Re: tty: deadlock between n_tracerouter_receivebuf and flush_to_ldisc

From	Dmitry Vyukov <dvyukov@google.com>
Newsgroups	linux.kernel
Subject	Re: tty: deadlock between n_tracerouter_receivebuf and flush_to_ldisc
Date	2016-01-20 13:00 +0100
Message-ID	<qSZAD-5Pv-15@gated-at.bofh.it> (permalink)
References	(1 earlier) <qR7sC-2Y8-13@gated-at.bofh.it> <qRfzR-gk-31@gated-at.bofh.it> <qRgme-OC-7@gated-at.bofh.it> <qSXp9-4uM-25@gated-at.bofh.it> <qSZqV-5Mi-9@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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


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);


Shouldn't it look like:

 Possible unsafe locking scenario:
      CPU0                    CPU1
       ----                    ----
  lock(&buf->lock);
                               lock(routelock);
                               lock(&buf->lock);
  lock(routelock);

?

Thread

Re: tty: deadlock between n_tracerouter_receivebuf and  flush_to_ldisc One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk> - 2016-01-15 17:40 +0100
  Re: tty: deadlock between n_tracerouter_receivebuf and flush_to_ldisc Dmitry Vyukov <dvyukov@google.com> - 2016-01-15 18:30 +0100
    Re: tty: deadlock between n_tracerouter_receivebuf and flush_to_ldisc Dmitry Vyukov <dvyukov@google.com> - 2016-01-20 10:40 +0100
      Re: tty: deadlock between n_tracerouter_receivebuf and flush_to_ldisc Peter Zijlstra <peterz@infradead.org> - 2016-01-20 12:50 +0100
        Re: tty: deadlock between n_tracerouter_receivebuf and flush_to_ldisc Dmitry Vyukov <dvyukov@google.com> - 2016-01-20 13:00 +0100
          Re: tty: deadlock between n_tracerouter_receivebuf and flush_to_ldisc Peter Zijlstra <peterz@infradead.org> - 2016-01-20 13:10 +0100
            Re: tty: deadlock between n_tracerouter_receivebuf and  flush_to_ldisc One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk> - 2016-01-20 16:00 +0100
              Re: tty: deadlock between n_tracerouter_receivebuf and flush_to_ldisc Dmitry Vyukov <dvyukov@google.com> - 2016-01-20 16:20 +0100
                Re: tty: deadlock between n_tracerouter_receivebuf and flush_to_ldisc Peter Zijlstra <peterz@infradead.org> - 2016-01-20 17:40 +0100
  Re: tty: deadlock between n_tracerouter_receivebuf and  flush_to_ldisc J Freyensee <james_p_freyensee@linux.intel.com> - 2016-01-20 03:10 +0100

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