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Date: Wed, 20 Jan 2016 12:44:30 +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>,
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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
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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.