We had another report that it happens on RHEL 8 host, which is a very much older (franken) kernel.

I noticed it hangs in similar way when you insert msleep anywhere before smp_prepare_cpus in kernel_init_freeable. But I have no idea whether sleeping is valid here.

Can confirm - just adding msleep(1) there causes the same failure. I'm also unclear if sleeping here is valid or not.