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

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Message-ID: <51B843C9.9090500@openwrt.org>
Date:	Wed, 12 Jun 2013 11:47:53 +0200
From:	Felix Fietkau <nbd@...nwrt.org>
To:	Calvin Owens <jcalvinowens@...il.com>
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
Subject: Re: [PATCH] mac80211: Use RCU protection in ieee80211_get_tx_rates()

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();
 }
 
 /***********/

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