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

From: Rob Clark <robdclark@gmail.com>
To: Doug Anderson <dianders@chromium.org>
Cc: dri-devel <dri-devel@lists.freedesktop.org>,
	Rob Clark <robdclark@chromium.org>, Sean Paul <sean@poorly.run>,
	David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>,
	Jordan Crouse <jordan@cosmicpenguin.net>,
	"Kristian H. Kristensen" <hoegsberg@google.com>,
	"open list:DRM DRIVER FOR MSM ADRENO GPU" 
	<linux-arm-msm@vger.kernel.org>,
	"open list:DRM DRIVER FOR MSM ADRENO GPU" 
	<freedreno@lists.freedesktop.org>,
	open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/4] drm/msm: Fix debugfs deadlock
Date: Wed, 31 Mar 2021 16:31:38 -0700	[thread overview]
Message-ID: <CAF6AEGvxQeTFHn_ztzP=4eTQa_B106+SZ-8NjFk2RGYgRYJgSw@mail.gmail.com> (raw)
In-Reply-To: <CAD=FV=UECYxLXQa_L572eLSVHq7pbxuA0zLvHzYHhCKy8K=9TA@mail.gmail.com>

On Wed, Mar 31, 2021 at 4:13 PM Doug Anderson <dianders@chromium.org> wrote:
>
> Hi,
>
> On Wed, Mar 31, 2021 at 3:14 PM Rob Clark <robdclark@gmail.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