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Date: Wed, 20 Jan 2016 14:58:43 +0000
From: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>, Ingo Molnar <mingo@redhat.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
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> 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.

There are two line disciplines using two different locking orders

The two line disciplines never execute at once. A given tty is either
using one or the other and there is a clear and correctly locked
changeover.


semantically its something a bit like


 foo(x)
 {
          if (x == 1) {
		lock(A)
		lock(B)
	  } else {
		lock(B)
		lock(A)
	  }

	  Do stuff();

	  if (x == 1) {
		unlock(B)
		unlock(A)
	  } else {
		unlock(A)
		unlock(B)
	  }
}

with the guarantee made elsewhere that no instances of foo(1) and foo(0)
are ever executing at the same time.

That's not by dumb design - it's an interesting "nobody ever noticed
this" turned up by the lock detector between two totaly unrelated bits of
code.

Alan