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Date: Wed, 24 Jun 2009 12:30:05 +0200
From: Marek Szyprowski <m.szyprowski@samsung.com>
Subject: RE: PROBLEM: kernel oops with g_serial USB gadget on 2.6.30
In-reply-to: <200906240149.41013.david-b@pacbell.net>
To: "'David Brownell'" <david-b@pacbell.net>
Cc: "'Alan Cox'" <alan@lxorguk.ukuu.org.uk>,
       "'Alan Stern'" <stern@rowland.harvard.edu>,
       "'Peter Korsgaard'" <jacmet@sunsite.dk>,
       "'USB list'" <linux-usb@vger.kernel.org>,
       "'Kernel development list'" <linux-kernel@vger.kernel.org>,
       kyungmin.park@samsung.com, Marek Szyprowski <m.szyprowski@samsung.com>
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Hello,

On Wednesday, June 24, 2009 10:50 AM, David Brownell wrote:

> > I did some additional tests and found another bug. When I enabled debug in my
> > low level udc driver then I can easily trigger the following bug:
> >
> > [ ? 55.630000] Unable to handle kernel NULL pointer dereference at virtual address 00000014
> > [ ? 55.630000] pgd = c0004000
> > [ ? 55.630000] [00000014] *pgd=00000000
> > [ ? 55.630000] Internal error: Oops: 17 [#1] PREEMPT
> > [ ? 55.630000] Modules linked in:
> > [ ? 55.630000] CPU: 0 ? ?Not tainted ?(2.6.30 #355)
> > [ ? 55.630000] PC is at __lock_acquire+0xa0/0xa6c
> > [ ? 55.630000] LR is at lock_acquire+0x58/0x6c
> > [ ? 55.630000] ...
> > [ ? 55.630000] [<c005786c>] (__lock_acquire+0xa0/0xa6c) from [<c0058290>] (lock_acquire+0x58/0x6c)
> > [ ? 55.630000] [<c0058290>] (lock_acquire+0x58/0x6c) from [<c0274e74>]
> (_spin_lock_irqsave+0x44/0x58)
> > [ ? 55.630000] [<c0274e74>] (_spin_lock_irqsave+0x44/0x58) from [<c019d608>]
> (gs_write_room+0x10/0x58)
> > [ ? 55.630000] [<c019d608>] (gs_write_room+0x10/0x58) from [<c0156058>] (tty_write_room+0x20/0x28)
> 
> So it's looking like tty->driver_data is somehow NULL.  That's
> never supposed to happen.  Did gs_open() fail or something?

I've triggered this bug by running a 'getty -L 115200 ttyGS0 vt100' and trying
to login on that usb console and then pressing 'enter' key for longer time.
getty might abort after a few failed logins, so the /dev/ttyGS0 file might be
closed before all characters that need to be echoed were processed (usb udc debug
messages really slows it down). This is however a pure guessing, I know nothing
about tty framework and internals.

Best regards
--
Marek Szyprowski
Samsung Poland R&D Center



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