Re: [PATCH RFC] tty_ldisc: add more limits to the @write_wakeup

[Huang Shijie <b32955@freescale.com>]

=D3=DA 2013=C4=EA11=D4=C213=C8=D5 15:30, Huang Shijie =D0=B4=B5=C0:
> 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_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 s=
till
> tries to send data with uart_write() which will try to grab the prot->l=
ock.
> A dead lock occurs, see the following log caught in the Bluetooth by ua=
rt:
>
> --------------------------------------------------------------------
> 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-ge4a=
1bef #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_spi=
n_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_wake=
up+0xa4/0x168)
> [<80455270>] (hci_uart_tx_wakeup+0xa4/0x168) from [<802dab18>] (tty_wak=
eup+0x50/0x5c)
> [<802dab18>] (tty_wakeup+0x50/0x5c) from [<802f81a4>] (imx_rtsint+0x50/=
0x80)
> [<802f81a4>] (imx_rtsint+0x50/0x80) from [<802f88f4>] (imx_int+0x158/0x=
17c)
> [<802f88f4>] (imx_int+0x158/0x17c) from [<8007abe0>] (handle_irq_event_=
percpu+0x50/0x194)
> [<8007abe0>] (handle_irq_event_percpu+0x50/0x194) from [<8007ad60>] (ha=
ndle_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 dea=
dlock.
>
> Signed-off-by: Huang Shijie <b32955@freescale.com>
> ---
>  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 *)
>   *
just a ping.

In actually, this is a BUG in the tty code or BT code.

thanks
Huang Shijie