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Message-ID: <3525aa03-4c5c-4d40-ad95-0bb1243d40f1@amd.com>
Date: Tue, 3 Sep 2024 17:27:32 -0400
From: Harry Wentland <harry.wentland@....com>
To: tjakobi@...h.uni-bielefeld.de, Leo Li <sunpeng.li@....com>,
Rodrigo Siqueira <Rodrigo.Siqueira@....com>,
Alex Deucher <alexander.deucher@....com>,
Christian König <christian.koenig@....com>,
"Pan, Xinhui" <Xinhui.Pan@....com>, David Airlie <airlied@...il.com>,
Daniel Vetter <daniel@...ll.ch>,
Mario Limonciello <mario.limonciello@....com>
Cc: amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] drm/amd/display: Avoid race between dcn10_set_drr()
and dc_state_destruct()
On 2024-09-02 05:40, tjakobi@...h.uni-bielefeld.de wrote:
> From: Tobias Jakobi <tjakobi@...h.uni-bielefeld.de>
>
> 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@...h.uni-bielefeld.de>
Thanks for this fix. It also makes the code more readable.
Reviewed-by: Harry Wentland <harry.wentland@....com>
Harry
> ---
> .../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, ¶ms);
> + /* 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, ¶ms);
> 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);
> }
> }
> }
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