[PATCH] media: cec-pin: Fix event FIFO ordering

[Gui-Dong Han]

cec_pin_update() fills work_pin_events[] and work_pin_ts[], then
increments work_pin_num_events. cec_pin_thread_func() uses that counter
to decide when to read the FIFO entries.

Do not let the counter update be observed without the event update. Also
do not let a freed slot be reused before the thread has finished reading
it. Use release operations when publishing an entry and releasing a slot,
and acquire operations when consuming those counter updates.

Leave the other work_pin_num_events users as they do not participate in
this FIFO publication path.

Fixes: ea5c8ef29668 ("media: cec-pin: add low-level pin hardware support")
Signed-off-by: Gui-Dong Han <hanguidong02@xxxxxxxxx>
---
Found by auditing atomic operations used for synchronization.
A similar fix can be found in 6df8e84aa6b5.
---
 drivers/media/cec/core/cec-pin.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/media/cec/core/cec-pin.c b/drivers/media/cec/core/cec-pin.c
index 6e1c39102832..085fc12067af 100644
--- a/drivers/media/cec/core/cec-pin.c
+++ b/drivers/media/cec/core/cec-pin.c
@@ -115,7 +115,7 @@ static void cec_pin_update(struct cec_pin *pin, bool v, bool force)
 		return;
 
 	pin->adap->cec_pin_is_high = v;
-	if (atomic_read(&pin->work_pin_num_events) < CEC_NUM_PIN_EVENTS) {
+	if (atomic_read_acquire(&pin->work_pin_num_events) < CEC_NUM_PIN_EVENTS) {
 		u8 ev = v;
 
 		if (pin->work_pin_events_dropped) {
@@ -126,7 +126,7 @@ static void cec_pin_update(struct cec_pin *pin, bool v, bool force)
 		pin->work_pin_ts[pin->work_pin_events_wr] = ktime_get();
 		pin->work_pin_events_wr =
 			(pin->work_pin_events_wr + 1) % CEC_NUM_PIN_EVENTS;
-		atomic_inc(&pin->work_pin_num_events);
+		atomic_inc_return_release(&pin->work_pin_num_events);
 	} else {
 		pin->work_pin_events_dropped = true;
 		pin->work_pin_events_dropped_cnt++;
@@ -1101,7 +1101,7 @@ static int cec_pin_thread_func(void *_adap)
 						     pin->work_tx_ts);
 		}
 
-		while (atomic_read(&pin->work_pin_num_events)) {
+		while (atomic_read_acquire(&pin->work_pin_num_events)) {
 			unsigned int idx = pin->work_pin_events_rd;
 			u8 v = pin->work_pin_events[idx];
 
@@ -1110,7 +1110,7 @@ static int cec_pin_thread_func(void *_adap)
 						v & CEC_PIN_EVENT_FL_DROPPED,
 						pin->work_pin_ts[idx]);
 			pin->work_pin_events_rd = (idx + 1) % CEC_NUM_PIN_EVENTS;
-			atomic_dec(&pin->work_pin_num_events);
+			atomic_dec_return_release(&pin->work_pin_num_events);
 		}
 
 		switch (atomic_xchg(&pin->work_irq_change,
-- 
2.34.1