linux-kernel - Re: [PATCH wireless] ar5523: Fix deadlock bugs caused by cancel_work_sync in ar5523

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Date:   Mon, 30 May 2022 14:24:04 +0300
From:   Kalle Valo <kvalo@...nel.org>
To:     Duoming Zhou <duoming@....edu.cn>
Cc:     linux-wireless@...r.kernel.org, pontus.fuchs@...il.com,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH wireless] ar5523: Fix deadlock bugs caused by cancel_work_sync in ar5523_stop

Duoming Zhou <duoming@....edu.cn> writes:

> 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@....edu.cn>

I assume you have found this with a static checker tool, it would be
good document what tool you are using. And if you have not tested this
with real hardware clearly mention that with "Compile tested only".

> ---
>  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);
>  }

Releasing a lock and taking it again looks like a hack to me. Please
test with a real device and try to find a better solution.

-- 
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https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches