Re: [syzbot] [cgroups?] possible deadlock in task_rq_lock

[Hillf Danton <hdanton@sina.com>]

On Fri, 16 Aug 2024 23:52:22 -0700
> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    367b5c3d53e5 Add linux-next specific files for 20240816
> git tree:       linux-next
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=142413c5980000

#syz test linux-next  367b5c3d53e5

--- x/kernel/sched/deadline.c
+++ y/kernel/sched/deadline.c
@@ -1498,6 +1498,9 @@ static void update_curr_dl_se(struct rq
 	 * starting a new period, pushing the activation.
 	 */
 	if (dl_se->dl_defer && dl_se->dl_throttled && dl_runtime_exceeded(dl_se)) {
+		bool lock = rq != dl_se->rq;
+		struct rq_flags rf;
+		struct rq *__rq = dl_se->rq;
 		/*
 		 * If the server was previously activated - the starving condition
 		 * took place, it this point it went away because the fair scheduler
@@ -1508,7 +1511,11 @@ static void update_curr_dl_se(struct rq
 
 		hrtimer_try_to_cancel(&dl_se->dl_timer);
 
+		if (lock)
+			rq_lock(__rq, &rf);
 		replenish_dl_new_period(dl_se, dl_se->rq);
+		if (lock)
+			rq_unlock(__rq, &rf);
 
 		/*
 		 * Not being able to start the timer seems problematic. If it could not
--- x/kernel/workqueue.c
+++ y/kernel/workqueue.c
@@ -5653,6 +5653,7 @@ static struct workqueue_struct *__alloc_
 	wq = kzalloc(wq_size, GFP_KERNEL);
 	if (!wq)
 		return NULL;
+	wq_init_lockdep(wq);
 
 	if (flags & WQ_UNBOUND) {
 		wq->unbound_attrs = alloc_workqueue_attrs();
@@ -5757,10 +5758,6 @@ struct workqueue_struct *alloc_workqueue
 	va_start(args, max_active);
 	wq = __alloc_workqueue(fmt, flags, max_active, args);
 	va_end(args);
-	if (!wq)
-		return NULL;
-
-	wq_init_lockdep(wq);
 
 	return wq;
 }
--