On 07/07/21 14:03, Frederic Weisbecker wrote: > On Wed, Jul 07, 2021 at 12:55:20AM +0100, Valentin Schneider wrote: >> Thanks for the report. >> >> So somehow the init task ends up with a non-zero preempt_count()? Per >> FORK_PREEMPT_COUNT we should exit __ret_from_fork() with a zero count, are >> you hitting the WARN_ONCE() in finish_task_switch()? >> >> Does CONFIG_DEBUG_PREEMPT=y yield anything interesting? >> >> I can't make sense of this right now, but it's a bit late :) I'll grab some >> toolchain+qemu tomorrow and go poke at it (and while at it I need to do the >> same with powerpc). > > One possible issue is that s390's init_idle_preempt_count() doesn't apply on the > target idle task but on the _current_ CPU. And since smp_init() -> > idle_threads_init() is actually called remotely, we are overwriting the current > CPU preempt_count() instead of the target one. Indeed, this becomes quite obvious when tracing the preemption count changes. This also means that s390 relied on the idle_thread_get() from the hotplug machinery to properly setup the preempt count, rather than init_idle_preempt_count() - which is quite yuck. I'll write a patch for that and likely one for powerpc.
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