linux-kernel - Re: 2.6.21-rc4-mm1: init hangs on tty

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Date:	Wed, 21 Mar 2007 14:41:15 +0000 (GMT)
From:	Hugh Dickins <hugh@...itas.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.21-rc4-mm1: init hangs on tty_mutex

On Tue, 20 Mar 2007, Eric W. Biederman wrote:
> Hugh Dickins <hugh@...itas.com> writes:
> 
> > I get a hang at startup or shutdown: tiocsctty() holds tty_mutex, and
> > now with Eric's tty-in-tiocsctty-when-we-steal-a-tty-hang-it-up.patch
> > it can end up calling con_close(), which tries to take tty_mutex.
> > So best revert that in hot-fixes, until Eric provides a better.
> 
> Well I'm confused I know I tested this, and I even added a printk and
> saw it trigger a couple of times.
> 
> Hmm.  Looking at the code it is clearly not safe to hold the tty_mutex
> when calling tty_vhangup.  I thought the other caller in tty_io.c was doing
> that but on a second glance it is clearly not.  And the hangup and close
> methods do seem to take the tty_mutex.
> 
> How this worked in my testing is beyond me..

I agree there's something weird about it, and wonder whether it's
exposing some other, unrelated bug e.g. something uninitialized.

What I omitted to mention is that I reliably had init hanging on
startup with my KDB kernels (on i386 UP and i386 SMP and x86_64 SMP);
but never saw my non-KDB kernels (nor my powerpc kernel) hanging
there at startup - they would hang (in the same way) at shutdown
(every time? I can't swear to that).  But I can't imagine what
KDB would have to do with it, other than shifting stuff around.

Hugh
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