Comment 2 for bug 843904

It looks like we may need to backport this fix from upstream. I'll look into the backport when I get time.

commit af6261031317f646d22f994c0b467521e47aa49f
Author: Chris Wilson <email address hidden>
Date: Mon Sep 20 10:31:40 2010 +0100

    drm/i915: Hold a reference to the object whilst unbinding the eviction list

    During heavy aperture thrashing we may be forced to wait upon several active
    objects during eviction. The active list may be the last reference to
    these objects and so the action of waiting upon one of them may cause
    another to be freed (and itself unbound). To prevent the object
    disappearing underneath us, we need to acquire and hold a reference
    whilst unbinding.

    This should fix the reported page refcount OOPS:

    kernel BUG at drivers/gpu/drm/i915/i915_gem.c:1444!
    ...
    RIP: 0010:[<ffffffffa0093026>] [<ffffffffa0093026>] i915_gem_object_put_pages+0x25/0
    Call Trace:
     [<ffffffffa009481d>] i915_gem_object_unbind+0xc5/0x1a7 [i915]
     [<ffffffffa0098ab2>] i915_gem_evict_something+0x3bd/0x409 [i915]
     [<ffffffffa0027923>] ? drm_gem_object_lookup+0x27/0x57 [drm]
     [<ffffffffa0093bc3>] i915_gem_object_bind_to_gtt+0x1d3/0x279 [i915]
     [<ffffffffa0095b30>] i915_gem_object_pin+0xa3/0x146 [i915]
     [<ffffffffa0027948>] ? drm_gem_object_lookup+0x4c/0x57 [drm]
     [<ffffffffa00961bc>] i915_gem_do_execbuffer+0x50d/0xe32 [i915]

    Reported-by: Shawn Starr <email address hidden>
    Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=18902
    Signed-off-by: Chris Wilson <email address hidden>