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

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Date:   Mon, 4 Nov 2019 10:29:21 +0100
From:   Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
To:     Sean Paul <sean@...rly.run>, Rob Clark <robdclark@...il.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Sean Paul <seanpaul@...omium.org>,
        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

Op 01-11-2019 om 21:06 schreef Sean Paul:
> 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

Yeah would make sense to have a bitmask, instead of making the property special. :)

Current solution seems a bit ugly.