[PATCH wireless] ar5523: Fix deadlock bugs caused by cancel_work_sync in ar5523_stop

[Duoming Zhou]

If the work item is running, the cancel_work_sync in ar5523_stop will
not return until work item is finished. If we hold mutex_lock and use
cancel_work_sync to wait the work item to finish, the work item such as
ar5523_tx_wd_work and ar5523_tx_work also require mutex_lock. As a result,
the ar5523_stop will be blocked forever. One of the race conditions is
shown below:

    (Thread 1)             |   (Thread 2)
ar5523_stop                |
  mutex_lock(&ar->mutex)   | ar5523_tx_wd_work
                           |   mutex_lock(&ar->mutex)
  cancel_work_sync         |   ...

This patch moves cancel_work_sync out of mutex_lock in order to mitigate
deadlock bugs.

Fixes: b7d572e1871d ("ar5523: Add new driver")
Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
---
 drivers/net/wireless/ath/ar5523/ar5523.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/wireless/ath/ar5523/ar5523.c b/drivers/net/wireless/ath/ar5523/ar5523.c
index 9cabd342d15..99d6b13ffcf 100644
--- a/drivers/net/wireless/ath/ar5523/ar5523.c
+++ b/drivers/net/wireless/ath/ar5523/ar5523.c
@@ -1071,8 +1071,10 @@ static void ar5523_stop(struct ieee80211_hw *hw)
 	ar5523_cmd_write(ar, WDCMSG_TARGET_STOP, NULL, 0, 0);
 
 	del_timer_sync(&ar->tx_wd_timer);
+	mutex_unlock(&ar->mutex);
 	cancel_work_sync(&ar->tx_wd_work);
 	cancel_work_sync(&ar->rx_refill_work);
+	mutex_lock(&ar->mutex);
 	ar5523_cancel_rx_bufs(ar);
 	mutex_unlock(&ar->mutex);
 }
-- 
2.17.1