On Thu, Jul 22, 2021 at 05:29:27PM +0800, Desmond Cheong Zhi Xi wrote:
> Inside drm_is_current_master, using the outer drm_device.master_mutex
> to protect reads of drm_file.master makes the function prone to creating
> lock hierarchy inversions. Instead, we can use the
> drm_file.master_lookup_lock that sits at the bottom of the lock
> hierarchy.
>
> Reported-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> ---
> drivers/gpu/drm/drm_auth.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> index f00354bec3fb..9c24b8cc8e36 100644
> --- a/drivers/gpu/drm/drm_auth.c
> +++ b/drivers/gpu/drm/drm_auth.c
> @@ -63,8 +63,9 @@
>
> static bool drm_is_current_master_locked(struct drm_file *fpriv)
> {
> - lockdep_assert_held_once(&fpriv->minor->dev->master_mutex);
> -
> + /* Either drm_device.master_mutex or drm_file.master_lookup_lock
> + * should be held here.
> + */
Disappointing that lockdep can't check or conditions for us, a
lockdep_assert_held_either would be really neat in some cases.
Adding lockdep folks, maybe they have ideas.
On the patch:
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
> }
>
> @@ -82,9 +83,9 @@ bool drm_is_current_master(struct drm_file *fpriv)
> {
> bool ret;
>
> - mutex_lock(&fpriv->minor->dev->master_mutex);
> + spin_lock(&fpriv->master_lookup_lock);
> ret = drm_is_current_master_locked(fpriv);
> - mutex_unlock(&fpriv->minor->dev->master_mutex);
> + spin_unlock(&fpriv->master_lookup_lock);
>
> return ret;
> }
> --
> 2.25.1
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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