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Date: Wed, 20 Jan 2016 13:07:52 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
        Ingo Molnar <mingo@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>, LKML <linux-kernel@vger.kernel.org>,
        J Freyensee <james_p_freyensee@linux.intel.com>,
        syzkaller <syzkaller@googlegroups.com>,
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        Alexander Potapenko <glider@google.com>,
        Sasha Levin <sasha.levin@oracle.com>,
        Eric Dumazet <edumazet@google.com>
Subject: Re: tty: deadlock between n_tracerouter_receivebuf and flush_to_ldisc
Message-ID: <20160120120752.GQ3421@worktop>
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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@infradead.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.