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Date: Thu, 10 Mar 2011 01:49:52 -0500
From: Chuck Ebbert <cebbert@redhat.com>
To: "Mario 'BitKoenig' Holbe" <Mario.Holbe@TU-Ilmenau.DE>
Cc: linux-kernel@vger.kernel.org
Subject: Re: kernel BUG and freeze on cat /proc/tty/driver/serial
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On Wed, 16 Feb 2011 17:17:28 +0100
"Mario 'BitKoenig' Holbe" <Mario.Holbe@TU-Ilmenau.DE> wrote:

> reading /proc/tty/driver/serial leads to a NULL pointer dereference
> BUG and freeze on a serial-console enabled 2.6.35.{4,10,11} and
> 2.6.37. 2.6.32.28 does fine without BUG and freeze.
> 
> Fresh boot 2.6.35.11 into emergency...
> # cat /proc/tty/driver/serial
> [   73.199568] BUG: unable to handle kernel NULL pointer dereference
> at 00000099 [   73.227373] IP: [<c11a8969>] tty_ldisc_try+0x10/0x35

The oops is here, in uart_handle_dcd_change() in serial_core.h:

        struct tty_ldisc *ld = tty_ldisc_ref(port->tty);

(port->tty is NULL)

Called from check_modem_status() in 8250.c:

                if (status & UART_MSR_DDCD)
                        uart_handle_dcd_change(&up->port, status & UART_MSR_DCD);

So apparently the port has no tty, until you run "setserial -g", or else
that somehow makes the DCD status appear unchanged later when reading
/proc/tty/driver/serial.
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