sched/wake_q: Add helper to call wake_up_q after unlock with preemption disabled

A common pattern seen when wake_qs are used to defer a wakeup
until after a lock is released is something like:
  preempt_disable();
  raw_spin_unlock(lock);
  wake_up_q(wake_q);
  preempt_enable();

So create some raw_spin_unlock*_wake() helper functions to clean
this up.

Applies on top of the fix I submitted here:
 https://lore.kernel.org/lkml/20241212222138.2400498-1-jstultz@google.com/

NOTE: I recognise the unlock()/unlock_irq()/unlock_irqrestore()
variants creates its own duplication, which we could use a macro
to generate the similar functions, but I often dislike how those
generation macros making finding the actual implementation
harder, so I left the three functions as is. If folks would
prefer otherwise, let me know and I'll switch it.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20241217040803.243420-1-jstultz@google.com

include/linux/sched/wake_q.h

@@ -63,4 +63,38 @@ extern void wake_q_add(struct wake_q_head *head, struct task_struct *task);
 extern void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task);
 extern void wake_up_q(struct wake_q_head *head);
 
+/* Spin unlock helpers to unlock and call wake_up_q with preempt disabled */
+static inline
+void raw_spin_unlock_wake(raw_spinlock_t *lock, struct wake_q_head *wake_q)
+{
+	guard(preempt)();
+	raw_spin_unlock(lock);
+	if (wake_q) {
+		wake_up_q(wake_q);
+		wake_q_init(wake_q);
+	}
+}
+
+static inline
+void raw_spin_unlock_irq_wake(raw_spinlock_t *lock, struct wake_q_head *wake_q)
+{
+	guard(preempt)();
+	raw_spin_unlock_irq(lock);
+	if (wake_q) {
+		wake_up_q(wake_q);
+		wake_q_init(wake_q);
+	}
+}
+
+static inline
+void raw_spin_unlock_irqrestore_wake(raw_spinlock_t *lock, unsigned long flags,
+				     struct wake_q_head *wake_q)
+{
+	guard(preempt)();
+	raw_spin_unlock_irqrestore(lock, flags);
+	if (wake_q) {
+		wake_up_q(wake_q);
+		wake_q_init(wake_q);
+	}
+}
 #endif /* _LINUX_SCHED_WAKE_Q_H */

kernel/futex/pi.c

@@ -1020,10 +1020,7 @@ retry_private:
 	 * it sees the futex_q::pi_state.
 	 */
 	ret = __rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current, &wake_q);
-	preempt_disable();
+	raw_spin_unlock_irq_wake(&q.pi_state->pi_mutex.wait_lock, &wake_q);
-	raw_spin_unlock_irq(&q.pi_state->pi_mutex.wait_lock);
-	wake_up_q(&wake_q);
-	preempt_enable();
 
 	if (ret) {
 		if (ret == 1)

kernel/locking/mutex.c

@@ -657,10 +657,7 @@ __mutex_lock_common(struct mutex *lock, unsigned int state, unsigned int subclas
 				goto err;
 		}
 
-		raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
+		raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
-		/* Make sure we do wakeups before calling schedule */
-		wake_up_q(&wake_q);
-		wake_q_init(&wake_q);
 
 		schedule_preempt_disabled();
 
@@ -710,8 +707,7 @@ skip_wait:
 	if (ww_ctx)
 		ww_mutex_lock_acquired(ww, ww_ctx);
 
-	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
+	raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
-	wake_up_q(&wake_q);
 	preempt_enable();
 	return 0;
 
@@ -720,10 +716,9 @@ err:
 	__mutex_remove_waiter(lock, &waiter);
 err_early_kill:
 	trace_contention_end(lock, ret);
-	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
+	raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
 	debug_mutex_free_waiter(&waiter);
 	mutex_release(&lock->dep_map, ip);
-	wake_up_q(&wake_q);
 	preempt_enable();
 	return ret;
 }
@@ -935,10 +930,7 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
 	if (owner & MUTEX_FLAG_HANDOFF)
 		__mutex_handoff(lock, next);
 
-	preempt_disable();
+	raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
-	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
-	wake_up_q(&wake_q);
-	preempt_enable();
 }
 
 #ifndef CONFIG_DEBUG_LOCK_ALLOC

kernel/locking/rtmutex.c

@@ -1292,13 +1292,7 @@ static int __sched task_blocks_on_rt_mutex(struct rt_mutex_base *lock,
 	 */
 	get_task_struct(owner);
 
-	preempt_disable();
+	raw_spin_unlock_irq_wake(&lock->wait_lock, wake_q);
-	raw_spin_unlock_irq(&lock->wait_lock);
-	/* wake up any tasks on the wake_q before calling rt_mutex_adjust_prio_chain */
-	wake_up_q(wake_q);
-	wake_q_init(wake_q);
-	preempt_enable();
-
 
 	res = rt_mutex_adjust_prio_chain(owner, chwalk, lock,
 					 next_lock, waiter, task);
@@ -1642,13 +1636,7 @@ static int __sched rt_mutex_slowlock_block(struct rt_mutex_base *lock,
 			owner = rt_mutex_owner(lock);
 		else
 			owner = NULL;
-		preempt_disable();
+		raw_spin_unlock_irq_wake(&lock->wait_lock, wake_q);
-		raw_spin_unlock_irq(&lock->wait_lock);
-		if (wake_q) {
-			wake_up_q(wake_q);
-			wake_q_init(wake_q);
-		}
-		preempt_enable();
 
 		if (!owner || !rtmutex_spin_on_owner(lock, waiter, owner))
 			rt_mutex_schedule();
@@ -1799,10 +1787,7 @@ static int __sched rt_mutex_slowlock(struct rt_mutex_base *lock,
 	 */
 	raw_spin_lock_irqsave(&lock->wait_lock, flags);
 	ret = __rt_mutex_slowlock_locked(lock, ww_ctx, state, &wake_q);
-	preempt_disable();
+	raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
-	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
-	wake_up_q(&wake_q);
-	preempt_enable();
 	rt_mutex_post_schedule();
 
 	return ret;
@@ -1860,11 +1845,7 @@ static void __sched rtlock_slowlock_locked(struct rt_mutex_base *lock,
 			owner = rt_mutex_owner(lock);
 		else
 			owner = NULL;
-		preempt_disable();
+		raw_spin_unlock_irq_wake(&lock->wait_lock, wake_q);
-		raw_spin_unlock_irq(&lock->wait_lock);
-		wake_up_q(wake_q);
-		wake_q_init(wake_q);
-		preempt_enable();
 
 		if (!owner || !rtmutex_spin_on_owner(lock, &waiter, owner))
 			schedule_rtlock();
@@ -1893,10 +1874,7 @@ static __always_inline void __sched rtlock_slowlock(struct rt_mutex_base *lock)
 
 	raw_spin_lock_irqsave(&lock->wait_lock, flags);
 	rtlock_slowlock_locked(lock, &wake_q);
-	preempt_disable();
+	raw_spin_unlock_irqrestore_wake(&lock->wait_lock, flags, &wake_q);
-	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
-	wake_up_q(&wake_q);
-	preempt_enable();
 }
 
 #endif /* RT_MUTEX_BUILD_SPINLOCKS */