[Xen-devel][PATCH 1/2] xen/gntdev: Do not destroy context while dma-bufs are in use

[Oleksandr Andrushchenko]

From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>

If there are exported DMA buffers which are still in use and
grant device is closed by either normal user-space close or by
a signal this leads to the grant device context to be destroyed,
thus making it not possible to correctly destroy those exported
buffers when they are returned back to gntdev and makes the module
crash:

[  339.617540] [<ffff00000854c0d8>] dmabuf_exp_ops_release+0x40/0xa8
[  339.617560] [<ffff00000867a6e8>] dma_buf_release+0x60/0x190
[  339.617577] [<ffff0000082211f0>] __fput+0x88/0x1d0
[  339.617589] [<ffff000008221394>] ____fput+0xc/0x18
[  339.617607] [<ffff0000080ed4e4>] task_work_run+0x9c/0xc0
[  339.617622] [<ffff000008089714>] do_notify_resume+0xfc/0x108

Fix this by referencing gntdev on each DMA buffer export and
unreferencing on buffer release.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
---
 drivers/xen/gntdev-dmabuf.c | 12 +++++++++++-
 drivers/xen/gntdev-dmabuf.h |  2 +-
 drivers/xen/gntdev.c        |  2 +-
 3 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/xen/gntdev-dmabuf.c b/drivers/xen/gntdev-dmabuf.c
index cba6b586bfbd..d97fcfc5e558 100644
--- a/drivers/xen/gntdev-dmabuf.c
+++ b/drivers/xen/gntdev-dmabuf.c
@@ -80,6 +80,12 @@ struct gntdev_dmabuf_priv {
 	struct list_head imp_list;
 	/* This is the lock which protects dma_buf_xxx lists. */
 	struct mutex lock;
+	/*
+	 * We reference this file while exporting dma-bufs, so
+	 * the grant device context is not destroyed while there are
+	 * external users alive.
+	 */
+	struct file *filp;
 };
 
 /* DMA buffer export support. */
@@ -311,6 +317,7 @@ static void dmabuf_exp_release(struct kref *kref)
 
 	dmabuf_exp_wait_obj_signal(gntdev_dmabuf->priv, gntdev_dmabuf);
 	list_del(&gntdev_dmabuf->next);
+	fput(gntdev_dmabuf->priv->filp);
 	kfree(gntdev_dmabuf);
 }
 
@@ -423,6 +430,7 @@ static int dmabuf_exp_from_pages(struct gntdev_dmabuf_export_args *args)
 	mutex_lock(&args->dmabuf_priv->lock);
 	list_add(&gntdev_dmabuf->next, &args->dmabuf_priv->exp_list);
 	mutex_unlock(&args->dmabuf_priv->lock);
+	get_file(gntdev_dmabuf->priv->filp);
 	return 0;
 
 fail:
@@ -834,7 +842,7 @@ long gntdev_ioctl_dmabuf_imp_release(struct gntdev_priv *priv,
 	return dmabuf_imp_release(priv->dmabuf_priv, op.fd);
 }
 
-struct gntdev_dmabuf_priv *gntdev_dmabuf_init(void)
+struct gntdev_dmabuf_priv *gntdev_dmabuf_init(struct file *filp)
 {
 	struct gntdev_dmabuf_priv *priv;
 
@@ -847,6 +855,8 @@ struct gntdev_dmabuf_priv *gntdev_dmabuf_init(void)
 	INIT_LIST_HEAD(&priv->exp_wait_list);
 	INIT_LIST_HEAD(&priv->imp_list);
 
+	priv->filp = filp;
+
 	return priv;
 }
 
diff --git a/drivers/xen/gntdev-dmabuf.h b/drivers/xen/gntdev-dmabuf.h
index 7220a53d0fc5..3d9b9cf9d5a1 100644
--- a/drivers/xen/gntdev-dmabuf.h
+++ b/drivers/xen/gntdev-dmabuf.h
@@ -14,7 +14,7 @@
 struct gntdev_dmabuf_priv;
 struct gntdev_priv;
 
-struct gntdev_dmabuf_priv *gntdev_dmabuf_init(void);
+struct gntdev_dmabuf_priv *gntdev_dmabuf_init(struct file *filp);
 
 void gntdev_dmabuf_fini(struct gntdev_dmabuf_priv *priv);
 
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index b0b02a501167..9d8e02cfd480 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -600,7 +600,7 @@ static int gntdev_open(struct inode *inode, struct file *flip)
 	mutex_init(&priv->lock);
 
 #ifdef CONFIG_XEN_GNTDEV_DMABUF
-	priv->dmabuf_priv = gntdev_dmabuf_init();
+	priv->dmabuf_priv = gntdev_dmabuf_init(flip);
 	if (IS_ERR(priv->dmabuf_priv)) {
 		ret = PTR_ERR(priv->dmabuf_priv);
 		kfree(priv);
-- 
2.20.1