[PATCH v15 17/23] drm/shmem-helper: Add and use drm_gem_shmem_resv_assert_held() helper

[Dmitry Osipenko]

In a preparation of adding drm-shmem memory shrinker, move all reservation
locking lockdep checks to use new drm_gem_shmem_resv_assert_held() that
will resolve spurious lockdep warning about wrong locking order vs
fs_reclam code paths during freeing of shmem GEM, where lockdep isn't
aware that it's impossible to have locking contention with the fs_reclam
at this special time.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko at collabora.com>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 37 +++++++++++++++++---------
 1 file changed, 25 insertions(+), 12 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index d96fee3d6166..ca5da976aafa 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -128,6 +128,23 @@ 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_resv_assert_held(struct drm_gem_shmem_object *shmem)
+{
+	/*
+	 * Destroying the object is a special case.. drm_gem_shmem_free()
+	 * calls many things that WARN_ON if the obj lock is not held.  But
+	 * acquiring the obj lock in drm_gem_shmem_free() can cause a locking
+	 * order inversion between reservation_ww_class_mutex and fs_reclaim.
+	 *
+	 * 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, we pretend it is already locked.
+	 */
+	if (kref_read(&shmem->base.refcount))
+		drm_gem_shmem_resv_assert_held(shmem);
+}
+
 /**
  * drm_gem_shmem_free - Free resources associated with a shmem GEM object
  * @shmem: shmem GEM object to free
@@ -142,8 +159,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
 	if (obj->import_attach) {
 		drm_prime_gem_destroy(obj, shmem->sgt);
 	} else if (!shmem->imported_sgt) {
-		dma_resv_lock(shmem->base.resv, NULL);
-
 		drm_WARN_ON(obj->dev, kref_read(&shmem->vmap_use_count));
 
 		if (shmem->sgt) {
@@ -156,8 +171,6 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
 			drm_gem_shmem_put_pages_locked(shmem);
 
 		drm_WARN_ON(obj->dev, kref_read(&shmem->pages_use_count));
-
-		dma_resv_unlock(shmem->base.resv);
 	}
 
 	drm_gem_object_release(obj);
@@ -170,7 +183,7 @@ static int drm_gem_shmem_get_pages_locked(struct drm_gem_shmem_object *shmem)
 	struct drm_gem_object *obj = &shmem->base;
 	struct page **pages;
 
-	dma_resv_assert_held(shmem->base.resv);
+	drm_gem_shmem_resv_assert_held(shmem);
 
 	if (kref_get_unless_zero(&shmem->pages_use_count))
 		return 0;
@@ -228,7 +241,7 @@ static void drm_gem_shmem_kref_release_pages(struct kref *kref)
  */
 void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
 {
-	dma_resv_assert_held(shmem->base.resv);
+	drm_gem_shmem_resv_assert_held(shmem);
 
 	kref_put(&shmem->pages_use_count, drm_gem_shmem_kref_release_pages);
 }
@@ -252,7 +265,7 @@ static int drm_gem_shmem_pin_locked(struct drm_gem_shmem_object *shmem)
 {
 	int ret;
 
-	dma_resv_assert_held(shmem->base.resv);
+	drm_gem_shmem_resv_assert_held(shmem);
 
 	if (kref_get_unless_zero(&shmem->pages_pin_count))
 		return 0;
@@ -276,7 +289,7 @@ static void drm_gem_shmem_kref_unpin_pages(struct kref *kref)
 
 static void drm_gem_shmem_unpin_locked(struct drm_gem_shmem_object *shmem)
 {
-	dma_resv_assert_held(shmem->base.resv);
+	drm_gem_shmem_resv_assert_held(shmem);
 
 	kref_put(&shmem->pages_pin_count, drm_gem_shmem_kref_unpin_pages);
 }
@@ -357,7 +370,7 @@ int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem,
 	} else {
 		pgprot_t prot = PAGE_KERNEL;
 
-		dma_resv_assert_held(shmem->base.resv);
+		drm_gem_shmem_resv_assert_held(shmem);
 
 		if (kref_get_unless_zero(&shmem->vmap_use_count)) {
 			iosys_map_set_vaddr(map, shmem->vaddr);
@@ -426,7 +439,7 @@ void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
 	if (obj->import_attach) {
 		dma_buf_vunmap(obj->import_attach->dmabuf, map);
 	} else {
-		dma_resv_assert_held(shmem->base.resv);
+		drm_gem_shmem_resv_assert_held(shmem);
 		kref_put(&shmem->vmap_use_count, drm_gem_shmem_kref_vunmap);
 	}
 
@@ -462,7 +475,7 @@ drm_gem_shmem_create_with_handle(struct drm_file *file_priv,
  */
 int drm_gem_shmem_madvise_locked(struct drm_gem_shmem_object *shmem, int madv)
 {
-	dma_resv_assert_held(shmem->base.resv);
+	drm_gem_shmem_resv_assert_held(shmem);
 
 	if (shmem->madv >= 0)
 		shmem->madv = madv;
@@ -478,7 +491,7 @@ void drm_gem_shmem_purge_locked(struct drm_gem_shmem_object *shmem)
 	struct drm_gem_object *obj = &shmem->base;
 	struct drm_device *dev = obj->dev;
 
-	dma_resv_assert_held(shmem->base.resv);
+	drm_gem_shmem_resv_assert_held(shmem);
 
 	drm_WARN_ON(obj->dev, !drm_gem_shmem_is_purgeable(shmem));
 
-- 
2.41.0