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

[Hillf Danton]

On Mon, 5 Oct 2020 18:17:01 Kristian H. Kristensen wrote:
> On Mon, Oct 5, 2020 at 4:02 PM Daniel Vetter <daniel at ffwll.ch> 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 at chromium.org>
> > > >
> > > > 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 at chromium.org>
> > > > ---
> > > >  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.

The trylock makes page reclaimers turn to their next target e.g. inode
cache instead of waiting for the mutex to be released. It makes sense
for instance in scenarios of mild memory pressure.

> >
> > 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.
> 
> This is definitely a tangible improvement though - very happy to see
> msm_gem_shrinker_lock() go.
> 
> Reviewed-by: Kristian H. Kristensen <hoegsberg at google.com>
> 
> > -Daniel
> > --
> > Daniel Vetter
> > Software Engineer, Intel Corporation
> > http://blog.ffwll.ch
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