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+0x25/0xf5 [i915]
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 <shawn.starr@rogers.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=18902
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem_evict.c

@@ -79,6 +79,7 @@ mark_free(struct drm_i915_gem_object *obj_priv,
 	   struct list_head *unwind)
 {
 	list_add(&obj_priv->evict_list, unwind);
+	drm_gem_object_reference(&obj_priv->base);
 	return drm_mm_scan_add_block(obj_priv->gtt_space);
 }
 
@@ -165,6 +166,7 @@ i915_gem_evict_something(struct drm_device *dev, int min_size, unsigned alignmen
 	list_for_each_entry(obj_priv, &unwind_list, evict_list) {
 		ret = drm_mm_scan_remove_block(obj_priv->gtt_space);
 		BUG_ON(ret);
+		drm_gem_object_unreference(&obj_priv->base);
 	}
 
 	/* We expect the caller to unpin, evict all and try again, or give up.
@@ -181,18 +183,21 @@ found:
 			 * scanning, therefore store to be evicted objects on a
 			 * temporary list. */
 			list_move(&obj_priv->evict_list, &eviction_list);
-		}
+		} else
+			drm_gem_object_unreference(&obj_priv->base);
 	}
 
 	/* Unbinding will emit any required flushes */
 	list_for_each_entry_safe(obj_priv, tmp_obj_priv,
 				 &eviction_list, evict_list) {
 #if WATCH_LRU
-		DRM_INFO("%s: evicting %p\n", __func__, obj);
+		DRM_INFO("%s: evicting %p\n", __func__, &obj_priv->base);
 #endif
 		ret = i915_gem_object_unbind(&obj_priv->base);
 		if (ret)
 			return ret;
+
+		drm_gem_object_unreference(&obj_priv->base);
 	}
 
 	/* The just created free hole should be on the top of the free stack