Re: hci_ldsic nested locking problem

[Peter Hurley <peter@hurleysoftware.com>]

On 03/20/2014 02:25 PM, Felipe Balbi wrote:
> On Thu, Mar 20, 2014 at 02:21:17PM -0400, Peter Hurley wrote:
>> On 03/20/2014 02:11 PM, Felipe Balbi wrote:
>>> 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:
>>
>> Where's the cancel_work_sync() on teardown?
>
> here, as a patch too this time:

Thanks. Minor edits below but, strictly speaking, not necessary.

Reviewed-by: Peter Hurley <peter@hurleysoftware.com>


>  From 3ee6b74833f154df64a6164476b854846206a3f2 Mon Sep 17 00:00:00 2001
> From: Felipe Balbi <balbi@ti.com>
> Date: Thu, 20 Mar 2014 13:20:10 -0500
> Subject: [PATCH] bluetooth: hci_ldisc: fix deadlock condition
>
> LDISCs shouldn't call tty->ops->write() from within
> ->write_wakeup().
>
> ->write_wakeup() is called with port lock taken and
> IRQs disabled, tty->ops->write() will try to acquire
> the same port lock and we will deadlock.
>

I know you found it independently but ?

Reported-by: Huang Shijie <b32955@freescale.com>

> Signed-off-by: Felipe Balbi <balbi@ti.com>
> ---
>   drivers/bluetooth/hci_ldisc.c | 20 +++++++++++++++-----
>   drivers/bluetooth/hci_uart.h  |  1 +
>   2 files changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
> index 6e06f6f..ecdd765 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;
> +

+	/* FIXME: if bad skb length or tty->ops->write() returns < 0 ??? */

>   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)
> @@ -281,6 +288,7 @@ 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);
>
> @@ -318,6 +326,8 @@ static void hci_uart_tty_close(struct tty_struct *tty)
>   	if (hdev)
>   		hci_uart_close(hdev);
>
> +	cancel_work_sync(&hy->write_work);
> +
>   	if (test_and_clear_bit(HCI_UART_PROTO_SET, &hu->flags)) {
>   		if (hdev) {
>   			if (test_bit(HCI_UART_REGISTERED, &hu->flags))
> 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;
>