Re: [PATCH 1/2] drm/amd/display: Avoid race between dcn10_set_drr() and dc_state_destruct()

From: Sefa Eyeoglu
Date: Mon Sep 02 2024 - 07:06:09 EST


On Mon, 2024-09-02 at 11:40 +0200, tjakobi@xxxxxxxxxxxxxxxxxxxxx wrote:
> From: Tobias Jakobi <tjakobi@xxxxxxxxxxxxxxxxxxxxx>
>
> dc_state_destruct() nulls the resource context of the DC state. The
> pipe
> context passed to dcn10_set_drr() is a member of this resource
> context.
>
> If dc_state_destruct() is called parallel to the IRQ processing
> (which
> calls dcn10_set_drr() at some point), we can end up using already
> nulled
> function callback fields of struct stream_resource.
>
> The logic in dcn10_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 <tjakobi@xxxxxxxxxxxxxxxxxxxxx>
> ---
>  .../amd/display/dc/hwss/dcn10/dcn10_hwseq.c   | 20 +++++++++++------
> --
>  1 file changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> b/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> index 3306684e805a..da8f2cb3c5db 100644
> --- a/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> +++ b/drivers/gpu/drm/amd/display/dc/hwss/dcn10/dcn10_hwseq.c
> @@ -3223,15 +3223,19 @@ void dcn10_set_drr(struct pipe_ctx
> **pipe_ctx,
>   * as well.
>   */
>   for (i = 0; i < num_pipes; i++) {
> - if ((pipe_ctx[i]->stream_res.tg != NULL) &&
> pipe_ctx[i]->stream_res.tg->funcs) {
> - 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);
> + /* 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) {
> + 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);
>   }
>   }
>  }

This fixes the panics with my RX 6800 XT on Sway with VRR enabled!

Tested-by: Sefa Eyeoglu <contact@xxxxxxxxxxxxx>

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