[PATCH RT 6/9] Bluetooth: avoid recursive locking in hci_send_to_channel()

[Steven Rostedt]

4.4.102-rt117-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

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.

Cc: Marcel Holtmann <marcel@xxxxxxxxxxxx>
Cc: Johan Hedberg <johan.hedberg@xxxxxxxxx>
Cc: stable-rt@xxxxxxxxxxxxxxx
Fixes: 38ceaa00d02d ("Bluetooth: Add support for sending MGMT commands and events to monitor")
Reported-by: Mart van de Wege <mvdwege@xxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 net/bluetooth/hci_sock.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/net/bluetooth/hci_sock.c b/net/bluetooth/hci_sock.c
index c842f40c1173..035a5f6e3de9 100644
--- a/net/bluetooth/hci_sock.c
+++ b/net/bluetooth/hci_sock.c
@@ -213,15 +213,13 @@ 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;
 
@@ -247,6 +245,13 @@ 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);
 }
 
@@ -299,8 +304,8 @@ void hci_send_to_monitor(struct hci_dev *hdev, struct sk_buff *skb)
 	hdr->index = cpu_to_le16(hdev->id);
 	hdr->len = cpu_to_le16(skb->len);
 
-	hci_send_to_channel(HCI_CHANNEL_MONITOR, skb_copy,
-			    HCI_SOCK_TRUSTED, NULL);
+	__hci_send_to_channel(HCI_CHANNEL_MONITOR, skb_copy,
+			      HCI_SOCK_TRUSTED, NULL);
 	kfree_skb(skb_copy);
 }
 
-- 
2.13.2