connector/edid probing races

[Sean Paul]

On Fri, Mar 10, 2017 at 05:52:34PM +0100, Daniel Vetter wrote:
> On Fri, Mar 10, 2017 at 11:01:54AM -0500, Sean Paul wrote:
> > On Fri, Mar 10, 2017 at 02:12:14PM +1000, Dave Airlie wrote:
> > > Talk to Jonas (jadahl) on irc, he had 3 mutters running and on hotplug
> > > all 3 of them were diving into the connector getting code. Now I think
> > > we can avoid this in userspace by not probing when not owning the VT,
> > > but it's still messy behaviour.
> > > 
> > > It looks like one thread does a getconnector, this fills out the EDID
> > > blob with a specific blob id,
> > > then second thread does getconnector, which replaces the EDID blob
> > > with another one
> > > the first thread calls get property blob, but the blob id it is
> > > looking for is destroyed.
> > > 
> > > Ideas?
> > 
> > I think you could probably fix this by introducing a new
> > drm_property_overwrite_global_blob() function. If the size of the current blob is
> > the same as the new blob, you could just overwrite the data and preserve the id
> > and references. You could then replace the drm_property_replace_global_blob() call in
> > drm_mode_connector_update_edid_property() to call the new function. Of course,
> > you would need to fall back on *replace_global_blob() if size differed, but it
> > would probably solve the issue above.
> 
> blobs are imutable (not that userspace can use that much since we can
> reuse ids), 

Pedantic distinction: blobs themselves aren't inherantly immutable. However, the
EDID property is defined as immutable. I suppose there could conceivably be some
userspace somewhere that relies on this. Bummer.


> I think if we do this we should compare the contents of the
> blob.
> 
> > That said, I'm not convinced it should be kernel's responsibility to really
> > solve this problem. How much effort should we put into mitigating the effects of
> > a racey and wasteful userspace?
> 
> It's a tradeoff between "why should the kernel care" and "every compositor
> will get this wrong". Maybe something like libweston will fix this
> eventually ....

So the question is whether it's worth it to penalize well-behaved compositors
with superfluous memcmp on EDID fetches to make up for the ill-behaved ones. I
don't think it is, but perhaps there are other benefits to preserving the single
blob for the lifetime of the connection.

Sean



> -Daniel
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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-- 
Sean Paul, Software Engineer, Google / Chromium OS