linux-kernel - Re: [PATCH 3/4] drm/msm: Fix debugfs deadlock

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Date:   Wed, 31 Mar 2021 16:31:38 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Doug Anderson <dianders@...omium.org>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jordan Crouse <jordan@...micpenguin.net>,
        "Kristian H. Kristensen" <hoegsberg@...gle.com>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@...r.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <freedreno@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] drm/msm: Fix debugfs deadlock

On Wed, Mar 31, 2021 at 4:13 PM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Wed, Mar 31, 2021 at 3:14 PM Rob Clark <robdclark@...il.com> wrote:
> >
> > @@ -111,23 +111,15 @@ static const struct file_operations msm_gpu_fops = {
> >  static int msm_gem_show(struct drm_device *dev, struct seq_file *m)
> >  {
> >         struct msm_drm_private *priv = dev->dev_private;
> > -       struct msm_gpu *gpu = priv->gpu;
> >         int ret;
> >
> > -       ret = mutex_lock_interruptible(&priv->mm_lock);
> > +       ret = mutex_lock_interruptible(&priv->obj_lock);
> >         if (ret)
> >                 return ret;
> >
> > -       if (gpu) {
> > -               seq_printf(m, "Active Objects (%s):\n", gpu->name);
> > -               msm_gem_describe_objects(&gpu->active_list, m);
> > -       }
> > -
> > -       seq_printf(m, "Inactive Objects:\n");
> > -       msm_gem_describe_objects(&priv->inactive_dontneed, m);
> > -       msm_gem_describe_objects(&priv->inactive_willneed, m);
> > +       msm_gem_describe_objects(&priv->objects, m);
>
> I guess we no longer sort the by Active and Inactive but that doesn't
> really matter?

It turned out to be less useful to sort by active/inactive, as much as
just having the summary at the bottom that the last patch adds.  We
can already tell from the per-object entries whether it is
active/purgable/purged.

I did initially try to come up with an approach that let me keep this,
but it would basically amount to re-writing the gem_submit path
(because you cannot do any memory allocation under mm_lock)

>
> > @@ -174,7 +174,13 @@ struct msm_drm_private {
> >         struct msm_rd_state *hangrd;   /* debugfs to dump hanging submits */
> >         struct msm_perf_state *perf;
> >
> > -       /*
> > +       /**
> > +        * List of all GEM objects (mainly for debugfs, protected by obj_lock
>
> It wouldn't hurt to talk about lock ordering here? Like: "If we need
> the "obj_lock" and a "gem_lock" at the same time we always grab the
> "obj_lock" first.

good point

>
> > @@ -60,13 +60,20 @@ struct msm_gem_object {
> >          */
> >         uint8_t vmap_count;
> >
> > -       /* And object is either:
> > -        *  inactive - on priv->inactive_list
> > +       /**
> > +        * Node in list of all objects (mainly for debugfs, protected by
> > +        * struct_mutex
>
> Not "struct_mutex" in comment, right? Maybe "obj_lock" I think?

oh, right, forgot to fix that from an earlier iteration

BR,
-R