mac80211: fix IBSS teardown race

Ignacy reports that sometimes after leaving an IBSS
joining a new one didn't work because there still
were stations on the list. He fixed it by flushing
stations when attempting to join a new IBSS, but
this shouldn't be happening in the first case. When
I looked into it I saw a race condition in teardown
that could cause stations to be added after flush,
and thus cause this situation. Ignacy confirms that
after applying my patch he hasn't seen this happen
again.
Reported-by: NIgnacy Gawedzki <i@lri.fr>
Debugged-by: NIgnacy Gawedzki <i@lri.fr>
Tested-by: NIgnacy Gawedzki <i@lri.fr>
Cc: stable@kernel.org
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/mac80211/ibss.c

@@ -965,6 +965,10 @@ int ieee80211_ibss_leave(struct ieee80211_sub_if_data *sdata)
 
 	mutex_lock(&sdata->u.ibss.mtx);
 
+	sdata->u.ibss.state = IEEE80211_IBSS_MLME_SEARCH;
+	memset(sdata->u.ibss.bssid, 0, ETH_ALEN);
+	sdata->u.ibss.ssid_len = 0;
+
 	active_ibss = ieee80211_sta_active_ibss(sdata);
 
 	if (!active_ibss && !is_zero_ether_addr(ifibss->bssid)) {
@@ -999,8 +1003,6 @@ int ieee80211_ibss_leave(struct ieee80211_sub_if_data *sdata)
 	kfree_skb(skb);
 
 	skb_queue_purge(&sdata->skb_queue);
-	memset(sdata->u.ibss.bssid, 0, ETH_ALEN);
-	sdata->u.ibss.ssid_len = 0;
 
 	del_timer_sync(&sdata->u.ibss.timer);