linux-kernel - Re: [2.6.22] circular lock detected

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Date:	Mon, 3 Sep 2007 16:01:35 +0200
From:	Jan Kara <jack@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	folkert@...heusden.com, linux-kernel@...r.kernel.org,
	a.p.zijlstra@...llo.nl
Subject: Re: [2.6.22] circular lock detected

On Mon 03-09-07 05:49:59, Andrew Morton wrote:
> > On Mon, 3 Sep 2007 14:27:02 +0200 Jan Kara <jack@...e.cz> wrote:
> > > > On Fri, 24 Aug 2007 23:00:33 +0200 Folkert van Heusden <folkert@...heusden.com> wrote:
> > > > Hi,
> > > > 
> > > > 2.6.22 kernel with hyperthreading enabled only ext3 filesystems (2).
> > > > 
> > > > [  346.314640] =======================================================
> > > > [  346.314758] [ INFO: possible circular locking dependency detected ]
> > > > [  346.314815] 2.6.22 #5
> > > > [  346.314862] -------------------------------------------------------
> > > > [  346.314920] tor/2421 is trying to acquire lock:
> > > > [  346.314973]  (tty_mutex){--..}, at: [<c121aa83>] mutex_lock+0x8/0xa
> > > > [  346.315193] 
> > > > [  346.315195] but task is already holding lock:
> > > > [  346.316203]  (&s->s_dquot.dqptr_sem){----}, at: [<c10a485f>] dquot_alloc_space+0x50/0x189
> > > > [  346.316457] 
> > > > [  346.316458] which lock already depends on the new lock.
> >
> > ...
> >
> > > > [  346.353603]  =======================
> > > > 
> > > Has been reported before, but I don't recall whether we fixed it.  Jan,
> > > do you know>?
> >   I think we at least found a solution: Teach lockdep that I_MUTEX for
> > different filesystems is different. Peter Zilstra wrote a patch for that
> > and Folkert even confirmed that it fixes the problem for him. I'm not
> > sure what happened with the patch afterwards though. Adding Peter to CC
> > :).
> 
> But this is a tty_lock-versus-dqptr_sem ranking error.  Unrelated to i_mutex?
  The final report is for this ranking but the locking chain (if I understand it
right) is:
tty_mutex (con_close) -> i_mutex (sysfs: remove_subdir)
i_mutex (do_truncate) -> i_alloc_sem (notify_change) -> truncate_mutex (ext3_truncate)
truncate_mutex (ext3_get_blocks_handle) -> dqptr_sem (dquot_alloc_space)

So it complains about tty_mutex vs dqptr_sem (I don't know why it does not
complain about tty_mutex vs i_mutex) but the wrong link in the chain is
that i_mutex from remove_subdir() [sysfs] and i_mutex from do_truncate()
[ext3] are different and should never depend on each other...

									Honza

-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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