[PATCH] drm/syncobj: reset file ptr to NULL when released.

[Chris Wilson]

Quoting Dave Airlie (2017-12-19 11:30:12)
> From: Dave Airlie <airlied at redhat.com>
> 
> The vk cts test:
> dEQP-VK.api.external.semaphore.opaque_fd.export_multiple_times_temporary
> 
> triggers a lot of
> VFS: Close: file count is 0
> 
> This patch fixes it, but I'm guessing it's racy and someone will
> smell rcu, but I just wanted to send out the proof of fixing it
> so I remember.
> 
> Signed-off-by: Dave Airlie <airlied at redhat.com>
> ---
>  drivers/gpu/drm/drm_syncobj.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> index f776fc1cc543..ffa5bbd75852 100644
> --- a/drivers/gpu/drm/drm_syncobj.c
> +++ b/drivers/gpu/drm/drm_syncobj.c
> @@ -361,6 +361,7 @@ static int drm_syncobj_file_release(struct inode *inode, struct file *file)
>  {
>         struct drm_syncobj *syncobj = file->private_data;
>  
> +       syncobj->file = NULL;

Oh. That's scary as the opposite side assumes that the syncobj->file is
assigned for the lifetime of the syncobj - once allocated is never
unset.

If we stop trying to reuse the struct file and just allocated one for
each fd, the complications just vanish?

diff --cc include/drm/drm_syncobj.h
index 3980602472c0,ba54e0e58bbc..000000000000
--- a/include/drm/drm_syncobj.h
+++ b/include/drm/drm_syncobj.h
@@@ -56,10 -57,8 +56,6 @@@ struct drm_syncobj 
  	 * @lock: Protects &cb_list and write-locks &fence.
  	 */
  	spinlock_t lock;
- 	/**
- 	 * @file: A file backing for this syncobj.
- 	 */
- 	struct file *file;
 -
 -	struct rcu_head rcu;
  };
  
  typedef void (*drm_syncobj_func_t)(struct drm_syncobj *syncobj,
diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index 131695915acd..0cca2e792719 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -399,23 +399,6 @@ static const struct file_operations drm_syncobj_file_fops = {
 	.release = drm_syncobj_file_release,
 };
 
-static int drm_syncobj_alloc_file(struct drm_syncobj *syncobj)
-{
-	struct file *file = anon_inode_getfile("syncobj_file",
-					       &drm_syncobj_file_fops,
-					       syncobj, 0);
-	if (IS_ERR(file))
-		return PTR_ERR(file);
-
-	drm_syncobj_get(syncobj);
-	if (cmpxchg(&syncobj->file, NULL, file)) {
-		/* lost the race */
-		fput(file);
-	}
-
-	return 0;
-}
-
 /**
  * drm_syncobj_get_fd - get a file descriptor from a syncobj
  * @syncobj: Sync object to export
@@ -427,21 +410,24 @@ static int drm_syncobj_alloc_file(struct drm_syncobj *syncobj)
  */
 int drm_syncobj_get_fd(struct drm_syncobj *syncobj, int *p_fd)
 {
-	int ret;
+	struct file *file;
 	int fd;
 
 	fd = get_unused_fd_flags(O_CLOEXEC);
 	if (fd < 0)
 		return fd;
 
-	if (!syncobj->file) {
-		ret = drm_syncobj_alloc_file(syncobj);
-		if (ret) {
-			put_unused_fd(fd);
-			return ret;
-		}
+	file = anon_inode_getfile("syncobj_file",
+				  &drm_syncobj_file_fops,
+				  syncobj, 0);
+	if (IS_ERR(file)) {
+		put_unused_fd(fd);
+		return PTR_ERR(file);
 	}
-	fd_install(fd, syncobj->file);
+
+	drm_syncobj_get(syncobj);
+	fd_install(fd, file);
+
 	*p_fd = fd;
 	return 0;
 }
@@ -461,31 +447,24 @@ static int drm_syncobj_handle_to_fd(struct drm_file *file_private,
 	return ret;
 }
 
-static struct drm_syncobj *drm_syncobj_fdget(int fd)
-{
-	struct file *file = fget(fd);
-
-	if (!file)
-		return NULL;
-	if (file->f_op != &drm_syncobj_file_fops)
-		goto err;
-
-	return file->private_data;
-err:
-	fput(file);
-	return NULL;
-};
-
 static int drm_syncobj_fd_to_handle(struct drm_file *file_private,
 				    int fd, u32 *handle)
 {
-	struct drm_syncobj *syncobj = drm_syncobj_fdget(fd);
+	struct drm_syncobj *syncobj;
+	struct file *file;
 	int ret;
 
-	if (!syncobj)
+	file = fget(fd);
+	if (!file)
 		return -EINVAL;
 
+	if (file->f_op != &drm_syncobj_file_fops) {
+		fput(file);
+		return -EINVAL;
+	}
+
 	/* take a reference to put in the idr */
+	syncobj = file->private_data;
 	drm_syncobj_get(syncobj);
 
 	idr_preload(GFP_KERNEL);
@@ -494,12 +473,11 @@ static int drm_syncobj_fd_to_handle(struct drm_file *file_private,
 	spin_unlock(&file_private->syncobj_table_lock);
 	idr_preload_end();
 
-	if (ret < 0) {
-		fput(syncobj->file);
-		return ret;
-	}
-	*handle = ret;
-	return 0;
+	if (ret > 0)
+		*handle = ret;
+
+	fput(file);
+	return ret;
 }
 
 static int drm_syncobj_import_sync_file_fence(struct drm_file *file_private,