Re: [PATCH 2/2] drm/amd/display: Avoid race between dcn35_set_drr() and dc_state_destruct()

[Harry Wentland]

On 2024-09-02 05:40, tjak...@math.uni-bielefeld.de wrote:
> From: Tobias Jakobi <tjak...@math.uni-bielefeld.de>
> 
> dc_state_destruct() nulls the resource context of the DC state. The pipe
> context passed to dcn35_set_drr() is a member of this resource context.
> 
> If dc_state_destruct() is called parallel to the IRQ processing (which
> calls dcn35_set_drr() at some point), we can end up using already nulled
> function callback fields of struct stream_resource.
> 
> The logic in dcn35_set_drr() already tries to avoid this, by checking tg
> against NULL. But if the nulling happens exactly after the NULL check and
> before the next access, then we get a race.
> 
> Avoid this by copying tg first to a local variable, and then use this
> variable for all the operations. This should work, as long as nobody
> frees the resource pool where the timing generators live.
> 
> Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3142
> Fixes: 06ad7e164256 ("drm/amd/display: Destroy DC context while keeping DML 
> and DML2")
> Signed-off-by: Tobias Jakobi <tjak...@math.uni-bielefeld.de>

Reviewed-by: Harry Wentland <harry.wentl...@amd.com>

Harry

> ---
>  .../amd/display/dc/hwss/dcn35/dcn35_hwseq.c   | 20 +++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/dc/hwss/dcn35/dcn35_hwseq.c 
> b/drivers/gpu/drm/amd/display/dc/hwss/dcn35/dcn35_hwseq.c
> index dcced89c07b3..4e77728dac10 100644
> --- a/drivers/gpu/drm/amd/display/dc/hwss/dcn35/dcn35_hwseq.c
> +++ b/drivers/gpu/drm/amd/display/dc/hwss/dcn35/dcn35_hwseq.c
> @@ -1370,7 +1370,13 @@ void dcn35_set_drr(struct pipe_ctx **pipe_ctx,
>       params.vertical_total_mid_frame_num = adjust.v_total_mid_frame_num;
>  
>       for (i = 0; i < num_pipes; i++) {
> -             if ((pipe_ctx[i]->stream_res.tg != NULL) && 
> pipe_ctx[i]->stream_res.tg->funcs) {
> +             /* dc_state_destruct() might null the stream resources, so 
> fetch tg
> +              * here first to avoid a race condition. The lifetime of the 
> pointee
> +              * itself (the timing_generator object) is not a problem here.
> +              */
> +             struct timing_generator *tg = pipe_ctx[i]->stream_res.tg;
> +
> +             if ((tg != NULL) && tg->funcs) {
>                       struct dc_crtc_timing *timing = 
> &pipe_ctx[i]->stream->timing;
>                       struct dc *dc = pipe_ctx[i]->stream->ctx->dc;
>  
> @@ -1383,14 +1389,12 @@ void dcn35_set_drr(struct pipe_ctx **pipe_ctx,
>                                       num_frames = 2 * (frame_rate % 60);
>                               }
>                       }
> -                     if (pipe_ctx[i]->stream_res.tg->funcs->set_drr)
> -                             pipe_ctx[i]->stream_res.tg->funcs->set_drr(
> -                                     pipe_ctx[i]->stream_res.tg, &params);
> +                     if (tg->funcs->set_drr)
> +                             tg->funcs->set_drr(tg, &params);
>                       if (adjust.v_total_max != 0 && adjust.v_total_min != 0)
> -                             if 
> (pipe_ctx[i]->stream_res.tg->funcs->set_static_screen_control)
> -                                     
> pipe_ctx[i]->stream_res.tg->funcs->set_static_screen_control(
> -                                             pipe_ctx[i]->stream_res.tg,
> -                                             event_triggers, num_frames);
> +                             if (tg->funcs->set_static_screen_control)
> +                                     tg->funcs->set_static_screen_control(
> +                                             tg, event_triggers, num_frames);
>               }
>       }
>  }