Re: hci_ldsic nested locking problem

From: Peter Hurley
Date: Thu Mar 20 2014 - 14:55:21 EST

On 03/20/2014 02:45 PM, Greg KH wrote:
On Thu, Mar 20, 2014 at 12:35:18PM -0500, Felipe Balbi wrote:
On Thu, Mar 20, 2014 at 01:34:57PM -0400, Peter Hurley wrote:
[ +cc Huang Shijie ]

On 03/20/2014 01:29 PM, Felipe Balbi wrote:
then we need updates to Documentation:


| Driver Side Interfaces:
| receive_buf() - Hand buffers of bytes from the driver to the ldisc
| for processing. Semantics currently rather
| mysterious 8(
| write_wakeup() - May be called at any point between open and close.
| The TTY_DO_WRITE_WAKEUP flag indicates if a call
| is needed but always races versus calls. Thus the
| ldisc must be careful about setting order and to
| handle unexpected calls. Must not sleep.
| The driver is forbidden from calling this directly
| from the ->write call from the ldisc as the ldisc
| is permitted to call the driver write method from
| this function. In such a situation defer it.

documentation says ldisc is allowed to call ->write() from
->write_wakeup(). huh ?

Patch submitted but never applied.

Thank you. For that patch:

Acked-by: Felipe Balbi <balbi@xxxxxx>

Can someone resend it, this is lost in my tree for some reason...

Apologies if my mailer mangles this.

--- >% ---
From: Huang Shijie <b32955@xxxxxxxxxxxxx>

In the uart_handle_cts_change(), uart_write_wakeup() is called after
we call @uart_port->ops->start_tx().

The Documentation/serial/driver tells us:
Start transmitting characters.

Locking: port->lock taken.
Interrupts: locally disabled.

So when the uart_write_wakeup() is called, the port->lock is taken by
the upper. See the following callstack:

|_ uart_write_wakeup
|_ tty_wakeup
|_ ld->ops->write_wakeup

With the port->lock held, we call the @write_wakeup. Some implemetation of
the @write_wakeup does not notice that the port->lock is held, and it still
tries to send data with uart_write() which will try to grab the prot->lock.
A dead lock occurs, see the following log caught in the Bluetooth by uart:

BUG: spinlock lockup suspected on CPU#0, swapper/0/0
lock: 0xdc3f4410, .magic: dead4ead, .owner: swapper/0/0, .owner_cpu: 0
CPU: 0 PID: 0 Comm: swapper/0 Tainted: G W 3.10.17-16839-ge4a1bef #1320
[<80014cbc>] (unwind_backtrace+0x0/0x138) from [<8001251c>] (show_stack+0x10/0x14)
[<8001251c>] (show_stack+0x10/0x14) from [<802816ac>] (do_raw_spin_lock+0x108/0x184)
[<802816ac>] (do_raw_spin_lock+0x108/0x184) from [<806a22b0>] (_raw_spin_lock_irqsave+0x54/0x60)
[<806a22b0>] (_raw_spin_lock_irqsave+0x54/0x60) from [<802f5754>] (uart_write+0x38/0xe0)
[<802f5754>] (uart_write+0x38/0xe0) from [<80455270>] (hci_uart_tx_wakeup+0xa4/0x168)
[<80455270>] (hci_uart_tx_wakeup+0xa4/0x168) from [<802dab18>] (tty_wakeup+0x50/0x5c)
[<802dab18>] (tty_wakeup+0x50/0x5c) from [<802f81a4>] (imx_rtsint+0x50/0x80)
[<802f81a4>] (imx_rtsint+0x50/0x80) from [<802f88f4>] (imx_int+0x158/0x17c)
[<802f88f4>] (imx_int+0x158/0x17c) from [<8007abe0>] (handle_irq_event_percpu+0x50/0x194)
[<8007abe0>] (handle_irq_event_percpu+0x50/0x194) from [<8007ad60>] (handle_irq_event+0x3c/0x5c)

This patch adds more limits to the @write_wakeup, the one who wants to
implemet the @write_wakeup should follow the limits which avoid the deadlock.

Signed-off-by: Huang Shijie <b32955@xxxxxxxxxxxxx>
include/linux/tty_ldisc.h | 5 ++++-
1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index f15c898..539ccc5 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -91,7 +91,10 @@
* This function is called by the low-level tty driver to signal
* that line discpline should try to send more characters to the
* low-level driver for transmission. If the line discpline does
- * not have any more data to send, it can just return.
+ * not have any more data to send, it can just return. If the line
+ * discipline does have some data to send, please arise a tasklet
+ * or workqueue to do the real data transfer. Do not send data in
+ * this hook, it may leads to a deadlock.
* int (*hangup)(struct tty_struct *)
-- 1.7.2.rc3
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