[PATCH v9 3/4] drm: lock drm_global_mutex earlier in the ioctl handler

[Desmond Cheong Zhi Xi]

In a future patch, a read lock on drm_device.master_rwsem is
held in the ioctl handler before the check for ioctl
permissions. However, this inverts the lock hierarchy of
drm_global_mutex --> master_rwsem.

To avoid this, we do some prep work to grab the drm_global_mutex
before checking for ioctl permissions.

Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx at gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/drm_ioctl.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index d25713b09b80..59c5aa850dd5 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -767,24 +767,27 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
 {
 	struct drm_file *file_priv = file->private_data;
 	struct drm_device *dev = file_priv->minor->dev;
+	bool locked_ioctl;
 	int retcode;
 
 	if (drm_dev_is_unplugged(dev))
 		return -ENODEV;
 
+	/* Enforce sane locking for modern driver ioctls. */
+	locked_ioctl = (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) &&
+			!(flags & DRM_UNLOCKED));
+	if (locked_ioctl)
+		mutex_lock(&drm_global_mutex);
+
 	retcode = drm_ioctl_permit(flags, file_priv);
 	if (unlikely(retcode))
-		return retcode;
+		goto out;
 
-	/* Enforce sane locking for modern driver ioctls. */
-	if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) ||
-	    (flags & DRM_UNLOCKED))
-		retcode = func(dev, kdata, file_priv);
-	else {
-		mutex_lock(&drm_global_mutex);
-		retcode = func(dev, kdata, file_priv);
+	retcode = func(dev, kdata, file_priv);
+
+out:
+	if (locked_ioctl)
 		mutex_unlock(&drm_global_mutex);
-	}
 	return retcode;
 }
 EXPORT_SYMBOL(drm_ioctl_kernel);
-- 
2.25.1