drm/i915: properly SIGBUS on I/O errors

... instead of looping endless with no hope of ever serving that
page-fault. We only need to break out of this loop when the gpu died,
to run the reset work (and hopefully resurrect it).

To clarify questions Chris raised on irc: This is about handling I/O
errors not from our own code, but e.g. when the disk died when trying
to swap in a gem bo. So this patch remidies the issue that the current
handling only handles gpu-death-induced cases of -EIO. Admittedly,
dying disks are much rarer than hanging gpus ...To clarify questions
Chris raised on irc: This is about handling I/O errors not from our
own code, but e.g. when the disk died when trying to swap in a gem bo.
So this patch remidies the issue that the current handling only
handles gpu-death-induced cases of -EIO. Admittedly, dying disks are
much rarer than hanging gpus ...

This seems to have been lost in:

commit d9bc7e9f32716901c617e1f0fb6ce0f74f172686
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Feb 7 13:09:31 2011 +0000

    drm/i915: Fix infinite loop regression from 21dd3734

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -1141,6 +1141,11 @@ unlock:
 out:
 	switch (ret) {
 	case -EIO:
+		/* If this -EIO is due to a gpu hang, give the reset code a
+		 * chance to clean up the mess. Otherwise return the proper
+		 * SIGBUS. */
+		if (!atomic_read(&dev_priv->mm.wedged))
+			return VM_FAULT_SIGBUS;
 	case -EAGAIN:
 		/* Give the error handler a chance to run and move the
 		 * objects off the GPU active list. Next time we service the