linux-kernel - Re: [PATCH 5/5] tty: n_gsm: Fixed NULL ptr OOPs in tty_write

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Date:	Fri, 3 Jun 2011 23:10:28 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Russ Gorby <russ.gorby@...el.com>
Cc:	Greg Kroah-Hartman <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	suhail.ahmed@...el.com
Subject: Re: [PATCH 5/5] tty: n_gsm: Fixed NULL ptr OOPs in tty_write_room()

On Fri,  3 Jun 2011 12:03:41 -0700
Russ Gorby <russ.gorby@...el.com> wrote:

> We saw a case where gsmld_output was called after the MUX
> was shutdown. In this case gsm->tty was null so tty_write_room(NULL)
> was called which resulted in an exception.

This is papering over a problem. It shouldn't be happening. Do you have a
copy of the backtrace of the event and does it occur a lot. If it's a one
off freak event so far then instead add

	if (gsm->tty == NULL) {
		WARN_ON(1);
		return -ENXIO;
	}

so we get dumps from it and the box survives when it occurs. That'll get
us traces to nail it properly. For one your patch is not a valid
ultimate fix because if it can occur what stops gsm->tty changing as it
occurs.

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