drm/prime: proper locking+refcounting for obj->dma_buf link

The export dma-buf cache is semantically similar to an flink name. So
semantically it makes sense to treat it the same and remove the name
(i.e. the dma_buf pointer) and its references when the last gem handle
disappears.

Again we need to be careful, but double so: Not just could someone
race and export with a gem close ioctl (so we need to recheck
obj->handle_count again when assigning the new name), but multiple
exports can also race against each another. This is prevented by
holding the dev->object_name_lock across the entire section which
touches obj->dma_buf.

With the new scheme we also need to reinstate the obj->dma_buf link at
import time (in case the only reference userspace has held in-between
was through the dma-buf fd and not through any native gem handle). For
simplicity we don't check whether it's a native object but
unconditionally set up that link - with the new scheme of removing the
obj->dma_buf reference when the last handle disappears we can do that.

To make it clear that this is not just for exported buffers anymore
als rename it from export_dma_buf to dma_buf.

To make sure that now one can race a fd_to_handle or handle_to_fd with
gem_close we use the same tricks as in flink of extending the
dev->object_name_locking critical section. With this change we finally
have a guaranteed 1:1 relationship (at least for native objects)
between gem objects and dma-bufs, even accounting for races (which can
happen since the dma-buf itself holds a reference while in-flight).

This prevent igt/prime_self_import/export-vs-gem_close-race from
Oopsing the kernel. There is still a leak though since the per-file
priv dma-buf/handle cache handling is racy. That will be fixed in a
later patch.

v2: Remove the bogus dma_buf_put from the export_and_register_object
failure path if we've raced with the handle count dropping to 0.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_fops.c

@@ -486,6 +486,7 @@ int drm_release(struct inode *inode, struct file *filp)
 	if (dev->driver->postclose)
 		dev->driver->postclose(dev, file_priv);
 
+
 	if (drm_core_check_feature(dev, DRIVER_PRIME))
 		drm_prime_destroy_file_private(&file_priv->prime);
 

drivers/gpu/drm/drm_gem.c

@@ -195,9 +195,14 @@ drm_gem_remove_prime_handles(struct drm_gem_object *obj, struct drm_file *filp)
 		drm_prime_remove_buf_handle(&filp->prime,
 				obj->import_attach->dmabuf);
 	}
-	if (obj->export_dma_buf) {
+
+	/*
+	 * Note: obj->dma_buf can't disappear as long as we still hold a
+	 * handle reference in obj->handle_count.
+	 */
+	if (obj->dma_buf) {
 		drm_prime_remove_buf_handle(&filp->prime,
-				obj->export_dma_buf);
+				obj->dma_buf);
 	}
 }
 
@@ -231,6 +236,15 @@ static void drm_gem_object_handle_free(struct drm_gem_object *obj)
 	}
 }
 
+static void drm_gem_object_exported_dma_buf_free(struct drm_gem_object *obj)
+{
+	/* Unbreak the reference cycle if we have an exported dma_buf. */
+	if (obj->dma_buf) {
+		dma_buf_put(obj->dma_buf);
+		obj->dma_buf = NULL;
+	}
+}
+
 static void
 drm_gem_object_handle_unreference_unlocked(struct drm_gem_object *obj)
 {
@@ -244,8 +258,10 @@ drm_gem_object_handle_unreference_unlocked(struct drm_gem_object *obj)
 	*/
 
 	mutex_lock(&obj->dev->object_name_lock);
-	if (--obj->handle_count == 0)
+	if (--obj->handle_count == 0) {
 		drm_gem_object_handle_free(obj);
+		drm_gem_object_exported_dma_buf_free(obj);
+	}
 	mutex_unlock(&obj->dev->object_name_lock);
 
 	drm_gem_object_unreference_unlocked(obj);
@@ -712,6 +728,8 @@ drm_gem_release(struct drm_device *dev, struct drm_file *file_private)
 void
 drm_gem_object_release(struct drm_gem_object *obj)
 {
+	WARN_ON(obj->dma_buf);
+
 	if (obj->filp)
 	    fput(obj->filp);
 }

drivers/gpu/drm/drm_prime.c

@@ -193,11 +193,8 @@ void drm_gem_dmabuf_release(struct dma_buf *dma_buf)
 {
 	struct drm_gem_object *obj = dma_buf->priv;
 
-	if (obj->export_dma_buf == dma_buf) {
 	/* drop the reference on the export fd holds */
-		obj->export_dma_buf = NULL;
 	drm_gem_object_unreference_unlocked(obj);
-	}
 }
 EXPORT_SYMBOL(drm_gem_dmabuf_release);
 
@@ -298,6 +295,37 @@ struct dma_buf *drm_gem_prime_export(struct drm_device *dev,
 }
 EXPORT_SYMBOL(drm_gem_prime_export);
 
+static struct dma_buf *export_and_register_object(struct drm_device *dev,
+						  struct drm_gem_object *obj,
+						  uint32_t flags)
+{
+	struct dma_buf *dmabuf;
+
+	/* prevent races with concurrent gem_close. */
+	if (obj->handle_count == 0) {
+		dmabuf = ERR_PTR(-ENOENT);
+		return dmabuf;
+	}
+
+	dmabuf = dev->driver->gem_prime_export(dev, obj, flags);
+	if (IS_ERR(dmabuf)) {
+		/* normally the created dma-buf takes ownership of the ref,
+		 * but if that fails then drop the ref
+		 */
+		return dmabuf;
+	}
+
+	/*
+	 * Note that callers do not need to clean up the export cache
+	 * since the check for obj->handle_count guarantees that someone
+	 * will clean it up.
+	 */
+	obj->dma_buf = dmabuf;
+	get_dma_buf(obj->dma_buf);
+
+	return dmabuf;
+}
+
 int drm_gem_prime_handle_to_fd(struct drm_device *dev,
 		struct drm_file *file_priv, uint32_t handle, uint32_t flags,
 		int *prime_fd)
@@ -313,15 +341,20 @@ int drm_gem_prime_handle_to_fd(struct drm_device *dev,
 	/* re-export the original imported object */
 	if (obj->import_attach) {
 		dmabuf = obj->import_attach->dmabuf;
+		get_dma_buf(dmabuf);
 		goto out_have_obj;
 	}
 
-	if (obj->export_dma_buf) {
-		dmabuf = obj->export_dma_buf;
+	mutex_lock(&dev->object_name_lock);
+	if (obj->dma_buf) {
+		get_dma_buf(obj->dma_buf);
+		dmabuf = obj->dma_buf;
+		mutex_unlock(&dev->object_name_lock);
 		goto out_have_obj;
 	}
 
-	dmabuf = dev->driver->gem_prime_export(dev, obj, flags);
+	dmabuf = export_and_register_object(dev, obj, flags);
+	mutex_unlock(&dev->object_name_lock);
 	if (IS_ERR(dmabuf)) {
 		/* normally the created dma-buf takes ownership of the ref,
 		 * but if that fails then drop the ref
@@ -329,14 +362,13 @@ int drm_gem_prime_handle_to_fd(struct drm_device *dev,
 		ret = PTR_ERR(dmabuf);
 		goto out;
 	}
-	obj->export_dma_buf = dmabuf;
 
 	mutex_lock(&file_priv->prime.lock);
 	/* if we've exported this buffer the cheat and add it to the import list
 	 * so we get the correct handle back
 	 */
 	ret = drm_prime_add_buf_handle(&file_priv->prime,
-				       obj->export_dma_buf, handle);
+				       dmabuf, handle);
 	if (ret)
 		goto fail_put_dmabuf;
 
@@ -349,7 +381,6 @@ int drm_gem_prime_handle_to_fd(struct drm_device *dev,
 	return 0;
 
 out_have_obj:
-	get_dma_buf(dmabuf);
 	ret = dma_buf_fd(dmabuf, flags);
 	if (ret < 0) {
 		dma_buf_put(dmabuf);
@@ -365,8 +396,6 @@ int drm_gem_prime_handle_to_fd(struct drm_device *dev,
 					   dmabuf);
 	mutex_unlock(&file_priv->prime.lock);
 fail_put_dmabuf:
-	/* clear NOT to be checked when releasing dma_buf */
-	obj->export_dma_buf = NULL;
 	dma_buf_put(dmabuf);
 out:
 	drm_gem_object_unreference_unlocked(obj);
@@ -448,13 +477,22 @@ int drm_gem_prime_fd_to_handle(struct drm_device *dev,
 		goto out_put;
 
 	/* never seen this one, need to import */
+	mutex_lock(&dev->object_name_lock);
 	obj = dev->driver->gem_prime_import(dev, dma_buf);
 	if (IS_ERR(obj)) {
 		ret = PTR_ERR(obj);
-		goto out_put;
+		goto out_unlock;
+	}
+
+	if (obj->dma_buf) {
+		WARN_ON(obj->dma_buf != dma_buf);
+	} else {
+		obj->dma_buf = dma_buf;
+		get_dma_buf(dma_buf);
 	}
 
-	ret = drm_gem_handle_create(file_priv, obj, handle);
+	/* drm_gem_handle_create_tail unlocks dev->object_name_lock. */
+	ret = drm_gem_handle_create_tail(file_priv, obj, handle);
 	drm_gem_object_unreference_unlocked(obj);
 	if (ret)
 		goto out_put;
@@ -475,6 +513,8 @@ int drm_gem_prime_fd_to_handle(struct drm_device *dev,
 	 * to detach.. which seems ok..
 	 */
 	drm_gem_handle_delete(file_priv, *handle);
+out_unlock:
+	mutex_lock(&dev->object_name_lock);
 out_put:
 	dma_buf_put(dma_buf);
 	mutex_unlock(&file_priv->prime.lock);

include/drm/drmP.h

@@ -667,8 +667,16 @@ struct drm_gem_object {
 
 	void *driver_private;
 
-	/* dma buf exported from this GEM object */
-	struct dma_buf *export_dma_buf;
+	/**
+	 * dma_buf - dma buf associated with this GEM object
+	 *
+	 * Pointer to the dma-buf associated with this gem object (either
+	 * through importing or exporting). We break the resulting reference
+	 * loop when the last gem handle for this object is released.
+	 *
+	 * Protected by obj->object_name_lock
+	 */
+	struct dma_buf *dma_buf;
 
 	/**
 	 * import_attach - dma buf attachment backing this object