On Fri, Jul 30, 2021 at 04:06:44PM +0800, Desmond Cheong Zhi Xi wrote:
> On 30/7/21 2:08 pm, Boqun Feng wrote:
> > On Fri, Jul 30, 2021 at 12:15:15PM +0800, Desmond Cheong Zhi Xi wrote:
> > > In drm_is_current_master_locked, accessing drm_file.master should be
> > > protected by either drm_file.master_lookup_lock or
> > > drm_device.master_mutex. This was previously awkward to assert with
> > > lockdep.
> > >
> > > Following patch ("locking/lockdep: Provide lockdep_assert{,_once}()
> > > helpers"), this assertion is now convenient so we add it in.
> > >
> > > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> > > ---
> > > drivers/gpu/drm/drm_auth.c | 6 +++---
> > > 1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> > > index 9c24b8cc8e36..6f4d7ff23c80 100644
> > > --- a/drivers/gpu/drm/drm_auth.c
> > > +++ b/drivers/gpu/drm/drm_auth.c
> > > @@ -63,9 +63,9 @@
> > > static bool drm_is_current_master_locked(struct drm_file *fpriv)
> > > {
> > > - /* Either drm_device.master_mutex or drm_file.master_lookup_lock
> > > - * should be held here.
> > > - */
> > > + lockdep_assert_once(lockdep_is_held(&fpriv->master_lookup_lock) ||
> > > + lockdep_is_held(&fpriv->minor->dev->master_mutex));
> > > +
> >
> > I think it's better to also add the lockdep_assert() of & (i.e. both
> > held) in the updater side, and have comments pointing to each other.
> >
> > Is it convenient to do in this patchset? If the updater side doesn't
> > need to put the lockdep_assert() (maybe the lock acquire code and the
> > update code are in the same function), it's still better to add some
>
> Thanks for the feedback, Boqun.
>
> Yeah, I think the updater side maybe doesn't need new lockdep_assert()
> because what currently happens is either
>
> lockdep_assert_held_once(&dev->master_mutex);
> /* 6 lines of prep */
> spin_lock(&fpriv->master_lookup_lock);
> fpriv->master = new_value;
> or
> mutex_lock(&dev->master_mutex);
> /* 3 lines of checks */
> spin_lock(&file_priv->master_lookup_lock);
> file_priv->master = new_value;
>
> > comments like:
> >
> > /*
> > * To update drm_file.master, both drm_file.master_lookup_lock
> > * and drm_device.master_mutex are needed, therefore holding
> > * either of them is safe and enough for the read side.
> > */
> >
> > Just feel it's better to explain the lock design either in the
> > lockdep_assert() or comments.
> >
>
> But clarifying the lock design in the documentation sounds like a really
> good idea.
>
> Probably a good place for this would be in the kerneldoc where we also
> explain the lifetime rules and usage of the pointer outside drm_auth.c:
>
> diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
> index 726cfe0ff5f5..a3acb7ac3550 100644
> --- a/include/drm/drm_file.h
> +++ b/include/drm/drm_file.h
> @@ -233,6 +233,10 @@ struct drm_file {
> * this only matches &drm_device.master if the master is the currently
> * active one.
> *
> + * To update @master, both &drm_device.master_mutex and
> + * @master_lookup_lock need to be held, therefore holding either of
> + * them is safe and enough for the read side.
> + *
> * When dereferencing this pointer, either hold struct
> * &drm_device.master_mutex for the duration of the pointer's use, or
> * use drm_file_get_master() if struct &drm_device.master_mutex is not
Works for me ;-)
For the whole series, feel free to add:
Acked-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Regards,
Boqun
>
> Best wishes,
> Desmond
>
> > Regards,
> > Boqun
> >
> > > return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
> > > }
> > > --
> > > 2.25.1
> > >
>
