Re: kvm+nouveau induced lockdep gripe

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2020-10-27 07:03:38 [+0100], Mike Galbraith wrote:
> On Mon, 2020-10-26 at 20:53 +0100, Sebastian Andrzej Siewior wrote:
> >
> > Could you try this, please?
> 
> Nogo, first call of sched_setscheduler() is via kthread_create().  I
> confirmed that nuking that (gratuitous user foot saving override) call
> on top of moving sched_set_fifo() does shut it up, but that won't fly.

mkay. but this then, too. Let me try if I can figure out when this
broke.

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 3edaa380dc7b4..64d6afb127239 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -244,6 +244,7 @@ EXPORT_SYMBOL_GPL(kthread_parkme);
 static int kthread(void *_create)
 {
 	/* Copy data: it's on kthread's stack */
+	static const struct sched_param param = { .sched_priority = 0 };
 	struct kthread_create_info *create = _create;
 	int (*threadfn)(void *data) = create->threadfn;
 	void *data = create->data;
@@ -273,6 +274,13 @@ static int kthread(void *_create)
 	init_completion(&self->parked);
 	current->vfork_done = &self->exited;
 
+	/*
+	 * root may have changed our (kthreadd's) priority or CPU mask.
+	 * The kernel thread should not inherit these properties.
+	 */
+	sched_setscheduler_nocheck(current, SCHED_NORMAL, &param);
+	set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_KTHREAD));
+
 	/* OK, tell user we're spawned, wait for stop or wakeup */
 	__set_current_state(TASK_UNINTERRUPTIBLE);
 	create->result = current;
@@ -370,7 +378,6 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
 	}
 	task = create->result;
 	if (!IS_ERR(task)) {
-		static const struct sched_param param = { .sched_priority = 0 };
 		char name[TASK_COMM_LEN];
 
 		/*
@@ -379,13 +386,6 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
 		 */
 		vsnprintf(name, sizeof(name), namefmt, args);
 		set_task_comm(task, name);
-		/*
-		 * root may have changed our (kthreadd's) priority or CPU mask.
-		 * The kernel thread should not inherit these properties.
-		 */
-		sched_setscheduler_nocheck(task, SCHED_NORMAL, &param);
-		set_cpus_allowed_ptr(task,
-				     housekeeping_cpumask(HK_FLAG_KTHREAD));
 	}
 	kfree(create);
 	return task;

Sebastian