Bluetooth: Restore locking semantics when looking up L2CAP channels

As the comment for l2cap_get_chan_by_scid indicated, the function used
to return a locked socket.  The lock for the socket was acquired while
the channel list was also locked.

When locking was moved over to the l2cap_chan structure, the channel
lock was no longer acquired with the channel list still locked.  This
made it possible for the l2cap_chan to be deleted after
conn->chan_lock was released but before l2cap_chan_lock was called.
Making the call to l2cap_chan_lock before releasing conn->chan_lock
makes it impossible for the l2cap_chan to be deleted at the wrong
time.
Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Reviewed-by: Ulisses Furquim <ulisses@profusion.mobi>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>

net/bluetooth/l2cap_core.c

@@ -98,13 +98,15 @@ static struct l2cap_chan *__l2cap_get_chan_by_scid(struct l2cap_conn *conn, u16
 }
 
 /* Find channel with given SCID.
- * Returns locked socket */
+ * Returns locked channel. */
 static struct l2cap_chan *l2cap_get_chan_by_scid(struct l2cap_conn *conn, u16 cid)
 {
 	struct l2cap_chan *c;
 
 	mutex_lock(&conn->chan_lock);
 	c = __l2cap_get_chan_by_scid(conn, cid);
+	if (c)
+		l2cap_chan_lock(c);
 	mutex_unlock(&conn->chan_lock);
 
 	return c;
@@ -3183,8 +3185,6 @@ static inline int l2cap_config_req(struct l2cap_conn *conn, struct l2cap_cmd_hdr
 	if (!chan)
 		return -ENOENT;
 
-	l2cap_chan_lock(chan);
-
 	if (chan->state != BT_CONFIG && chan->state != BT_CONNECT2) {
 		struct l2cap_cmd_rej_cid rej;
 
@@ -3297,8 +3297,6 @@ static inline int l2cap_config_rsp(struct l2cap_conn *conn, struct l2cap_cmd_hdr
 	if (!chan)
 		return 0;
 
-	l2cap_chan_lock(chan);
-
 	switch (result) {
 	case L2CAP_CONF_SUCCESS:
 		l2cap_conf_rfc_get(chan, rsp->data, len);
@@ -4631,8 +4629,6 @@ static inline int l2cap_data_channel(struct l2cap_conn *conn, u16 cid, struct sk
 		return 0;
 	}
 
-	l2cap_chan_lock(chan);
-
 	BT_DBG("chan %p, len %d", chan, skb->len);
 
 	if (chan->state != BT_CONNECTED)