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Date:	Thu, 20 Mar 2014 14:25:33 -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

Hi,

On Thu, Mar 20, 2014 at 03:16:35PM -0400, Peter Hurley wrote:
> 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@...leysoftware.com>
> 
> 
> > From 3ee6b74833f154df64a6164476b854846206a3f2 Mon Sep 17 00:00:00 2001
> >From: Felipe Balbi <balbi@...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@...escale.com>

I will never add any *-by tags without seeing it in the mailing list.
Now I can add it to the patch and send it as a real patch (git
send-email it).

> >Signed-off-by: Felipe Balbi <balbi@...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;
> >
> 

-- 
balbi

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