, owner)) {
+			if (first_loop) {
+				first_loop = 0;
+				__set_current_state(TASK_RUNNING);
+			}
 			lockevent_inc(rtmutex_slow_sleep);
 			rt_mutex_schedule();
 		}

Sebastian