[Intel-gfx] PROBLEM: i915 causes complete desktop freezes in 4.15-rc5

[Chris Wilson]

Quoting Alexandru Chirvasitu (2018-01-06 18:44:29)
> Thanks!
> 
> It's also a mystery to me why I never had any crashes on any of the
> other systems running on this machine running the same (unpatched)
> kernels.
> 
> I'm assuming the window manager might have something to do with it:
> all of the others are on i3 and the buggy one's openbox, so perhaps
> tiling vs. stacking makes a difference?

It just takes the right pattern of activity. The logic upon retiring a
request does try to strip the fences from all the objects listening to
the fence, but we can only do that so long as all the fences on the
object have been signaled. So for starters, it needs an object being
used by multiple timelines (different processes and/or engines) and then
retirement has to be run at just the right frequency to not see all of
those fences not to be completed. (Even more for this failure, it must
have a retired exclusive/write fence paired with active shared/read
fences.) It is that timing issue that has made it so rare, it's a
pattern that we definitely do not expose in our testing.
-Chris