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Date:	Thu, 20 Mar 2014 13:11:45 -0500
From:	Felipe Balbi <balbi@...com>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	<balbi@...com>, Marcel Holtmann <marcel@...tmann.org>,
	Alan Cox <alan@...ux.intel.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Muralidharan Karicheri <m-karicheri2@...com>,
	<linux-bluetooth@...r.kernel.org>, <linux-serial@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Huang Shijie <b32955@...escale.com>
Subject: Re: hci_ldsic nested locking problem

On Thu, Mar 20, 2014 at 01:31:40PM -0400, Peter Hurley wrote:
> [ +cc Huang Shijie ]
> 
> On 03/20/2014 01:16 PM, Felipe Balbi wrote:
> >On Thu, Mar 20, 2014 at 04:42:16PM +0000, Alan Cox wrote:
> >>On Thu, 2014-03-20 at 11:34 -0500, Felipe Balbi wrote:
> >>>Hi,
> >>>
> >>>when 8250 driver calls uart_write_wakeup(), the tty port lock is already
> >>>taken. hci_ldisc.c's implementation of ->write_wakeup() calls
> >>>tty->ops->write() to actually send the characters, but that call will
> >>>try to acquire the same port lock again.
> >>>
> >>>Looking at other line disciplines that looks like a bug in hci_ldisc.c.
> >>>Am I correct to assume that ->write_wakeup() is supposed to *just*
> >>>wakeup the bottom half so we handle ->write() in another context ?
> >>>
> >>>Is it legal to call tty->ops->write() from within ->write_wakeup() ?
> >>
> >>It isn't because you might send all the bytes and go
> >>
> >>	write
> >>		write_wakeup
> >>			write
> >>				write wakeup
> >>					...
> >>
> >>and recurse
> >
> >cool, so there really is a bug in hci_ldisc. Marcel, any tips on how do
> >you want this to be sorted out ?
> 
> hci_uart_tx_wakeup() should perform the I/O as work.
> FWIW, this was reported by Huang Shijie back on Dec 6.
> 
> I'd fix it but I have no way to test it.

here's a build-tested only patch which is waiting for testing from other
colleagues who've got a platform to reproduce the problem:

diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
index bc68a44..789000d 100644
--- a/drivers/bluetooth/hci_ldisc.c
+++ b/drivers/bluetooth/hci_ldisc.c
@@ -118,10 +118,6 @@ static inline struct sk_buff *hci_uart_dequeue(struct hci_uart *hu)
 
 int hci_uart_tx_wakeup(struct hci_uart *hu)
 {
-	struct tty_struct *tty = hu->tty;
-	struct hci_dev *hdev = hu->hdev;
-	struct sk_buff *skb;
-
 	if (test_and_set_bit(HCI_UART_SENDING, &hu->tx_state)) {
 		set_bit(HCI_UART_TX_WAKEUP, &hu->tx_state);
 		return 0;
@@ -129,6 +125,18 @@ int hci_uart_tx_wakeup(struct hci_uart *hu)
 
 	BT_DBG("");
 
+	schedule_work(&hu->write_work);
+
+	return 0;
+}
+
+static void hci_uart_write_work(struct work_struct *work)
+{
+	struct hci_uart *hu = container_of(work, struct hci_uart, init_ready);
+	struct tty_struct *tty = hu->tty;
+	struct hci_dev *hdev = hu->hdev;
+	struct sk_buff *skb;
+
 restart:
 	clear_bit(HCI_UART_TX_WAKEUP, &hu->tx_state);
 
@@ -153,7 +161,6 @@ restart:
 		goto restart;
 
 	clear_bit(HCI_UART_SENDING, &hu->tx_state);
-	return 0;
 }
 
 static void hci_uart_init_work(struct work_struct *work)
@@ -289,6 +296,7 @@ static int hci_uart_tty_open(struct tty_struct *tty)
 	tty->receive_room = 65536;
 
 	INIT_WORK(&hu->init_ready, hci_uart_init_work);
+	INIT_WORK(&hu->write_work, hci_uart_write_work);
 
 	spin_lock_init(&hu->rx_lock);
 
diff --git a/drivers/bluetooth/hci_uart.h b/drivers/bluetooth/hci_uart.h
index fffa61f..12df101 100644
--- a/drivers/bluetooth/hci_uart.h
+++ b/drivers/bluetooth/hci_uart.h
@@ -68,6 +68,7 @@ struct hci_uart {
 	unsigned long		hdev_flags;
 
 	struct work_struct	init_ready;
+	struct work_struct	write_work;
 
 	struct hci_uart_proto	*proto;
 	void			*priv;

-- 
balbi

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