Modesetting lock changes breaks vmwgfx since 3.18-rc1

[Thomas Hellstrom]

Hi!

On 10/31/2014 06:33 PM, Daniel Vetter wrote:
> On Thu, Oct 30, 2014 at 03:41:31PM +0100, Thomas Hellstrom wrote:
>> Hi!
>>
>> Details in
>>
>> https://urldefense.proofpoint.com/v2/url?u=https-3A__bugzilla.redhat.com_show-5Fbug.cgi-3Fid-3D1155825&d=AAIBAg&c=Sqcl0Ez6M0X8aeM67LKIiDJAXVeAw-YihVMNtXt-uEs&r=vpukPkBtpoNQp2IUKuFviOmPNYWVKmen3Jeeu55zmEA&m=lsrU5TbpNlecpBoc0CpsmwopYh2VrihduvgGEIFPsXw&s=uzLKWffads4edz_fJN5-3K2IObzmVFUcH6kty4ObySs&e= 
>>
>> Can whoever broke the driver please make an effort to fix it up?
>>
>> Daniel, You've been named as a suspect. I'm not sure if that is true.
> Yeah the recent locking frobbing didn't account for the FIXME comment in
> vmwgfx. Might be good to address that long-term to get vmwgfx more in line
> with everyone else again - I simply don't understand the interactions well
> enough to make that call. There's two of those in total.
>
> Can we just remove them perhaps? The question is whether you can do the
> cursor operation just with the per-crtc lock as protection, or whether
> there's any shared resources and you need the full exclusion provided by
> modeset_lock_all. If the crtc locking is good enough then we could rip
> this out in both function and be done.
>
> Note that this FIXME has been in vmwgfx since almost two years now.
>
> If that's not possible then I have a major kludge of a hack in my atomic
> helpers branch which will keep this alive for a bit longer. It ain't
> pretty though.
>
> Cheers, Daniel

Hi.

Actually we haven't looked much at the new modesetting locking at all
but been pretty happy with what others have put in the code, but it
sounds like we need to actually deal with this now. I've basically never
been really convinced that we'd see any modesetting locking contention
with a single lock, but OTOH I guess with a single lock we'd eventually
start to run into locking recursion problems.

/Thomas