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Re: [PATCH] mac80211: Fix a race on enabling power save.

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On Wed, 2011-02-23 at 13:04 +0530, Vivek Natarajan wrote:
> There is a race on sending a data frame before the tx completion
> of nullfunc frame for enabling power save. As the data quickly
> follows the nullfunc frame, the AP thinks that the station is out
> of power save and continues to send the frames. Whereas in the
> station, the nullfunc ack will be processed after the tx completion
> of data frame and mac80211 goes to powersave. Thus the power
> save state mismatch between the station and the AP causes some
> data loss and some applications fail because of that. This patch
> fixes this issue.
> 
> Signed-off-by: Vivek Natarajan <vnatarajan@xxxxxxxxxxx>
> ---
>  net/mac80211/mlme.c   |   14 +++++++++++++-
>  net/mac80211/status.c |    2 --
>  2 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/net/mac80211/mlme.c b/net/mac80211/mlme.c
> index 7b3f9df..abb0116 100644
> --- a/net/mac80211/mlme.c
> +++ b/net/mac80211/mlme.c
> @@ -738,9 +738,19 @@ void ieee80211_dynamic_ps_enable_work(struct work_struct *work)
>  		return;
>  
>  	if ((local->hw.flags & IEEE80211_HW_PS_NULLFUNC_STACK) &&
> -	    (!(ifmgd->flags & IEEE80211_STA_NULLFUNC_ACKED)))
> +	    (!(ifmgd->flags & IEEE80211_STA_NULLFUNC_ACKED))) {
> +		netif_tx_stop_all_queues(sdata->dev);
> +		/*
> +		 * Flush all the frames queued in the driver before
> +		 * going to power save
> +		 */
> +		drv_flush(local, false);
>  		ieee80211_send_nullfunc(local, sdata, 1);
>  
> +		/* Flush once again to get the tx status of nullfunc frame */
> +		drv_flush(local, false);
> +	}
> +
>  	if (!((local->hw.flags & IEEE80211_HW_REPORTS_TX_ACK_STATUS) &&
>  	      (local->hw.flags & IEEE80211_HW_PS_NULLFUNC_STACK)) ||
>  	    (ifmgd->flags & IEEE80211_STA_NULLFUNC_ACKED)) {
> @@ -748,6 +758,8 @@ void ieee80211_dynamic_ps_enable_work(struct work_struct *work)
>  		local->hw.conf.flags |= IEEE80211_CONF_PS;
>  		ieee80211_hw_config(local, IEEE80211_CONF_CHANGE_PS);
>  	}
> +
> +	netif_tx_start_all_queues(sdata->dev);
>  }
>  
>  void ieee80211_dynamic_ps_timer(unsigned long data)
> diff --git a/net/mac80211/status.c b/net/mac80211/status.c
> index 010a559..8651851 100644
> --- a/net/mac80211/status.c
> +++ b/net/mac80211/status.c
> @@ -318,8 +318,6 @@ void ieee80211_tx_status(struct ieee80211_hw *hw, struct sk_buff *skb)
>  		if (info->flags & IEEE80211_TX_STAT_ACK) {
>  			local->ps_sdata->u.mgd.flags |=
>  					IEEE80211_STA_NULLFUNC_ACKED;
> -			ieee80211_queue_work(&local->hw,
> -					&local->dynamic_ps_enable_work);

Is the patch still correct if you don't remove this? I realise it won't
usually be necessary, but I think we don't flush the TX status tasklet
on flush right now so in theory there could be a race there that this
code would clean up after, right?

johannes

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