linux-kernel - Re: tty: deadlock between n_tracerouter_receivebuf and flush_to

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Date:	Wed, 20 Jan 2016 13:07:52 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>,
	LKML <linux-kernel@...r.kernel.org>,
	J Freyensee <james_p_freyensee@...ux.intel.com>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: tty: deadlock between n_tracerouter_receivebuf and flush_to_ldisc

On Wed, Jan 20, 2016 at 12:54:55PM +0100, Dmitry Vyukov wrote:
> On Wed, Jan 20, 2016 at 12:44 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Wed, Jan 20, 2016 at 10:36:01AM +0100, Dmitry Vyukov wrote:
> >
> >> +Peter, Ingo, can you please suggest a way to annotate this locking
> >> pattern to avoid lockdep false positive?
> >
> > No, because I haven't a clue what the actual problem is.
> >
> > The report is fairly clear on how:
> >
> >  routelock
> >    &port->buf.lock/1
> >      &o_tty->termios_rwsem/1
> >        &buf->lock
> >
> > gets established, and shows where we try:
> >
> >   &buf->lock
> >     routelock
> >
> > Which gives a circle, ergo deadlock.
> >
> > Show which link is wrong and why, and I can suggest ways of annotating
> > that.
> 
> 
> Alan provided an explanation calling this "false report":
> https://groups.google.com/d/msg/syzkaller/YrV0bzdfa-g/cCVoUf1OFQAJ

I read that, I didn't understand it. Which link is wrong and why?

> And I don't understand how the following is a deadlock, since there is
> no cycle...
> 
>  Possible unsafe locking scenario:
>       CPU0                    CPU1
>        ----                    ----
>   lock(&buf->lock);
>                                lock(&o_tty->termios_rwsem/1);
>                                lock(&buf->lock);
>   lock(routelock);

Ignore the stupid picture, it only really works for simple cases.