Re: [PATCH drm-misc-next v9 01/11] drm/gem: fix lockdep check for dma-resv lock

[Danilo Krummrich]

On 8/8/23 09:21, Boris Brezillon wrote:
> On Thu,  3 Aug 2023 18:52:20 +0200
> Danilo Krummrich <dakr@xxxxxxxxxx> wrote:
>
> > When no custom lock is set to protect a GEMs GPUVA list, lockdep checks
> > should fall back to the GEM objects dma-resv lock. With the current
> > implementation we're setting the lock_dep_map of the GEM objects 'resv'
> > pointer (in case no custom lock_dep_map is set yet) on
> > drm_gem_private_object_init().
> >
> > However, the GEM objects 'resv' pointer might still change after
> > drm_gem_private_object_init() is called, e.g. through
> > ttm_bo_init_reserved(). This can result in the wrong lock being tracked.
> >
> > To fix this, call dma_resv_held() directly from
> > drm_gem_gpuva_assert_lock_held() and fall back to the GEMs lock_dep_map
> > pointer only if an actual custom lock is set.
> >
> > Fixes: e6303f323b1a ("drm: manager to keep track of GPUs VA mappings")
> > Signed-off-by: Danilo Krummrich <dakr@xxxxxxxxxx>
>
> Reviewed-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
>
> but I'm wondering if it wouldn't be a good thing to add a
> drm_gem_set_resv() helper, so the core can control drm_gem_object::resv
> re-assignments (block them if it's happening after the GEM has been
> exposed to the outside world or update auxiliary data if it's happening
> before that).

I agree, this might be a good idea. There are quite a few places where 
drm_gem_object::resv is set from external code.

> > ---
> >   include/drm/drm_gem.h | 15 +++++++++------
> >   1 file changed, 9 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> > index c0b13c43b459..bc9f6aa2f3fe 100644
> > --- a/include/drm/drm_gem.h
> > +++ b/include/drm/drm_gem.h
> > @@ -551,15 +551,17 @@ int drm_gem_evict(struct drm_gem_object *obj);
> >    * @lock: the lock used to protect the gpuva list. The locking primitive
> >    * must contain a dep_map field.
> >    *
> > - * Call this if you're not proctecting access to the gpuva list
> > - * with the dma-resv lock, otherwise, drm_gem_gpuva_init() takes care
> > - * of initializing lock_dep_map for you.
> > + * Call this if you're not proctecting access to the gpuva list with the
> > + * dma-resv lock, but with a custom lock.
> >    */
> >   #define drm_gem_gpuva_set_lock(obj, lock) \
> > -	if (!(obj)->gpuva.lock_dep_map) \
> > +	if (!WARN((obj)->gpuva.lock_dep_map, \
> > +		  "GEM GPUVA lock should be set only once.")) \
> >   		(obj)->gpuva.lock_dep_map = &(lock)->dep_map
> >   #define drm_gem_gpuva_assert_lock_held(obj) \
> > -	lockdep_assert(lock_is_held((obj)->gpuva.lock_dep_map))
> > +	lockdep_assert((obj)->gpuva.lock_dep_map ? \
> > +		       lock_is_held((obj)->gpuva.lock_dep_map) : \
> > +		       dma_resv_held((obj)->resv))
> >   #else
> >   #define drm_gem_gpuva_set_lock(obj, lock) do {} while (0)
> >   #define drm_gem_gpuva_assert_lock_held(obj) do {} while (0)
> > @@ -573,11 +575,12 @@ int drm_gem_evict(struct drm_gem_object *obj);
> >    *
> >    * Calling this function is only necessary for drivers intending to support the
> >    * &drm_driver_feature DRIVER_GEM_GPUVA.
> > + *
> > + * See also drm_gem_gpuva_set_lock().
> >    */
> >   static inline void drm_gem_gpuva_init(struct drm_gem_object *obj)
> >   {
> >   	INIT_LIST_HEAD(&obj->gpuva.list);
> > -	drm_gem_gpuva_set_lock(obj, &obj->resv->lock.base);
> >   }
> >   
> >   /**