rtl8187: Fix lockups due to concurrent access to config routine

With the rtl8187 driver, the config routine is not protected against
access before a previous call has completed. When this happens, the
TX loopback that is needed to change channels may cause the chip to
be locked with a reset needed to restore communications. This patch
entered mainline as commit 7dcdd073.

The problem was found by Herton Ronaldo Krzesinski <herton@mandriva.com.br>,
who also suggested this type of fix.
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Acked-by: Herton Ronaldo Krzesinski <herton@mandriva.com.br>
Acked-by: Hin-Tak Leung <htl10@users.sourceforge.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/net/wireless/rtl8187.h

@@ -67,6 +67,10 @@ struct rtl8187_priv {
 	const struct rtl818x_rf_ops *rf;
 	struct ieee80211_vif *vif;
 	int mode;
+	/* The mutex protects the TX loopback state.
+	 * Any attempt to set channels concurrently locks the device.
+	 */
+	struct mutex conf_mutex;
 
 	/* rtl8187 specific */
 	struct ieee80211_channel channels[14];

drivers/net/wireless/rtl8187_dev.c

@@ -580,6 +580,7 @@ static int rtl8187_config(struct ieee80211_hw *dev, struct ieee80211_conf *conf)
 	struct rtl8187_priv *priv = dev->priv;
 	u32 reg;
 
+	mutex_lock(&priv->conf_mutex);
 	reg = rtl818x_ioread32(priv, &priv->map->TX_CONF);
 	/* Enable TX loopback on MAC level to avoid TX during channel
 	 * changes, as this has be seen to causes problems and the
@@ -610,6 +611,7 @@ static int rtl8187_config(struct ieee80211_hw *dev, struct ieee80211_conf *conf)
 	rtl818x_iowrite16(priv, &priv->map->ATIMTR_INTERVAL, 100);
 	rtl818x_iowrite16(priv, &priv->map->BEACON_INTERVAL, 100);
 	rtl818x_iowrite16(priv, &priv->map->BEACON_INTERVAL_TIME, 100);
+	mutex_unlock(&priv->conf_mutex);
 	return 0;
 }
 
@@ -814,6 +816,7 @@ static int __devinit rtl8187_probe(struct usb_interface *intf,
 		printk(KERN_ERR "rtl8187: Cannot register device\n");
 		goto err_free_dev;
 	}
+	mutex_init(&priv->conf_mutex);
 
 	printk(KERN_INFO "%s: hwaddr %s, rtl8187 V%d + %s\n",
 	       wiphy_name(dev->wiphy), print_mac(mac, dev->wiphy->perm_addr),