Re: [PATCH v3 1/2] wifi: ath9k: fix races between ath9k_wmi_cmd and ath9k_wmi_ctrl_rx

[Fedor Pchelkin <pchelkin@ispras.ru>]

On Wed, Apr 26, 2023 at 07:07:08AM +0800, Hillf Danton wrote: 
> Given similar wait timeout[1], just taking lock on the waiter side is not
> enough wrt fixing the race, because in case job done on the waker side,
> waiter needs to wait again after timeout.
> 

As I understand you correctly, you mean the case when a timeout occurs
during ath9k_wmi_ctrl_rx() callback execution. I suppose if a timeout has
occurred on a waiter's side, it should return immediately and doesn't have
to care in which state the callback has been at that moment.

AFAICS, this is controlled properly with taking a wmi_lock on waiter and
waker sides, and there is no data corruption.

If a callback has not managed to do its work entirely (performing a
completion and subsequently waking waiting thread is included here), then,
well, it is considered a timeout, in my opinion.

Your suggestion makes a wmi_cmd call to give a little more chance for the
belated callback to complete (although timeout has actually expired). That
is probably good, but increasing a timeout value makes that job, too. I
don't think it makes any sense on real hardware.

Or do you mean there is data corruption that is properly fixed with your
patch?

That is, I agree there can be a situation when a callback makes all the
logical work it should and it just hasn't got enough time to perform a
completion before a timeout on waiter's side occurs. And this behaviour
can be named "racy". But, technically, this seems to be a rather valid
timeout.

> [1] https://lore.kernel.org/lkml/9d9b9652-c1ac-58e9-2eab-9256c17b1da2@I-love.SAKURA.ne.jp/
> 

I don't think it's a similar case because wait_for_completion_state() is
interruptible while wait_for_completion_timeout() is not.

> A correct fix looks like after putting pieces together
> 
> +++ b/drivers/net/wireless/ath/ath9k/wmi.c
> @@ -238,6 +238,7 @@ static void ath9k_wmi_ctrl_rx(void *priv
>  		spin_unlock_irqrestore(&wmi->wmi_lock, flags);
>  		goto free_skb;
>  	}
> +	wmi->last_seq_id = 0;
>  	spin_unlock_irqrestore(&wmi->wmi_lock, flags);
>  
>  	/* WMI command response */
> @@ -339,9 +340,20 @@ int ath9k_wmi_cmd(struct wmi *wmi, enum
>  
>  	time_left = wait_for_completion_timeout(&wmi->cmd_wait, timeout);
>  	if (!time_left) {
> +		unsigned long flags;
> +		int wait = 0;
> +
>  		ath_dbg(common, WMI, "Timeout waiting for WMI command: %s\n",
>  			wmi_cmd_to_name(cmd_id));
> -		wmi->last_seq_id = 0;
> +
> +		spin_lock_irqsave(&wmi->wmi_lock, flags);
> +		if (wmi->last_seq_id == 0) /* job done on the waker side? */
> +			wait = 1;
> +		else
> +			wmi->last_seq_id = 0;
> +		spin_unlock_irqrestore(&wmi->wmi_lock, flags);
> +		if (wait)
> +			wait_for_completion(&wmi->cmd_wait);
>  		mutex_unlock(&wmi->op_mutex);
>  		return -ETIMEDOUT;
>  	}