Ubuntu Wily / VMWare graphics boot regression

[Kamal Mostafa]

Hi Thomas, Sinclair, and my team-

Here's a weird one.  It appears that this Linux commit which was
recently applied to Ubuntu Wily 15.10 (via 4.2-stable):

  [mainline] 025af18 drm/ttm: Fixed a read/write lock imbalance

is the trigger for this rather major Ubuntu/VMWare graphics boot
regression:

  https://bugs.launchpad.net/ubuntu/wily/+source/linux/+bug/1548587
  Ubuntu 15.10 VMWare guest won't show UI after upgrading to 4.2.0-30

(In Comment #33 I produced a test kernel with that commit reverted  
which was confirmed as fixing the regression).


But the thing is...

025af18 (attached) just looks so *obviously* valid, in that the thing
it fixes looks like it was obviously wrong.  I was reluctant to even
try reverting it, and was surprised when multiple testers confirmed
that it fixed the problem.

Furthermore, backports of 025af18 have been deployed in many other
stable kernels (and of course, mainline) but the reported boot problem
** only seems to occur with v4.2-based kernels **.  The problem does
occur with 4.2-stable (including the 025af18 backport), but does _not_
occur with a 4.4 kernel (which always contained 025af18).  That commit
been shipping in pre-4.2 Ubuntu Trusty and Vivid for at cycle or two
with no reports of problems there either.

So despite the indication that 025af18 is the troublemaker for 4.2-
stable based kernels, I'm not very happy with the idea of just
reverting it from 4.2-stable or from Wily without a better
understanding of why.

Any thoughts on this topic will be much appreciated.

 -Kamal
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