[PATCH v3 REPOST] sched: Remove a preempt-disable section in rt_mutex_setprio()

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

rt_mutex_setprio() has only one caller: rt_mutex_adjust_prio(). It
expects that task_struct::pi_lock and rt_mutex_base::wait_lock are held.
Both locks are raw_spinlock_t and are acquired with disabled interrupts.

Nevertheless rt_mutex_setprio() disables preemption while invoking
__balance_callbacks() and raw_spin_rq_unlock(). Even if one of the
balance callbacks unlocks the rq then it must not enable interrupts
because rt_mutex_base::wait_lock is still locked.
Therefore interrupts should remain disabled and disabling preemption is
not needed.

Commit 4c9a4bc89a9cc ("sched: Allow balance callbacks for check_class_changed()")
adds a preempt-disable section to rt_mutex_setprio() and
__sched_setscheduler(). In __sched_setscheduler() the preemption is
disabled before rq is unlocked and interrupts enabled but I don't see
why it makes a difference in rt_mutex_setprio().

Remove the preempt_disable() section from rt_mutex_setprio().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
v2…v3: https://lore.kernel.org/all/20250610144700.uBK6RS95@linutronix.de/
  - Add a comment why IRQs are still disabled.

v1…v2: https://lore.kernel.org/all/20250513142954.ZM5QSQNc@linutronix.de/
  - Repost without RFC

 kernel/sched/core.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f754a60de8484..61c3297b43756 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7440,14 +7440,11 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
 
 	check_class_changed(rq, p, prev_class, oldprio);
 out_unlock:
-	/* Avoid rq from going away on us: */
-	preempt_disable();
+	/* Caller holds task_struct::pi_lock, IRQs are still disabled */
 
 	rq_unpin_lock(rq, &rf);
 	__balance_callbacks(rq);
 	raw_spin_rq_unlock(rq);
-
-	preempt_enable();
 }
 #endif /* CONFIG_RT_MUTEXES */
 
-- 
2.51.0