Re: [PATCH 13/14] drm/msm: Drop struct_mutex in shrinker path

[Rob Clark]

On Mon, Oct 5, 2020 at 7:02 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Mon, Oct 05, 2020 at 05:24:19PM +0800, Hillf Danton wrote:
> >
> > On Sun,  4 Oct 2020 12:21:45
> > > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> > >
> > > Now that the inactive_list is protected by mm_lock, and everything
> > > else on per-obj basis is protected by obj->lock, we no longer depend
> > > on struct_mutex.
> > >
> > > Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> > > ---
> > >  drivers/gpu/drm/msm/msm_gem.c          |  1 -
> > >  drivers/gpu/drm/msm/msm_gem_shrinker.c | 54 --------------------------
> > >  2 files changed, 55 deletions(-)
> > >
> > [...]
> >
> > > @@ -71,13 +33,8 @@ msm_gem_shrinker_scan(struct shrinker *shrinker, struct shrink_control *sc)
> > >  {
> > >     struct msm_drm_private *priv =
> > >             container_of(shrinker, struct msm_drm_private, shrinker);
> > > -   struct drm_device *dev = priv->dev;
> > >     struct msm_gem_object *msm_obj;
> > >     unsigned long freed = 0;
> > > -   bool unlock;
> > > -
> > > -   if (!msm_gem_shrinker_lock(dev, &unlock))
> > > -           return SHRINK_STOP;
> > >
> > >     mutex_lock(&priv->mm_lock);
> >
> > Better if the change in behavior is documented that SHRINK_STOP will
> > no longer be needed.
>
> btw I read through this and noticed you have your own obj lock, plus
> mutex_lock_nested. I strongly recommend to just cut over to dma_resv_lock
> for all object lock needs (soc drivers have been terrible with this
> unfortuntaly), and in the shrinker just use dma_resv_trylock instead of
> trying to play clever games outsmarting lockdep.
>
> I recently wrote an entire blog length rant on why I think
> mutex_lock_nested is too dangerous to be useful:
>
> https://blog.ffwll.ch/2020/08/lockdep-false-positives.html
>
> Not anything about this here, just general comment. The problem extends to
> shmem helpers and all that also having their own locks for everything.

the shrinker lock class has existed for a while.. and is based on the
idea that anything in the get-pages/vmap path cannot happen on a
WONTNEED bo.. although perhaps there should be a few more 'if
(WARN_ON(obj->madv != WILLNEED)) return -EBUSY'..

replacing obj->lock with dma_resv lock, might be a nice cleanup.. but
I think it will be a bit churny..

BR,
-R

> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch