Re: [PATCH 06/13] drm: add minor-lookup/release helpers

[Thierry Reding <thierry.reding@gmail.com>]

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On Fri, Feb 21, 2014 at 08:34:15AM +0100, David Herrmann wrote:
> Hi
> 
> On Fri, Feb 21, 2014 at 8:09 AM, Thierry Reding
> <thierry.reding@gmail.com> wrote:
> > On Wed, Jan 29, 2014 at 03:01:53PM +0100, David Herrmann wrote:
> > [...]
> >> diff --git a/drivers/gpu/drm/drm_stub.c b/drivers/gpu/drm/drm_stub.c
> >> index c51333e..d3232b6 100644
> >> --- a/drivers/gpu/drm/drm_stub.c
> >> +++ b/drivers/gpu/drm/drm_stub.c
> >> @@ -356,6 +356,45 @@ static void drm_unplug_minor(struct drm_minor *minor)
> >>  }
> >>
> >>  /**
> >> + * drm_minor_acquire - Acquire a DRM minor
> >> + * @minor_id: Minor ID of the DRM-minor
> >> + *
> >> + * Looks up the given minor-ID and returns the respective DRM-minor object. The
> >> + * refence-count of the underlying device is increased so you must release this
> >> + * object with drm_minor_release().
> >> + *
> >> + * As long as you hold this minor, it is guaranteed that the object and the
> >> + * minor->dev pointer will stay valid! However, the device may get unplugged and
> >> + * unregistered while you hold the minor.
> >> + *
> >> + * Returns:
> >> + * Pointer to minor-object with increased device-refcount, or PTR_ERR on
> >> + * failure.
> >> + */
> >> +struct drm_minor *drm_minor_acquire(unsigned int minor_id)
> >> +{
> >> +     struct drm_minor *minor;
> >> +
> >> +     minor = idr_find(&drm_minors_idr, minor_id);
> >> +     if (!minor)
> >> +             return ERR_PTR(-ENODEV);
> >> +
> >> +     drm_dev_ref(minor->dev);
> >
> > Is it possible that somebody would drop the last reference on the device
> > right between the idr_find() call and drm_dev_ref()? In which case both
> > the device and the minor will have become invalid when drm_dev_ref() is
> > called.
> 
> No, it's locked via the drm_global_mutex. And later I introduce a
> spinlock here that protects this in case we remove the global lock.

Indeed. Thanks for clarifying.

Thierry

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