[PATCH 1/9] sched/core: Drop mutex locks before proxy rescheduling

[Andrea Righi]

find_proxy_task() can call proxy_resched_idle() while holding a mutex
wait_lock and the blocked task lock. Guard cleanup does not run until
after the return expression is evaluated.

proxy_resched_idle() invokes scheduling-class callbacks through
put_prev_set_next_task(). Calling those callbacks with the mutex locks
held prevents them from safely inspecting the proxy chain, as doing so
may require acquiring the same locks.

This is a preparatory change to support proxy execution in sched_ext. It
leaves the guard scope before calling proxy_resched_idle() so sched_ext
callbacks can safely inspect the proxy chain.

Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
---
 kernel/sched/core.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3cc6fb1d20547..0139cd4a8be7e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6923,7 +6923,7 @@ 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 */
 			if (curr_in_chain)
-				return proxy_resched_idle(rq);
+				goto resched_idle;
 			__clear_task_blocked_on(p, NULL);
 			goto deactivate;
 		}
@@ -6935,7 +6935,7 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
 			 * and leave that CPU to sort things out.
 			 */
 			if (curr_in_chain)
-				return proxy_resched_idle(rq);
+				goto resched_idle;
 			goto migrate_task;
 		}
 
@@ -6948,7 +6948,7 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
 			 * case we should end up back in find_proxy_task(), this time
 			 * hopefully with all relevant tasks already enqueued.
 			 */
-			return proxy_resched_idle(rq);
+			goto resched_idle;
 		}
 
 		/*
@@ -6985,7 +6985,7 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
 			 * So schedule rq->idle so that ttwu_runnable() can get the rq
 			 * lock and mark owner as running.
 			 */
-			return proxy_resched_idle(rq);
+			goto resched_idle;
 		}
 		/*
 		 * OK, now we're absolutely sure @owner is on this
@@ -6997,6 +6997,8 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
 	WARN_ON_ONCE(owner && !owner->on_rq);
 	return owner;
 
+resched_idle:
+	return proxy_resched_idle(rq);
 deactivate:
 	proxy_deactivate(rq, p);
 	return NULL;
-- 
2.55.0