Re: [syzbot] WARNING in kthread_is_per_cpu

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Apr 19, 2021 at 12:31:22PM +0100, Valentin Schneider wrote:

>   if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))
>                                  `\
>                                    to_kthread(p);
>                                     `\
>                                       WARN_ON(!(p->flags & PF_KTHREAD));
> 
> ... Huh?

Something like so perhaps?

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 1578973c5740..eeba40df61ac 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -78,6 +78,14 @@ static inline void set_kthread_struct(void *kthread)
 	current->set_child_tid = (__force void __user *)kthread;
 }
 
+static inline struct kthread *__to_kthread(struct task_struct *k)
+{
+	void *kthread = (__force void *)k->set_child_tid;
+	if (kthread && !(k->flags & PF_KTHREAD))
+		kthread = NULL;
+	return kthread;
+}
+
 static inline struct kthread *to_kthread(struct task_struct *k)
 {
 	WARN_ON(!(k->flags & PF_KTHREAD));
@@ -516,7 +524,7 @@ void kthread_set_per_cpu(struct task_struct *k, int cpu)
 
 bool kthread_is_per_cpu(struct task_struct *k)
 {
-	struct kthread *kthread = to_kthread(k);
+	struct kthread *kthread = __to_kthread(k);
 	if (!kthread)
 		return false;
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3384ea74cad4..dc6311bd6986 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7658,7 +7658,7 @@ static void balance_push(struct rq *rq)
 	 * histerical raisins.
 	 */
 	if (rq->idle == push_task ||
-	    ((push_task->flags & PF_KTHREAD) && kthread_is_per_cpu(push_task)) ||
+	    kthread_is_per_cpu(push_task) ||
 	    is_migration_disabled(push_task)) {
 
 		/*