linux-kernel - Re: [PATCH] mac80211: Use RCU protection in ieee80211_get_tx

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Message-ID: <20130613152735.GA1822@gmail.com>
Date:	Thu, 13 Jun 2013 10:27:35 -0500
From:	Calvin Owens <jcalvinowens@...il.com>
To:	Felix Fietkau <nbd@...nwrt.org>
Cc:	Johannes Berg <johannes@...solutions.net>,
	"Luis R. Rodriguez" <mcgrof@....qualcomm.com>,
	"John W. Linville" <linville@...driver.com>,
	linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org,
	ath9k-devel@...ts.ath9k.org, netdev@...r.kernel.org,
	jcalvinowens@...il.com
Subject: Re: [PATCH] mac80211: Use RCU protection in ieee80211_get_tx_rates()

On Wednesday 06/12 at 11:47 +0200, Felix Fietkau wrote:
> On 2013-06-12 10:00 AM, Calvin Owens wrote:
> > Copying the rate table should be done in an RCU read-side critical
> > section.
> I think this approach is wrong. The sta entry is also under RCU
> protection (no locking for read access in that part of the code.
> In a normal driver tx path, no extra rcu_read_lock/rcu_read_unlock is
> needed. Only if the driver does some scheduling outside of the tx
> function (which ath9k does), this RCU warning appears.
> 
> How about this change instead:
> ---
> --- a/drivers/net/wireless/ath/ath9k/xmit.c
> +++ b/drivers/net/wireless/ath/ath9k/xmit.c
> @@ -1570,6 +1570,8 @@ void ath_txq_schedule(struct ath_softc *
>  	    txq->axq_ampdu_depth >= ATH_AGGR_MIN_QDEPTH)
>  		return;
>  
> +	rcu_read_lock();
> +
>  	ac = list_first_entry(&txq->axq_acq, struct ath_atx_ac, list);
>  	last_ac = list_entry(txq->axq_acq.prev, struct ath_atx_ac, list);
>  
> @@ -1608,8 +1610,10 @@ void ath_txq_schedule(struct ath_softc *
>  
>  		if (ac == last_ac ||
>  		    txq->axq_ampdu_depth >= ATH_AGGR_MIN_QDEPTH)
> -			return;
> +			break;
>  	}
> +
> +	rcu_read_unlock();
>  }
>  
>  /***********/
>

Yep, that stops the RCU warning for me.

Tested-by: Calvin Owens <jcalvinowens@...il.com>
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