I have a busy ppc64le KVM box where guests sometimes hit the infamous "kernel BUG at kernel/smpboot.c:134!" issue during boot:
BUG_ON(td->cpu != smp_processor_id()); Basically a per CPU hotplug thread scheduled on the wrong CPU. The oops output confirms it: CPU: 0 Comm: watchdog/130 The issue is in kthread_bind where we set the cpus_allowed mask, but do not touch task_thread_info(p)->cpu. The scheduler assumes the previously scheduled CPU is in the cpus_allowed mask, but in this case we are moving a thread to another CPU so it is not. We used to call set_task_cpu which sets task_thread_info(p)->cpu (in fact kthread_bind still has a comment suggesting this). That was removed in e2912009fb7b ("sched: Ensure set_task_cpu() is never called on blocked tasks"). Since we cannot call set_task_cpu (the task is in a sleeping state), just do an explicit set of task_thread_info(p)->cpu. Fixes: e2912009fb7b ("sched: Ensure set_task_cpu() is never called on blocked tasks") Cc: sta...@vger.kernel.org Signed-off-by: Anton Blanchard <an...@samba.org> --- kernel/kthread.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/kthread.c b/kernel/kthread.c index 10e489c..e40ab1d 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -327,13 +327,14 @@ EXPORT_SYMBOL(kthread_create_on_node); static void __kthread_bind(struct task_struct *p, unsigned int cpu, long state) { - /* Must have done schedule() in kthread() before we set_task_cpu */ + /* Must have done schedule() in kthread() before we change affinity */ if (!wait_task_inactive(p, state)) { WARN_ON(1); return; } /* It's safe because the task is inactive. */ do_set_cpus_allowed(p, cpumask_of(cpu)); + task_thread_info(p)->cpu = cpu; p->flags |= PF_NO_SETAFFINITY; } -- 2.1.0 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/