linux-kernel - Re: dquot.c: possible circular locking Re: [2.6.20] BUG: workqueue leaked lock

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Date:	Tue, 20 Mar 2007 14:44:46 +0100
From:	Jarek Poplawski <jarkao2@...pl>
To:	Jan Kara <jack@...e.cz>
Cc:	Folkert van Heusden <folkert@...heusden.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...sign.ru>,
	Neil Brown <neilb@...e.de>, Christoph Hellwig <hch@....de>
Subject: Re: dquot.c: possible circular locking Re: [2.6.20] BUG: workqueue leaked lock

On Tue, Mar 20, 2007 at 01:19:09PM +0100, Jan Kara wrote:
> On Tue 20-03-07 12:31:51, Jarek Poplawski wrote:
> > On Tue, Mar 20, 2007 at 12:22:53PM +0100, Jarek Poplawski wrote:
> > > On Tue, Mar 20, 2007 at 12:17:01PM +0100, Jarek Poplawski wrote:
> > > ...
> > > > IMHO lockdep found that two locks are taken in different order:
> > > > 
> > > > -> #1: 1) tty_mutex in con_console() 2) dqptr_sem (somewhere later)
> > > > -> #0: 1) dqptr_sem 2) tty_console in dquot_alloc_space() with print_warning()
> > 
> > Once more - should be:
> >  -> #1: 1) tty_mutex in con_close() 2) dqptr_sem (somewhere later)
> >  -> #0: 1) dqptr_sem 2) tty_mutex in dquot_alloc_space() with print_warning()
>   Yes, I was looking at it. Hmm, we can possibly get rid of tty_mutex being
> acquired under dqptr_sem in quota code. But looking at the path from
> con_close() there's another inversion with i_mutex which is also acquired
> along the path for sysfs. And we can hardly get rid of it in the quota code.
>   Now none of these is a real deadlock as quota should never call
> print_warning() for sysfs (it doesn't use quota) but still it's nasty. I
> suppose tty_mutex is above i_mutex because of those sysfs calls and it
> seems sysfs must be called under tty_mutex because of races with
> init_dev(). So it's not easy to get rid of that dependency either.

I wonder if this cannot be done with a workqueue (message to a buffer,
maybe after try_lock on tty_mutex) and BTW isn't there any "modern"
way to queue console messages like these?

Jarek P.
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