sched_ext: Defer sub_kset base put to scx_sched_free_rcu_work

scx_sub_enable_workfn() pins parent->kobj before dropping scx_sched_lock,
but that does not pin parent->sub_kset. Concurrent disable can
kset_unregister and free sub_kset before scx_alloc_and_add_sched()
dereferences it.

Split sub_kset teardown: kobject_del() at disable keeps sysfs removal; defer
kobject_put() to scx_sched_free_rcu_work so the memory survives. A racing
child sees state_in_sysfs=0 with valid memory, sysfs_create_dir() fails, and
the existing exit_kind gate in scx_link_sched() turns it away with -ENOENT.

Fixes: 411d3ef ("sched_ext: Unregister sub_kset on scheduler disable")
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: htejun

kernel/sched/ext.c

@@ -4821,6 +4821,8 @@ static void scx_sched_free_rcu_work(struct work_struct *work)
 	kfree(sch->cgrp_path);
 	if (sch_cgroup(sch))
 		cgroup_put(sch_cgroup(sch));
+	if (sch->sub_kset)
+		kobject_put(&sch->sub_kset->kobj);
 #endif	/* CONFIG_EXT_SUB_SCHED */
 
 	for_each_possible_cpu(cpu) {
@@ -5861,7 +5863,7 @@ static void scx_sub_disable(struct scx_sched *sch)
 	if (sch->ops.exit)
 		SCX_CALL_OP(sch, exit, NULL, sch->exit_info);
 	if (sch->sub_kset)
-		kset_unregister(sch->sub_kset);
+		kobject_del(&sch->sub_kset->kobj);
 	kobject_del(&sch->kobj);
 }
 #else	/* CONFIG_EXT_SUB_SCHED */
@@ -5995,7 +5997,7 @@ static void scx_root_disable(struct scx_sched *sch)
 	 */
 #ifdef CONFIG_EXT_SUB_SCHED
 	if (sch->sub_kset)
-		kset_unregister(sch->sub_kset);
+		kobject_del(&sch->sub_kset->kobj);
 #endif
 	kobject_del(&sch->kobj);