Re: hci_ldsic nested locking problem

[Peter Hurley]

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:
> > > >
> > > > Documentation/serial/tty.txt::
> > > >
> > > > |  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.
> > >
> > > http://www.spinics.net/lists/linux-serial/msg11144.html
> >
> > 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_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@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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