drm/amd/display: Avoid race between dcn35_set_drr() and dc_state_destruct()

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@math.uni-bielefeld.de>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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 a4c6decee0f8a47ff526490424c80d88d50d5ef6..5c5b2dd86f5866eaf14a6c1c98a3426867e53f17 100644 (file)
--- a/drivers/gpu/drm/amd/display/dc/hwss/dcn35/dcn35_hwseq.c
+++ b/drivers/gpu/drm/amd/display/dc/hwss/dcn35/dcn35_hwseq.c
@@ -1414,7 +1414,13 @@ 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) {
                        if (pipe_ctx[i]->stream && pipe_ctx[i]->stream->ctx->dc->debug.static_screen_wait_frames) {
                                struct dc_crtc_timing *timing = &pipe_ctx[i]->stream->timing;
                                struct dc *dc = pipe_ctx[i]->stream->ctx->dc;
@@ -1426,14 +1432,12 @@ 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);
                }
        }
 }