linux-kernel - [PATCH v24 02/11] sched: Fix modifying donor->blocked on without proper locking

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Message-ID: <20251124223111.3616950-3-jstultz@google.com>
Date: Mon, 24 Nov 2025 22:30:54 +0000
From: John Stultz <jstultz@...gle.com>
To: LKML <linux-kernel@...r.kernel.org>
Cc: John Stultz <jstultz@...gle.com>, K Prateek Nayak <kprateek.nayak@....com>, 
	Joel Fernandes <joelagnelf@...dia.com>, Qais Yousef <qyousef@...alina.io>, 
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Valentin Schneider <vschneid@...hat.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, 
	Zimuzo Ezeozue <zezeozue@...gle.com>, Mel Gorman <mgorman@...e.de>, Will Deacon <will@...nel.org>, 
	Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Metin Kaya <Metin.Kaya@....com>, 
	Xuewen Yan <xuewen.yan94@...il.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Daniel Lezcano <daniel.lezcano@...aro.org>, Suleiman Souhlal <suleiman@...gle.com>, 
	kuyo chang <kuyo.chang@...iatek.com>, hupu <hupu.gm@...il.com>, kernel-team@...roid.com
Subject: [PATCH v24 02/11] sched: Fix modifying donor->blocked on without
 proper locking

Introduce an action enum in find_proxy_task() which allows
us to handle work needed to be done outside the mutex.wait_lock
and task.blocked_lock guard scopes.

This ensures proper locking when we clear the donor's blocked_on
pointer in proxy_deactivate(), and the switch statement will be
useful as we add more cases to handle later in this series.

Reviewed-by: K Prateek Nayak <kprateek.nayak@....com>
Signed-off-by: John Stultz <jstultz@...gle.com>
---
v23:
* Split out from earlier patch.
v24:
* Minor re-ordering local variables to keep with style
  as suggested by K Prateek

Cc: Joel Fernandes <joelagnelf@...dia.com>
Cc: Qais Yousef <qyousef@...alina.io>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Valentin Schneider <vschneid@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Zimuzo Ezeozue <zezeozue@...gle.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Will Deacon <will@...nel.org>
Cc: Waiman Long <longman@...hat.com>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Metin Kaya <Metin.Kaya@....com>
Cc: Xuewen Yan <xuewen.yan94@...il.com>
Cc: K Prateek Nayak <kprateek.nayak@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
Cc: Suleiman Souhlal <suleiman@...gle.com>
Cc: kuyo chang <kuyo.chang@...iatek.com>
Cc: hupu <hupu.gm@...il.com>
Cc: kernel-team@...roid.com
---
 kernel/sched/core.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b3f9bc20b7e1f..1b6fd173daadd 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6591,7 +6591,7 @@ static struct task_struct *proxy_deactivate(struct rq *rq, struct task_struct *d
 		 * as unblocked, as we aren't doing proxy-migrations
 		 * yet (more logic will be needed then).
 		 */
-		donor->blocked_on = NULL;
+		clear_task_blocked_on(donor, NULL);
 	}
 	return NULL;
 }
@@ -6615,6 +6615,7 @@ static struct task_struct *proxy_deactivate(struct rq *rq, struct task_struct *d
 static struct task_struct *
 find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
 {
+	enum { FOUND, DEACTIVATE_DONOR } action = FOUND;
 	struct task_struct *owner = NULL;
 	int this_cpu = cpu_of(rq);
 	struct task_struct *p;
@@ -6652,12 +6653,14 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
 
 		if (!READ_ONCE(owner->on_rq) || owner->se.sched_delayed) {
 			/* XXX Don't handle blocked owners/delayed dequeue yet */
-			return proxy_deactivate(rq, donor);
+			action = DEACTIVATE_DONOR;
+			break;
 		}
 
 		if (task_cpu(owner) != this_cpu) {
 			/* XXX Don't handle migrations yet */
-			return proxy_deactivate(rq, donor);
+			action = DEACTIVATE_DONOR;
+			break;
 		}
 
 		if (task_on_rq_migrating(owner)) {
@@ -6715,6 +6718,13 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
 		 */
 	}
 
+	/* Handle actions we need to do outside of the guard() scope */
+	switch (action) {
+	case DEACTIVATE_DONOR:
+		return proxy_deactivate(rq, donor);
+	case FOUND:
+		/* fallthrough */;
+	}
 	WARN_ON_ONCE(owner && !owner->on_rq);
 	return owner;
 }
-- 
2.52.0.487.g5c8c507ade-goog