linux-kernel - Re: [PATCH] drm/atomic: swap_state should stall on cleanup

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Message-ID: <CAF6AEGs9CwDLv7O4ymvTsK1+Bjopy8Q+DzOqqfeW9jM=n5beUQ@mail.gmail.com>
Date:   Fri, 1 Nov 2019 07:59:16 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Rob Clark <robdclark@...omium.org>,
        Sean Paul <seanpaul@...omium.org>,
        Maxime Ripard <mripard@...nel.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/atomic: swap_state should stall on cleanup_done

On Fri, Nov 1, 2019 at 7:47 AM Maarten Lankhorst
<maarten.lankhorst@...ux.intel.com> wrote:
>
> Op 31-10-2019 om 23:36 schreef Rob Clark:
> > From: Rob Clark <robdclark@...omium.org>
> >
> > Stalling on cleanup_done ensures that any atomic state related to a
> > nonblock commit no longer has dangling references to per-object state
> > that can be freed.
> >
> > Otherwise, if a !nonblock commit completes after a nonblock commit has
> > swapped state (ie. the synchronous part of the nonblock commit comes
> > before the !nonblock commit), but before the asynchronous part of the
> > nonblock commit completes, what was the new per-object state in the
> > nonblock commit can be freed.
> >
> > This shows up with the new self-refresh helper, as _update_avg_times()
> > dereferences the original old and new crtc_state.
> >
> > Fixes: d4da4e33341c ("drm: Measure Self Refresh Entry/Exit times to avoid thrashing")
> > Cc: Sean Paul <seanpaul@...omium.org>
> > Signed-off-by: Rob Clark <robdclark@...omium.org>
> > ---
> > Other possibilities:
> > 1) maybe block later before freeing atomic state?
> > 2) refcount individual per-object state
> >
> >  drivers/gpu/drm/drm_atomic_helper.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > index 3ef2ac52ce94..a5d95429f91b 100644
> > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > @@ -2711,7 +2711,7 @@ int drm_atomic_helper_swap_state(struct drm_atomic_state *state,
> >                       if (!commit)
> >                               continue;
> >
> > -                     ret = wait_for_completion_interruptible(&commit->hw_done);
> > +                     ret = wait_for_completion_interruptible(&commit->cleanup_done);
> >                       if (ret)
> >                               return ret;
> >               }
> > @@ -2722,7 +2722,7 @@ int drm_atomic_helper_swap_state(struct drm_atomic_state *state,
> >                       if (!commit)
> >                               continue;
> >
> > -                     ret = wait_for_completion_interruptible(&commit->hw_done);
> > +                     ret = wait_for_completion_interruptible(&commit->cleanup_done);
> >                       if (ret)
> >                               return ret;
> >               }
> > @@ -2733,7 +2733,7 @@ int drm_atomic_helper_swap_state(struct drm_atomic_state *state,
> >                       if (!commit)
> >                               continue;
> >
> > -                     ret = wait_for_completion_interruptible(&commit->hw_done);
> > +                     ret = wait_for_completion_interruptible(&commit->cleanup_done);
> >                       if (ret)
> >                               return ret;
> >               }
>
> Nack, hw_done means all new_crtc_state (from the old commit pov) dereferences are done.
>

hmm, it would be nice if the for_each_blah_in_state() iterators would
splat on incorrect usage, then..  it tool a while to track down what
was going wrong.  And Sean claimed the self refresh helpers worked for
him on rockchip/i915 (although I'm starting to suspect maybe he just
didn't have enough debug options enabled to poison freed memory?)

> Self refresh helpers should be fixed. :)

Looks like what they need out of crtc_state is pretty minimal, maybe
they could extract out crtc_state->self_refresh_active earlier..

BR,
-R