[PATCH] kernel panic, pty.c: remove direct call to tty_wakup in pty_write

[Andre Naujoks]

Hello.

This patch removes the direct call to tty_wakeup in pty_write. I have
not noticed any drawbacks with this but I am not familiar with the pty
driver at all. I think what happens is a recursive loop,
write_wakeup->write->write_wakeup ...

The documentation for the tty interface forbids this direct call:

(from Documentation/serial/tty.txt)
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.



The direct call caused a reproducable kernel panic (see bottom of this
mail) for me with the following setup:

- using can-utils from git://gitorious.org/linux-can/can-utils.git
  slcan_attach and cangen are used

- create a network link between two serial CAN interfaces with:
  $ socat PTY,link=/tmp/slcan0,raw TCP4-LISTEN:50000 &
  $ socat TCP4:localhost:50000 PTY,link=/tmp/slcan1,raw &
  $ slcan_attach /tmp/slcan0
  $ slcan_attach /tmp/slcan1
  $ ip link set slcan0 up
  $ ip link set slcan1 up

- produce a kernel panic by overloading the CAN interfaces:
  $ cangen slcan0 -g0


Please keep me in CC. I am not subscribed to the list.
If I can provide any more information, I will be glad to do so.

This is the patch. It applies to the current linux master branch: