Re: [PATCH 3/6] drm: Define a conditional guard for drm_dev_{enter,exit}()

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 14 May 2026 11:34:52 -0700
Chia-I Wu <olvaffe@gmail.com> wrote:

> On Wed, May 13, 2026 at 10:24 AM Boris Brezillon
> <boris.brezillon@collabora.com> wrote:
> >
> > Define a conditional drm_dev_access guard to automate the
> > drm_dev_{enter,exit}() sequence.
> >
> > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> > ---
> >  include/drm/drm_drv.h | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
> > index 42fc085f986d..79d1958f93e4 100644
> > --- a/include/drm/drm_drv.h
> > +++ b/include/drm/drm_drv.h
> > @@ -490,6 +490,15 @@ void drm_dev_unplug(struct drm_device *dev);
> >  int drm_dev_wedged_event(struct drm_device *dev, unsigned long method,
> >                          struct drm_wedge_task_info *info);
> >
> > +/*
> > + * Only the conditional drm_dev_access guard is valid. The drm_dev one is
> > + * here so we can extend it with a conditional variant.
> > + */
> > +DEFINE_LOCK_GUARD_1(drm_dev, struct drm_device,
> > +                   { WARN_ON("Use cond guards"); _T->idx = -1; },
> > +                   drm_dev_exit(_T->idx), int idx);  
> If this is ever mis-used, drm_dev_exit(-1) seems to cause OOB access.
> Is BUG more appropriate than WARN_ON?

I actually had

			if (_T->idx >= 0) drm_dev_exit(_T->idx),

at some point, and I ditched it thinking the WARN_ON_ONCE()
in srcu_read_unlock() would cover for that. I can add it back, of
course.

I'd be fine with a BUG_ON() too, but every time I tried to add one I've
been encouraged to handle the unexpected case instead.

Ideally, we would have a DEFINE_LOCK_GUARD_COND() variant that, instead
of expanding a non-conditional one, would define the whole thing so
that the non-conditional variant is never exposed.

> 
> > +DEFINE_LOCK_GUARD_1_COND(drm_dev, _access, drm_dev_enter(_T->lock, &_T->idx));
> > +
> >  /**
> >   * drm_dev_is_unplugged - is a DRM device unplugged
> >   * @dev: DRM device
> >
> > --
> > 2.54.0
> >