[tip: sched/core] sched: Fix modifying donor->blocked on without proper locking

[tip-bot2 for John Stultz]

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

Commit-ID:     56f4b24267a643b0b9ab73f09feaaabfee5a37ae
Gitweb:        https://git.kernel.org/tip/56f4b24267a643b0b9ab73f09feaaabfee5a37ae
Author:        John Stultz <jstultz@xxxxxxxxxx>
AuthorDate:    Tue, 24 Mar 2026 19:13:20 
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 03 Apr 2026 14:23:39 +02:00

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.

Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Link: https://patch.msgid.link/20260324191337.1841376-6-jstultz@xxxxxxxxxx
---
 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 1913dbc..bf4338f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6568,7 +6568,7 @@ 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;
 }
@@ -6592,6 +6592,7 @@ 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;
@@ -6625,12 +6626,14 @@ 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)) {
@@ -6688,6 +6691,13 @@ 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;
 }