Re: stable 5.4.135 REGRESSION / once-a-day WARNING: at kthread_is_per_cpu+0x1c/0x20

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 31, 2021 at 06:46:40AM +0200, Patrick Schaaf wrote:
> Looking into this again.
> 
> Unfortunately, couldn't see how I would do bisection on the issue, as
> it appears with that 5.4.118 commit implicated by the call stack,and
> with tha tremoved,is obviously gone(that I tested, 5.4.135 with
> b56ad4febe67b8c0647c0a3e427e935a76dedb59 reverted runs smoothly for
> me, while the original 5.4.135 with that 5.4.118 time commit in, now
> on a dozen machines, throws the WARNING.
> 
> I got email on the side of someone (Igor, on Cc) who sees the same
> with DELL servers, a newer 5.10 kernel, for him running IPVS + he sees
> actual operational impact there.
> 
> I just had a look at Linus mainlinetree, and see there is this
> followup / further fix from Peter Zijlstra,
> https://github.com/torvalds/linux/commit/3a7956e25e1d7b3c148569e78895e1f3178122a9
> ; now I'm much too incompetent to try and backport that, as it looks
> more involved, but I imagine such a backport would be needed to fix
> the WARNING (or IPVS breakage of Igor) we see.

3a7956e25e1d ("kthread: Fix PF_KTHREAD vs to_kthread() race") munged
into 5.4.135

Never even seen a compiler, please tests.

---
 kernel/kthread.c    | 43 +++++++++++++++++++++++++++++--------------
 kernel/sched/fair.c |  2 +-
 2 files changed, 30 insertions(+), 15 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index b2bac5d929d2..22750a8af83e 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -76,6 +76,25 @@ static inline struct kthread *to_kthread(struct task_struct *k)
 	return (__force void *)k->set_child_tid;
 }
 
+/*
+ * Variant of to_kthread() that doesn't assume @p is a kthread.
+ *
+ * Per construction; when:
+ *
+ *   (p->flags & PF_KTHREAD) && p->set_child_tid
+ *
+ * the task is both a kthread and struct kthread is persistent. However
+ * PF_KTHREAD on it's own is not, kernel_thread() can exec() (See umh.c and
+ * begin_new_exec()).
+ */
+static inline struct kthread *__to_kthread(struct task_struct *p)
+{
+	void *kthread = (__force void *)p->set_child_tid;
+	if (kthread && !(p->flags & PF_KTHREAD))
+		kthread = NULL;
+	return kthread;
+}
+
 void free_kthread_struct(struct task_struct *k)
 {
 	struct kthread *kthread;
@@ -176,10 +195,11 @@ void *kthread_data(struct task_struct *task)
  */
 void *kthread_probe_data(struct task_struct *task)
 {
-	struct kthread *kthread = to_kthread(task);
+	struct kthread *kthread = __to_kthread(task);
 	void *data = NULL;
 
-	probe_kernel_read(&data, &kthread->data, sizeof(data));
+	if (kthread)
+		probe_kernel_read(&data, &kthread->data, sizeof(data));
 	return data;
 }
 
@@ -490,9 +510,9 @@ void kthread_set_per_cpu(struct task_struct *k, int cpu)
 	set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
 }
 
-bool kthread_is_per_cpu(struct task_struct *k)
+bool kthread_is_per_cpu(struct task_struct *p)
 {
-	struct kthread *kthread = to_kthread(k);
+	struct kthread *kthread = __to_kthread(p);
 	if (!kthread)
 		return false;
 
@@ -1272,11 +1292,9 @@ EXPORT_SYMBOL(kthread_destroy_worker);
  */
 void kthread_associate_blkcg(struct cgroup_subsys_state *css)
 {
-	struct kthread *kthread;
+	struct kthread *kthread = __to_kthread(current);
+
 
-	if (!(current->flags & PF_KTHREAD))
-		return;
-	kthread = to_kthread(current);
 	if (!kthread)
 		return;
 
@@ -1298,13 +1316,10 @@ EXPORT_SYMBOL(kthread_associate_blkcg);
  */
 struct cgroup_subsys_state *kthread_blkcg(void)
 {
-	struct kthread *kthread;
+	struct kthread *kthread = __to_kthread(current)
 
-	if (current->flags & PF_KTHREAD) {
-		kthread = to_kthread(current);
-		if (kthread)
-			return kthread->blkcg_css;
-	}
+	if (kthread)
+		return kthread->blkcg_css;
 	return NULL;
 }
 EXPORT_SYMBOL(kthread_blkcg);
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 74cb20f32f72..87d9fad9d01d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7301,7 +7301,7 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 		return 0;
 
 	/* Disregard pcpu kthreads; they are where they need to be. */
-	if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p))
+	if (kthread_is_per_cpu(p))
 		return 0;
 
 	if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) {