[PATCH 01/18] sched/core: Drop mutex locks before proxy rescheduling

[Andrea Righi]

find_proxy_task() can call proxy_resched_idle() from return
expressions inside mutex wait_lock and blocked task lock guard scopes.
Return expressions are evaluated before scoped guards are released, so
proxy_resched_idle() invokes scheduling-class callbacks while both raw
spinlocks are held.

Route these cases through a common label outside the guard scopes. The
common label is required because a direct return would evaluate
proxy_resched_idle() before guard cleanup and run the class callbacks
under the proxy-exec locks again.

This is a preparatory change to support proxy execution with sched_ext.

Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
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 f78275192036b..5817d1a4cea2c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6965,7 +6965,7 @@ 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;
 		}
@@ -6977,7 +6977,7 @@ 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;
 		}
 
@@ -6990,7 +6990,7 @@ 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;
 		}
 
 		/*
@@ -7027,7 +7027,7 @@ 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
@@ -7039,6 +7039,8 @@ 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