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

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Message-ID: <20160120114430.GA25010@twins.programming.kicks-ass.net>
Date:	Wed, 20 Jan 2016 12:44:30 +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 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.