sched_ext: TASK_DEAD tasks must be switched out of SCX on ops_disable

scx_ops_disable_workfn() only switches !TASK_DEAD tasks out of SCX while
calling scx_ops_exit_task() on all tasks including dead ones. This can leave
a dead task on SCX but with SCX_TASK_NONE state, which is inconsistent.

If another task was in the process of changing the TASK_DEAD task's
scheduling class and grabs the rq lock after scx_ops_disable_workfn() is
done with the task, the task ends up calling scx_ops_disable_task() on the
dead task which is in an inconsistent state triggering a warning:

  WARNING: CPU: 6 PID: 3316 at kernel/sched/ext.c:3411 scx_ops_disable_task+0x12c/0x160
  ...
  RIP: 0010:scx_ops_disable_task+0x12c/0x160
  ...
  Call Trace:
   <TASK>
   check_class_changed+0x2c/0x70
   __sched_setscheduler+0x8a0/0xa50
   do_sched_setscheduler+0x104/0x1c0
   __x64_sys_sched_setscheduler+0x18/0x30
   do_syscall_64+0x7b/0x140
   entry_SYSCALL_64_after_hwframe+0x76/0x7e
  RIP: 0033:0x7f140d70ea5b

There is no reason to leave dead tasks on SCX when unloading the BPF
scheduler. Fix by making scx_ops_disable_workfn() eject all tasks including
the dead ones from SCX.
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/sched/ext.c

@@ -3998,30 +3998,22 @@ static void scx_ops_disable_workfn(struct kthread_work *work)
 	spin_lock_irq(&scx_tasks_lock);
 	scx_task_iter_init(&sti);
 	/*
-	 * Invoke scx_ops_exit_task() on all non-idle tasks, including
-	 * TASK_DEAD tasks. Because dead tasks may have a nonzero refcount,
-	 * we may not have invoked sched_ext_free() on them by the time a
-	 * scheduler is disabled. We must therefore exit the task here, or we'd
-	 * fail to invoke ops.exit_task(), as the scheduler will have been
-	 * unloaded by the time the task is subsequently exited on the
-	 * sched_ext_free() path.
+	 * The BPF scheduler is going away. All tasks including %TASK_DEAD ones
+	 * must be switched out and exited synchronously.
 	 */
 	while ((p = scx_task_iter_next_locked(&sti, true))) {
 		const struct sched_class *old_class = p->sched_class;
 		struct sched_enq_and_set_ctx ctx;
 
-		if (READ_ONCE(p->__state) != TASK_DEAD) {
-			sched_deq_and_put_task(p, DEQUEUE_SAVE | DEQUEUE_MOVE,
-					       &ctx);
+		sched_deq_and_put_task(p, DEQUEUE_SAVE | DEQUEUE_MOVE, &ctx);
 
-			p->scx.slice = min_t(u64, p->scx.slice, SCX_SLICE_DFL);
-			__setscheduler_prio(p, p->prio);
-			check_class_changing(task_rq(p), p, old_class);
+		p->scx.slice = min_t(u64, p->scx.slice, SCX_SLICE_DFL);
+		__setscheduler_prio(p, p->prio);
+		check_class_changing(task_rq(p), p, old_class);
 
-			sched_enq_and_set_task(&ctx);
+		sched_enq_and_set_task(&ctx);
 
-			check_class_changed(task_rq(p), p, old_class, p->prio);
-		}
+		check_class_changed(task_rq(p), p, old_class, p->prio);
 		scx_ops_exit_task(p);
 	}
 	scx_task_iter_exit(&sti);