Hello, Just took a look out of curiosity.
On Thu, May 12, 2022 at 02:25:57PM +0900, Byungchul Park wrote: > PROCESS A PROCESS B WORKER C > > __do_sys_reboot() > __do_sys_reboot() > mutex_lock(&system_transition_mutex) > ... mutex_lock(&system_transition_mutex) <- stuck > ... > request_firmware_work_func() > _request_firmware() > firmware_fallback_sysfs() > usermodehelper_read_lock_wait() > down_read(&umhelper_sem) > ... > fw_load_sysfs_fallback() > fw_sysfs_wait_timeout() > > wait_for_completion_killable_timeout(&fw_st->completion) <- stuck > kernel_halt() > __usermodehelper_disable() > down_write(&umhelper_sem) <- stuck > > -------------------------------------------------------- > All the 3 contexts are stuck at this point. > -------------------------------------------------------- > > PROCESS A PROCESS B WORKER C > > ... > up_write(&umhelper_sem) > ... > mutex_unlock(&system_transition_mutex) <- cannot wake up B > > ... > kernel_halt() > notifier_call_chain() > hw_shutdown_notify() > kill_pending_fw_fallback_reqs() > __fw_load_abort() > complete_all(&fw_st->completion) <- cannot wake up C > > ... > usermodeheler_read_unlock() > up_read(&umhelper_sem) <- cannot wake up A I'm not sure I'm reading it correctly but it looks like "process B" column is superflous given that it's waiting on the same lock to do the same thing that A is already doing (besides, you can't really halt the machine twice). What it's reporting seems to be ABBA deadlock between A waiting on umhelper_sem and C waiting on fw_st->completion. The report seems spurious: 1. wait_for_completion_killable_timeout() doesn't need someone to wake it up to make forward progress because it will unstick itself after timeout expires. 2. complete_all() from __fw_load_abort() isn't the only source of wakeup. The fw loader can be, and mainly should be, woken up by firmware loading actually completing instead of being aborted. I guess the reason why B shows up there is because the operation order is such that just between A and C, the complete_all() takes place before __usermodehlper_disable(), so the whole thing kinda doesn't make sense as you can't block a past operation by a future one. Inserting process B introduces the reverse ordering. Thanks. -- tejun
