sched/rt: Remove a preempt-disable section in rt_mutex_setprio()

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 4c9a4bc ("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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://patch.msgid.link/20251127155529.t_sTatE4@linutronix.de

Author: Sebastian Andrzej Siewior

kernel/sched/core.c

@@ -7352,15 +7352,12 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
 		p->prio = prio;
 	}
 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);
 	rq_repin_lock(rq, &rf);
 	__task_rq_unlock(rq, p, &rf);
-
-	preempt_enable();
 }
 #endif /* CONFIG_RT_MUTEXES */