linux-kernel - [tip: sched/urgent] sched/rt: Remove a preempt-disable section in rt_mutex

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Message-ID: <176478631197.498.9342089450906538199.tip-bot2@tip-bot2>
Date: Wed, 03 Dec 2025 18:25:11 -0000
From: "tip-bot2 for Sebastian Andrzej Siewior" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Ingo Molnar <mingo@...nel.org>, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: sched/urgent] sched/rt: Remove a preempt-disable section in
 rt_mutex_setprio()

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

Commit-ID:     81a325c5b4ebc387181c5aca7586ccc049a2e96e
Gitweb:        https://git.kernel.org/tip/81a325c5b4ebc387181c5aca7586ccc049a2e96e
Author:        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
AuthorDate:    Thu, 27 Nov 2025 16:55:29 +01:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 02 Dec 2025 15:27:16 +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@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Ingo Molnar <mingo@...nel.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 0c4ff93..7dfb6a9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7358,15 +7358,12 @@ 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 */