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

[tjakobi at math.uni-bielefeld.de]

From: Tobias Jakobi <tjakobi at 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 <tjakobi at math.uni-bielefeld.de>
---
 .../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);
 		}
 	}
 }
-- 
2.44.2