2021-07-24 11:22:11

by Desmond Cheong Zhi Xi

Subject: [PATCH v2 1/3] drm: use the lookup lock in drm_is_current_master

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 <[email protected]>
Signed-off-by: Desmond Cheong Zhi Xi <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
---
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.
+ */
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

2021-07-27 13:07:35

by Daniel Vetter

Subject: Re: [PATCH v2 1/3] drm: use the lookup lock in drm_is_current_master

On Sat, Jul 24, 2021 at 07:18:22PM +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 <[email protected]>
> Signed-off-by: Desmond Cheong Zhi Xi <[email protected]>
> Reviewed-by: Daniel Vetter <[email protected]>

Applied to drm-misc-next, thanks for your patch.
-Daniel

> ---
> 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.
> + */
> 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