[tip: sched/urgent] sched/rt: Remove a preempt-disable section in rt_mutex_setprio()

["tip-bot2 for Sebastian Andrzej Siewior" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     22abd832776b1317ae4c3f8a097c8b71bf83fb38
Gitweb:        https://git.kernel.org/tip/22abd832776b1317ae4c3f8a097c8b71bf83fb38
Author:        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate:    Thu, 27 Nov 2025 16:55:29 +01:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Sat, 06 Dec 2025 10:03:13 +01:00

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 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>
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
---
 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 1711e9e..ee7dfbf 100644
--- a/kernel/sched/core.c
+++ b/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 */