panic by unlocking of mutex in KLD

Julian Elischer julian at
Mon Jan 12 12:41:01 PST 2009

Alexej Sokolov wrote:
> 2009/1/12 Mateusz Guzik <mjguzik at>
>> On Mon, Jan 12, 2009 at 07:16:51PM +0100, Alexej Sokolov wrote:
>>> 2009/1/12 Mateusz Guzik <mjguzik at>
>>>> On Mon, Jan 12, 2009 at 05:19:56PM +0100, Alexej Sokolov wrote:
>>>>> 2009/1/12 Mateusz Guzik <mjguzik at>
>>>>>> Mutexes have owners. It panics on loading because processes cannot
>>>>>> return to userland with locks held.
>>>>> i am not sure about it. Some time ago i implemented a charecter
>> device
>>>> with
>>>>> two syscalls: write, read. "write" lock the mutex and  "read" unlock
>> it.
>>>> The
>>>>> user space programm opens device, then mekes "write" (mutex will held
>> in
>>>>> kernel), goes back to user space, then makes "read" (mutex will
>> unlocked
>>>> in
>>>>> kernel) and it all run without panic. If needed i can post the source
>>>> code.
>>>> Do you have kernel compiled with WITNESS? At least on -CURRENT the
>>>> kernel panicked like this (while loading your module):
>>>> System call kldload returning with 1 locks held
>>> My kernel is compiled without WITNESS. And it allows to lock mutex in one
>>> systcall (for example "write") and to unlock it in other ("read").
>>> Do you mean it is "very bad idea" to do something like this ?
>>> I could not find anywhere in the documentation that a it is not allowed
>> to
>>> return in the user space with a locked mutex.
>>> Can you give me some reference on man page, source code or something
>> other
>>> from where can I understand it ?
>> Locks are used to synchronize access to data changeable by other
>> threads. I don't know if I'm correct here, but let's consider the
>> following situation: your process grabs a mutex and returns to userland,
>> then it's killed due to segmentation violation. This mutex should (and
>> can be) unlocked on exit, but the state of data protected by it is
>> unknown. (For example your process was killed while inserting something
>> into linked list.) So even if the kernel could be guided to unlock it on
>> exit, the data could be in inconsistent state.
>> Also your locking scheme doesn't make much sense. Consider this:
>>        proc1 calls write on your cdev
>> but in the meantime
>>        proc2 calls read on your cdev
>> So you get panic because proc1 was writing some data. (attempt to unlock
>> mutex locked by proc1) Even if the kernel wouldn't panic, proc2 would
>> read inconsistend data because proc1 was writing. Proper solution is to
>> lock mutex before and after reading/writing data. For working example
>> you can check how devctl was implemented (sys/kern/subr_bus.c).
>> --
>> Mateusz Guzik <mjguzik at>
> Ok , now I understaand it.
> If a thread return to user space with locked mutex, kernel doesen't know if
> the thread will come back to unlock it. It is really unsafe return to
> userspace without unlocking of helding mutexes.

The system should panic if you try, and at least in debu mode it does 
chack and panic. I'm not sure if it does if all debugging is turned off.

> Thanks a lot for your help.
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