[stable 9] panic on reboot: ipmi_wd_event()

[Attilio Rao]

On 8/1/12, John Baldwin <jhb at freebsd.org> wrote:
> On Tuesday, July 31, 2012 4:51:19 pm Attilio Rao wrote:
>> On 7/31/12, John Baldwin <jhb at freebsd.org> wrote:
>> > On Thursday, July 19, 2012 7:58:14 pm Sean Bruno wrote:
>> >> Working on the Dell R420 today, got most of it working, even the
>> >> broadcom ethernet cards!  However, I get the following when I reboot
>> >> the
>> >> system:
>> >>
>> >> Syncing disks, vnodes remaining...4 Sleeping thread (tid 100107, pid
>> >> 9)
>> >> owns a non-sleepable lock
>> >> KDB: stack backtrace of thread 100107:
>> >> sched_switch() at sched_switch+0x19f
>> >> mi_switch() at mi_switch+0x208
>> >> sleepq_switch() at sleepq_switch+0xfc
>> >> sleepq_wait() at sleepq_wait+0x4d
>> >> _sleep() at _sleep+0x3f6
>> >> ipmi_submit_driver_request() at ipmi_submit_driver_request+0x97
>> >> ipmi_set_watchdog() at ipmi_set_watchdog+0xb1
>> >> ipmi_wd_event() at ipmi_wd_event+0x8f
>> >> kern_do_pat() at kern_do_pat+0x10f
>> >> sched_sync() at sched_sync+0x1ea
>> >> fork_exit() at fork_exit+0x135
>> >> fork_trampoline() at fork_trampoline+0xe
>> >
>> > Hmmm, the watchdog pat should probably happen without holding locks if
>> > possible.  This is related to the IPMI watchdog being special and
>> > wanting
>> > to schedule a thread to work.
>>
>> The watchdog pat without the locks is not easy to do because we
>> register the watchdog callbacks in eventhandlers, which are indeed
>> locked (and you may also end up racing against watchdog detach, if you
>> don't use any lock at all).
>
> No, eventhandlers go through several hoops to not hold any locks while
> the eventhandler functions are running.  It seems in this case that a
> lock is held in a higher layer (sched_sync()) and that is what I was
> talking about.  Yes, it is the 'sync_mtx' that is held.  Something like this

No, EVENTHANDLER_INVOKE() acquires eventhandler internal locks.
Look at eventhandler_find_list() for details.

Attilio


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