https://bugzilla.kernel.org/show_bug.cgi?id=206619

            Bug ID: 206619
           Summary: reboot hung since request_firmware
           Product: ACPI
           Version: 2.5
    Kernel Version: 4.19
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Power-Off
          Assignee: acpi_power-...@kernel-bugs.osdl.org
          Reporter: zhaoqia...@xiaomi.com
        Regression: No

I got an issue in reboot pressure test,reboot command called by user first,all
userspace task has been killed,then a kworker thread grab cpu then invoking
request_firmware,but since uevent thread has been killed,request_firmware will
never load successfully,blocked in "wait_for_completion_killable_timeout"
,before wait complete,this thread has calling usermodehelper_read_trylock and
held "umhelper_sem" lock,at this time,init reboot continue to running,try to
__usermodehelper_disable,this needs hold umhelper_sem,but it's held by
kworker,casue reboot process blocked until request_firmware timeout and release
umhelper_sem,if timeout not be setting,this timeout maybe MAX_JIFFY_OFFSET,so i
think it's a risk.

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