Re: [PATCH v3 2/9] drm: hold master_lookup_lock when releasing a drm_file's master

[Desmond Cheong Zhi Xi]

On 18/8/21 6:05 pm, Daniel Vetter wrote:
> On Wed, Aug 18, 2021 at 03:38:17PM +0800, Desmond Cheong Zhi Xi wrote:
> > When drm_file.master changes value, the corresponding
> > drm_device.master_lookup_lock should be held.
> >
> > In drm_master_release, a call to drm_master_put sets the
> > file_priv->master to NULL, so we protect this section with
> > drm_device.master_lookup_lock.
> >
> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
>
> At this points all refcounts to drm_file have disappeared, so yeah this is
> a lockless access, but also no one can observe it anymore. See also next
> patch.
>
> Hence I think the current code is fine.
> -Daniel

Ah ok got it, I didn't realize that. I'll drop patch 2 and 3 from the 
series then.

> > ---
> >   drivers/gpu/drm/drm_auth.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> > index 8efb58aa7d95..8c0e0dba1611 100644
> > --- a/drivers/gpu/drm/drm_auth.c
> > +++ b/drivers/gpu/drm/drm_auth.c
> > @@ -373,8 +373,11 @@ void drm_master_release(struct drm_file *file_priv)
> >   	}
> >   
> >   	/* drop the master reference held by the file priv */
> > -	if (file_priv->master)
> > +	if (file_priv->master) {
> > +		spin_lock(&dev->master_lookup_lock);
> >   		drm_master_put(&file_priv->master);
> > +		spin_unlock(&dev->master_lookup_lock);
> > +	}
> >   	mutex_unlock(&dev->master_mutex);
> >   }
> >   
> > --
> > 2.25.1