drm/i915: Pin the pages first in shmem prepare read/write

There is an improbable, but not impossible, case that if we leave the
pages unpin as we operate on the object, then somebody via the shrinker
may steal the lock (which lock? right now, it is struct_mutex, THE lock)
and change the cache domains after we have already inspected them.

(Whilst here, avail ourselves of the opportunity to take a couple of
steps to make the two functions look more similar.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818161718.27187-11-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -622,6 +622,12 @@ int i915_gem_obj_prepare_shmem_read(struct drm_i915_gem_object *obj,
 	if (ret)
 		return ret;
 
+	ret = i915_gem_object_get_pages(obj);
+	if (ret)
+		return ret;
+
+	i915_gem_object_pin_pages(obj);
+
 	i915_gem_object_flush_gtt_write_domain(obj);
 
 	/* If we're not in the cpu read domain, set ourself into the gtt
@@ -633,22 +639,20 @@ int i915_gem_obj_prepare_shmem_read(struct drm_i915_gem_object *obj,
 		*needs_clflush = !cpu_cache_is_coherent(obj->base.dev,
 							obj->cache_level);
 
-	ret = i915_gem_object_get_pages(obj);
-	if (ret)
-		return ret;
-
-	i915_gem_object_pin_pages(obj);
-
 	if (*needs_clflush && !static_cpu_has(X86_FEATURE_CLFLUSH)) {
 		ret = i915_gem_object_set_to_cpu_domain(obj, false);
-		if (ret) {
-			i915_gem_object_unpin_pages(obj);
-			return ret;
-		}
+		if (ret)
+			goto err_unpin;
+
 		*needs_clflush = 0;
 	}
 
+	/* return with the pages pinned */
 	return 0;
+
+err_unpin:
+	i915_gem_object_unpin_pages(obj);
+	return ret;
 }
 
 int i915_gem_obj_prepare_shmem_write(struct drm_i915_gem_object *obj,
@@ -664,6 +668,12 @@ int i915_gem_obj_prepare_shmem_write(struct drm_i915_gem_object *obj,
 	if (ret)
 		return ret;
 
+	ret = i915_gem_object_get_pages(obj);
+	if (ret)
+		return ret;
+
+	i915_gem_object_pin_pages(obj);
+
 	i915_gem_object_flush_gtt_write_domain(obj);
 
 	/* If we're not in the cpu write domain, set ourself into the
@@ -681,18 +691,11 @@ int i915_gem_obj_prepare_shmem_write(struct drm_i915_gem_object *obj,
 		*needs_clflush |= !cpu_cache_is_coherent(obj->base.dev,
 							 obj->cache_level);
 
-	ret = i915_gem_object_get_pages(obj);
-	if (ret)
-		return ret;
-
-	i915_gem_object_pin_pages(obj);
-
 	if (*needs_clflush && !static_cpu_has(X86_FEATURE_CLFLUSH)) {
 		ret = i915_gem_object_set_to_cpu_domain(obj, true);
-		if (ret) {
-			i915_gem_object_unpin_pages(obj);
-			return ret;
-		}
+		if (ret)
+			goto err_unpin;
+
 		*needs_clflush = 0;
 	}
 
@@ -701,7 +704,12 @@ int i915_gem_obj_prepare_shmem_write(struct drm_i915_gem_object *obj,
 
 	intel_fb_obj_invalidate(obj, ORIGIN_CPU);
 	obj->dirty = 1;
+	/* return with the pages pinned */
 	return 0;
+
+err_unpin:
+	i915_gem_object_unpin_pages(obj);
+	return ret;
 }
 
 /* Per-page copy function for the shmem pread fastpath.