linux-kernel - Re: [PATCH 1/2] drm/atomic: fix self-refresh helpers crtc state dereference

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Message-ID: <CAMavQKKMjge6MwH=-DS5Ngs8ZE5G_x_Vncy+YoqYrC0s0AfX=Q@mail.gmail.com>
Date:   Fri, 1 Nov 2019 16:06:28 -0400
From:   Sean Paul <sean@...rly.run>
To:     Rob Clark <robdclark@...il.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Sean Paul <seanpaul@...omium.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Rob Clark <robdclark@...omium.org>,
        Maxime Ripard <mripard@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] drm/atomic: fix self-refresh helpers crtc state dereference

On Fri, Nov 1, 2019 at 2:09 PM Rob Clark <robdclark@...il.com> wrote:
>
> From: Rob Clark <robdclark@...omium.org>
>
> drm_self_refresh_helper_update_avg_times() was incorrectly accessing the
> new incoming state after drm_atomic_helper_commit_hw_done().  But this
> state might have already been superceeded by an !nonblock atomic update
> resulting in dereferencing an already free'd 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>
> ---
> TODO I *think* this will more or less do the right thing.. althought I'm
> not 100% sure if, for example, we enter psr in a nonblock commit, and
> then leave psr in a !nonblock commit that overtakes the completion of
> the nonblock commit.  Not sure if this sort of scenario can happen in
> practice.  But not crashing is better than crashing, so I guess we
> should either take this patch or rever the self-refresh helpers until
> Sean can figure out a better solution.
>
>  drivers/gpu/drm/drm_atomic_helper.c       |  2 ++
>  drivers/gpu/drm/drm_self_refresh_helper.c | 11 ++++++-----
>  include/drm/drm_atomic.h                  |  8 ++++++++
>  3 files changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index 3ef2ac52ce94..732bd0ce9241 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -2240,6 +2240,8 @@ void drm_atomic_helper_commit_hw_done(struct drm_atomic_state *old_state)
>         int i;
>
>         for_each_oldnew_crtc_in_state(old_state, crtc, old_crtc_state, new_crtc_state, i) {
> +               old_state->crtcs[i].new_self_refresh_active = new_crtc_state->self_refresh_active;
> +
>                 commit = new_crtc_state->commit;
>                 if (!commit)
>                         continue;
> diff --git a/drivers/gpu/drm/drm_self_refresh_helper.c b/drivers/gpu/drm/drm_self_refresh_helper.c
> index 68f4765a5896..77b9079fa578 100644
> --- a/drivers/gpu/drm/drm_self_refresh_helper.c
> +++ b/drivers/gpu/drm/drm_self_refresh_helper.c
> @@ -143,19 +143,20 @@ void drm_self_refresh_helper_update_avg_times(struct drm_atomic_state *state,
>                                               unsigned int commit_time_ms)
>  {
>         struct drm_crtc *crtc;
> -       struct drm_crtc_state *old_crtc_state, *new_crtc_state;
> +       struct drm_crtc_state *old_crtc_state;
>         int i;
>
> -       for_each_oldnew_crtc_in_state(state, crtc, old_crtc_state,
> -                                     new_crtc_state, i) {
> +       for_each_old_crtc_in_state(state, crtc, old_crtc_state, i) {
> +               bool new_self_refresh_active =
> +                               state->crtcs[i].new_self_refresh_active;
>                 struct drm_self_refresh_data *sr_data = crtc->self_refresh_data;
>                 struct ewma_psr_time *time;
>
>                 if (old_crtc_state->self_refresh_active ==
> -                   new_crtc_state->self_refresh_active)
> +                   new_self_refresh_active)
>                         continue;
>
> -               if (new_crtc_state->self_refresh_active)
> +               if (new_self_refresh_active)
>                         time = &sr_data->entry_avg_ms;
>                 else
>                         time = &sr_data->exit_avg_ms;
> diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> index 927e1205d7aa..86baf2b38bb3 100644
> --- a/include/drm/drm_atomic.h
> +++ b/include/drm/drm_atomic.h
> @@ -155,6 +155,14 @@ struct __drm_crtcs_state {
>         struct drm_crtc *ptr;
>         struct drm_crtc_state *state, *old_state, *new_state;
>
> +       /**
> +        * @new_self_refresh_active:
> +        *
> +        * drm_atomic_helper_commit_hw_done() stashes new_crtc_state->self_refresh_active
> +        * so that it can be accessed late in drm_self_refresh_helper_update_avg_times().
> +        */
> +       bool new_self_refresh_active;
> +

Instead of stashing this in crtc, we could generate a bitmask local to
commit_tail and pass that to calc_avg_times? That way we don't have to
worry about someone using this when they shouldn't

Sean

>         /**
>          * @commit:
>          *
> --
> 2.21.0
>