[PATCH] drm/mediatek: fix race condition of plugged_cb and codec_dev

* [PATCH] drm/mediatek: fix race condition of plugged_cb and codec_dev
@ 2020-02-12 11:29 Tzung-Bi Shih
  2020-02-13  8:04 ` Tzung-Bi Shih
  0 siblings, 1 reply; 2+ messages in thread
From: Tzung-Bi Shih @ 2020-02-12 11:29 UTC (permalink / raw)
  To: broonie, airlied, daniel
  Cc: alsa-devel, cychiang, tzungbi, dri-devel, linux-mediatek,
	matthias.bgg, dgreid, linux-arm-kernel

hdmi_conn_detect and mtk_hdmi_audio_hook_plugged_cb woule be called
by different threads.  plugged_cb and codec_dev are in danger of race
condition.

Fixes by:
- Checks NULLs first.
- Uses WRITE_ONCE() to prevent store tearing (i.e. write to plugged_cb
  after codec_dev).
- Uses codec_dev as a signal to report HDMI jack status.

The patch intends to not use mutex to protect the shared variables.
In runtime, mtk_hdmi_audio_hook_plugged_cb would be called only once
if no one rebinds the sound card.  In contrast, hdmi_conn_detect could
be called multiple times.  Uses a mutex within hdmi_conn_detect calling
path is a waste (no lock contention).

Fixes: 5d3c64477392 ("drm/mediatek: support HDMI jack status reporting")
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
---
 drivers/gpu/drm/mediatek/mtk_hdmi.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/mediatek/mtk_hdmi.c b/drivers/gpu/drm/mediatek/mtk_hdmi.c
index 03aeb73005ef..746567c91bb7 100644
--- a/drivers/gpu/drm/mediatek/mtk_hdmi.c
+++ b/drivers/gpu/drm/mediatek/mtk_hdmi.c
@@ -1201,7 +1201,7 @@ mtk_hdmi_update_plugged_status(struct mtk_hdmi *hdmi)
 {
 	bool connected = mtk_cec_hpd_high(hdmi->cec_dev);
 
-	if (hdmi->plugged_cb && hdmi->codec_dev)
+	if (hdmi->codec_dev)
 		hdmi->plugged_cb(hdmi->codec_dev, connected);
 
 	return connected ?
@@ -1669,8 +1669,12 @@ static int mtk_hdmi_audio_hook_plugged_cb(struct device *dev, void *data,
 {
 	struct mtk_hdmi *hdmi = data;
 
-	hdmi->plugged_cb = fn;
-	hdmi->codec_dev = codec_dev;
+	if (!fn || !codec_dev)
+		return -EINVAL;
+
+	/* Use WRITE_ONCE() to prevent store tearing. */
+	WRITE_ONCE(hdmi->plugged_cb, fn);
+	WRITE_ONCE(hdmi->codec_dev, codec_dev);
 	mtk_hdmi_update_plugged_status(hdmi);
 
 	return 0;
-- 
2.25.0.225.g125e21ebc7-goog

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