Bluetooth: avoid recursive locking in hci_send_to_channel()

Mart reported a deadlock in -RT in the call path:
  hci_send_monitor_ctrl_event() -> hci_send_to_channel()

because both functions acquire the same read lock hci_sk_list.lock. This
is also a mainline issue because the qrwlock implementation is writer
fair (the traditional rwlock implementation is reader biased).

To avoid the deadlock there is now __hci_send_to_channel() which expects
the readlock to be held.

Fixes: 38ceaa00d02d ("Bluetooth: Add support for sending MGMT commands and events to monitor")
Reported-by: Mart van de Wege <mvdwege@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/hci_sock.c

@@ -251,15 +251,13 @@ void hci_send_to_sock(struct hci_dev *hdev, struct sk_buff *skb)
 }
 
 /* Send frame to sockets with specific channel */
-void hci_send_to_channel(unsigned short channel, struct sk_buff *skb,
-			 int flag, struct sock *skip_sk)
+static void __hci_send_to_channel(unsigned short channel, struct sk_buff *skb,
+				  int flag, struct sock *skip_sk)
 {
 	struct sock *sk;
 
 	BT_DBG("channel %u len %d", channel, skb->len);
 
-	read_lock(&hci_sk_list.lock);
-
 	sk_for_each(sk, &hci_sk_list.head) {
 		struct sk_buff *nskb;
 
@@ -285,6 +283,13 @@ void hci_send_to_channel(unsigned short channel, struct sk_buff *skb,
 			kfree_skb(nskb);
 	}
 
+}
+
+void hci_send_to_channel(unsigned short channel, struct sk_buff *skb,
+			 int flag, struct sock *skip_sk)
+{
+	read_lock(&hci_sk_list.lock);
+	__hci_send_to_channel(channel, skb, flag, skip_sk);
 	read_unlock(&hci_sk_list.lock);
 }
 
@@ -388,8 +393,8 @@ void hci_send_monitor_ctrl_event(struct hci_dev *hdev, u16 event,
 		hdr->index = index;
 		hdr->len = cpu_to_le16(skb->len - HCI_MON_HDR_SIZE);
 
-		hci_send_to_channel(HCI_CHANNEL_MONITOR, skb,
-				    HCI_SOCK_TRUSTED, NULL);
+		__hci_send_to_channel(HCI_CHANNEL_MONITOR, skb,
+				      HCI_SOCK_TRUSTED, NULL);
 		kfree_skb(skb);
 	}