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Oleg Nesterov authored
Now that to_kthread() is always validm change kthread_park() and kthread_unpark() to use it and kill to_live_kthread(). The conversion of kthread_unpark() is trivial. If KTHREAD_IS_PARKED is set then the task has called complete(&self->parked) and there the function cannot race against a concurrent kthread_stop() and exit. kthread_park() is more tricky, because its semantics are not well defined. It returns -ENOSYS if the thread exited but this can never happen and as Roman pointed out kthread_park() can obviously block forever if it would race with the exiting kthread. The usage of kthread_park() in cpuhp code (cpu.c, smpboot.c, stop_machine.c) is fine. It can never see an exiting/exited kthread, smpboot_destroy_threads() clears *ht->store, smpboot_park_thread() checks it is not NULL under the same smpboot_threads_lock. cpuhp_threads and cpu_stop_threads never exit, so other callers are fine too. But it has two more users: - watchdog_park_threads(): The code is actually correct, get_online_cpus() ensures that kthread_park() can't race with itself (note that kthread_park() can't handle this race correctly), but it should not use kthread_park() directly. - drivers/gpu/drm/amd/scheduler/gpu_scheduler.c should not use kthread_park() either. kthread_park() must not be called after amd_sched_fini() which does kthread_stop(), otherwise even to_live_kthread() is not safe because task_struct can be already freed and sched->thread can point to nowhere. The usage of kthread_park/unpark should either be restricted to core code which is properly protected against the exit race or made more robust so it is safe to use it in drivers. To catch eventual exit issues, add a WARN_ON(PF_EXITING) for now. Signed-off-by:
Oleg Nesterov <oleg@redhat.com> Acked-by:
cf380a4a