Re: hci_ldsic nested locking problem

From: Peter Hurley
Date: Thu Mar 20 2014 - 15:16:50 EST

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:

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 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@xxxxxxxxxxxxxxxxxx>

From 3ee6b74833f154df64a6164476b854846206a3f2 Mon Sep 17 00:00:00 2001
From: Felipe Balbi <balbi@xxxxxx>
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() 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@xxxxxxxxxxxxx>

Signed-off-by: Felipe Balbi <balbi@xxxxxx>
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)


+ 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 ??? */

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);


@@ -318,6 +326,8 @@ static void hci_uart_tty_close(struct tty_struct *tty)
if (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;

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