VM lockdep warning

[Jerome Glisse]

2012/4/21 Christian König <deathsimple at vodafone.de>:
> On 21.04.2012 17:57, Dave Airlie wrote:
>>
>> 2012/4/21 Jerome Glisse<j.glisse at gmail.com>:
>>>
>>> 2012/4/21 Christian König<deathsimple at vodafone.de>:
>>>>
>>>> On 21.04.2012 16:08, Jerome Glisse wrote:
>>>>>
>>>>> 2012/4/21 Christian König<deathsimple at vodafone.de>:
>>>>>>
>>>>>> Interesting, I'm pretty sure that I haven't touched the locking order
>>>>>> of
>>>>>> the
>>>>>> cs_mutex vs. vm_mutex.
>>>>>>
>>>>>> Maybe it is just some kind of side effect, going to locking into it
>>>>>> anyway.
>>>>>>
>>>>>> Christian.
>>>>>>
>>>>> It's the using, init path take lock in different order than cs path
>>>>
>>>> Well, could you explain to me why the vm code takes cs mutex in the
>>>> first
>>>> place?
>>>>
>>>> It clearly has it's own mutex and it doesn't looks like that it deals
>>>> with
>>>> any cs related data anyway.
>>>>
>>>> Christian.
>>>
>>> Lock simplification is on my todo. The issue is that vm manager is
>>> protected by
>>> cs_mutex The vm.mutex is specific to each vm it doesn't protect the
>>> global vm
>>> management. I didn't wanted to introduce a new global vm mutex as vm
>>> activity
>>> is mostly trigger on behalf of cs so i dediced to use the cs mutex.
>>>
>>> That's why non cs path of vm need to take the cs mutex.
>>
>> So if one app is adding a bo, and another doing CS, isn't deadlock a
>> real possibility?
>
> Yeah, I think so.

No it's not. Look at the code.

>> I expect the VM code need to take CS mutex earlier then.

No it does not. The idea is that when adding a bo we only need to take the
cs mutex if we need to resize the vm size (and even that can be worked around).

So we will need to take the cs ioctl in very few case (suspend, increasing vm
size).

>
> I would strongly suggest to give the vm code their own global mutex and
> remove the per vm mutex, cause the later is pretty superfluous if the
> cs_mutex is also taken most of the time.
>
> The attached patch is against drm-fixes and does exactly that.
>
> Christian.

NAK with your change there will be lock contention if one app is in cs and
another try to create bo. Currently there is allmost never contention. Once
i ironed out the DP->VGA i will work on something to remove the cs mutex
from vm path (ie remove it from bo creation/del path).

Cheers,
Jerome