linux-kernel - [PATCH v3] sched: Remove a preempt-disable section in rt_mutex

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Message-ID: <20250611095400.g8lgs-Tq@linutronix.de>
Date: Wed, 11 Jun 2025 11:54:00 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, Ben Segall <bsegall@...gle.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Mel Gorman <mgorman@...e.de>, Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleinxer <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH v3] sched: 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 the possible
balance callbacks unlock the rq they must not enable interrupts as I
doubt that they also unlock rt_mutex_base::wait_lock.
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().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.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 dce50fa57471d..537212b33fd84 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7292,14 +7292,11 @@ 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

-- 
2.49.0