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Subject[PATCH 08/11] In the uart_handle_cts_change(), uart_write_wakeup() is called after we call @uart_port->ops->start_tx().
Date
From: Huang Shijie <b32955@freescale.com>

The Documentation/serial/driver tells us:
-----------------------------------------------
start_tx(port)
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@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
---
include/linux/tty_ldisc.h | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index b8347c2..8946e31 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -92,7 +92,10 @@
* 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 *)
*
--
1.9.1.286.g5172cb3


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