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

tip-bot2 for John Stultz
2026-04-03T12:30:26Z

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@google.com>
AuthorDate: Tue, 24 Mar 2026 19:13:20
Committer: Peter Zijlstra <peterz@infradead.org>
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@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://patch.msgid.link/20260324191337.1841376-6-jstultz@google.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 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;
}