[PATCH v17 12/18] drm/shmem-helper: Prepare drm_gem_shmem_free() to shrinker addition

[Dmitry Osipenko]

Prepare drm_gem_shmem_free() to addition of memory shrinker support
to drm-shmem by adding and using variant of put_pages() that doesn't
touch reservation lock. Reservation shouldn't be touched because lockdep
will trigger a bogus warning about locking contention with fs_reclaim
code paths that can't happen during the time when GEM is freed and
lockdep doesn't know about that.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko at collabora.com>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 55 +++++++++++++++++---------
 1 file changed, 37 insertions(+), 18 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 8a8eab4d0332..4959f51b647a 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -128,6 +128,41 @@ struct drm_gem_shmem_object *drm_gem_shmem_create(struct drm_device *dev, size_t
 }
 EXPORT_SYMBOL_GPL(drm_gem_shmem_create);
 
+static void
+__drm_gem_shmem_release_pages(struct drm_gem_shmem_object *shmem)
+{
+	struct drm_gem_object *obj = &shmem->base;
+
+#ifdef CONFIG_X86
+	if (shmem->map_wc)
+		set_pages_array_wb(shmem->pages, obj->size >> PAGE_SHIFT);
+#endif
+
+	drm_gem_put_pages(obj, shmem->pages,
+			  shmem->pages_mark_dirty_on_put,
+			  shmem->pages_mark_accessed_on_put);
+	shmem->pages = NULL;
+}
+
+static void
+__drm_gem_shmem_put_pages(struct drm_gem_shmem_object *shmem)
+{
+	/*
+	 * Destroying the object is a special case.  Acquiring the obj
+	 * lock in drm_gem_shmem_put_pages_locked() can cause a locking
+	 * order inversion between reservation_ww_class_mutex and fs_reclaim
+	 * when called from drm_gem_shmem_free().
+	 *
+	 * This deadlock is not actually possible, because no one should
+	 * be already holding the lock when drm_gem_shmem_free() is called.
+	 * Unfortunately lockdep is not aware of this detail.  So when the
+	 * refcount drops to zero, make sure that the reservation lock
+	 * isn't touched here.
+	 */
+	if (refcount_dec_and_test(&shmem->pages_use_count))
+		__drm_gem_shmem_release_pages(shmem);
+}
+
 /**
  * drm_gem_shmem_free - Free resources associated with a shmem GEM object
  * @shmem: shmem GEM object to free
@@ -142,8 +177,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
 	if (obj->import_attach) {
 		drm_prime_gem_destroy(obj, shmem->sgt);
 	} else {
-		dma_resv_lock(shmem->base.resv, NULL);
-
 		drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count));
 
 		if (shmem->sgt) {
@@ -153,11 +186,9 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
 			kfree(shmem->sgt);
 		}
 		if (shmem->pages)
-			drm_gem_shmem_put_pages_locked(shmem);
+			__drm_gem_shmem_put_pages(shmem);
 
 		drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count));
-
-		dma_resv_unlock(shmem->base.resv);
 	}
 
 	drm_gem_object_release(obj);
@@ -207,21 +238,9 @@ static int drm_gem_shmem_get_pages_locked(struct drm_gem_shmem_object *shmem)
  */
 void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
 {
-	struct drm_gem_object *obj = &shmem->base;
-
 	dma_resv_assert_held(shmem->base.resv);
 
-	if (refcount_dec_and_test(&shmem->pages_use_count)) {
-#ifdef CONFIG_X86
-		if (shmem->map_wc)
-			set_pages_array_wb(shmem->pages, obj->size >> PAGE_SHIFT);
-#endif
-
-		drm_gem_put_pages(obj, shmem->pages,
-				  shmem->pages_mark_dirty_on_put,
-				  shmem->pages_mark_accessed_on_put);
-		shmem->pages = NULL;
-	}
+	__drm_gem_shmem_put_pages(shmem);
 }
 EXPORT_SYMBOL_GPL(drm_gem_shmem_put_pages_locked);
 
-- 
2.41.0