[Intel-gfx] [PATCH 6/8] drm/i915: Pin the pages first in shmem prepare read/write

[Chris Wilson]

There is an improbable, but not impossible, case that if we leave the
pages unpin as we operate on the object, then somebody may steal 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 at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c | 88 ++++++++++++++++++++++++-----------------
 1 file changed, 51 insertions(+), 37 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index ffe3d3e9d69d..8c90b6a12d45 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -571,13 +571,22 @@ int i915_gem_obj_prepare_shmem_read(struct drm_i915_gem_object *obj,
 	if ((obj->ops->flags & I915_GEM_OBJECT_HAS_STRUCT_PAGE) == 0)
 		return -EINVAL;
 
+	ret = i915_gem_object_get_pages(obj);
+	if (ret)
+		return ret;
+
+	i915_gem_object_pin_pages(obj);
+
+	if (obj->base.write_domain == I915_GEM_DOMAIN_CPU)
+		goto out;
+
+	ret = i915_gem_object_wait_rendering(obj, true);
+	if (ret)
+		goto err_unpin;
+
 	i915_gem_object_flush_gtt_write_domain(obj);
 
 	if (!(obj->base.read_domains & I915_GEM_DOMAIN_CPU)) {
-		ret = i915_gem_object_wait_rendering(obj, true);
-		if (ret)
-			return ret;
-
 		/* If we're not in the cpu read domain, set ourself into the gtt
 		 * read domain and manually flush cachelines (if required). This
 		 * optimizes for the case when the gpu will dirty the data
@@ -586,26 +595,25 @@ int i915_gem_obj_prepare_shmem_read(struct drm_i915_gem_object *obj,
 							obj->cache_level);
 	}
 
-	ret = i915_gem_object_get_pages(obj);
-	if (ret)
-		return ret;
-
-	i915_gem_object_pin_pages(obj);
-
 	if (*needs_clflush && !boot_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;
 	}
 
+out:
+	/* 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,
-				    unsigned *needs_clflush)
+				     unsigned *needs_clflush)
 {
 	int ret;
 
@@ -613,20 +621,27 @@ int i915_gem_obj_prepare_shmem_write(struct drm_i915_gem_object *obj,
 	if ((obj->ops->flags & I915_GEM_OBJECT_HAS_STRUCT_PAGE) == 0)
 		return -EINVAL;
 
-	i915_gem_object_flush_gtt_write_domain(obj);
+	ret = i915_gem_object_get_pages(obj);
+	if (ret)
+		return ret;
 
-	if (obj->base.write_domain != I915_GEM_DOMAIN_CPU) {
-		ret = i915_gem_object_wait_rendering(obj, false);
-		if (ret)
-			return ret;
+	i915_gem_object_pin_pages(obj);
 
-		/* If we're not in the cpu write domain, set ourself into the
-		 * gtt write domain and manually flush cachelines (as required).
-		 * This optimizes for the case when the gpu will use the data
-		 * right away and we therefore have to clflush anyway.
-		 */
-		*needs_clflush |= cpu_write_needs_clflush(obj) << 1;
-	}
+	if (obj->base.write_domain == I915_GEM_DOMAIN_CPU)
+		goto out;
+
+	ret = i915_gem_object_wait_rendering(obj, false);
+	if (ret)
+		goto err_unpin;
+
+	i915_gem_object_flush_gtt_write_domain(obj);
+
+	/* If we're not in the cpu write domain, set ourself into the
+	 * gtt write domain and manually flush cachelines (as required).
+	 * This optimizes for the case when the gpu will use the data
+	 * right away and we therefore have to clflush anyway.
+	 */
+	*needs_clflush |= cpu_write_needs_clflush(obj) << 1;
 
 	/* Same trick applies to invalidate partially written cachelines read
 	 * before writing.
@@ -635,27 +650,26 @@ 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 && !boot_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;
 	}
 
 	if ((*needs_clflush & CLFLUSH_AFTER) == 0)
 		obj->cache_dirty = true;
 
+out:
 	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.
-- 
2.8.1