linux-kernel - Re: [PATCH] sysfs: Add lockdep annotations for the sysfs active reference

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Date:	Sun, 17 Jan 2010 19:03:51 +0100
From:	Dominik Brodowski <linux@...inikbrodowski.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Ming Lei <tom.leiming@...il.com>, Greg KH <greg@...ah.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Borislav Petkov <petkovbb@...glemail.com>,
	David Airlie <airlied@...ux.ie>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...IV.linux.org.uk>, Tejun Heo <tj@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	linux-pcmcia@...ts.infradead.org
Subject: Re: [PATCH] sysfs: Add lockdep annotations for the sysfs active reference

Hey,

On Sun, Jan 17, 2010 at 09:18:34AM -0800, Eric W. Biederman wrote:
> Ming Lei <tom.leiming@...il.com> writes:
> 
> > On Sat, 02 Jan 2010 13:37:12 -0800
> > ebiederm@...ssion.com (Eric W. Biederman) wrote:
> >
> >> 
> >> Holding locks over device_del -> kobject_del -> sysfs_deactivate can
> >> cause deadlocks if those same locks are grabbed in sysfs show or store
> >> methods.
> >> 
> >> The I model s_active count + completion as a sleeping read/write lock.
> >> I describe to lockdep sysfs_get_active as a read_trylock,
> >> sysfs_put_active as a read_unlock, and sysfs_deactivate as a
> >> write_lock and write_unlock pair.  This seems to capture the essence
> >> for purposes of finding deadlocks, and in my testing gives finds real
> >> issues and ignores non-issues.
> >> 
> >> This brings us back to holding locks over kobject_del is a problem
> >> that ideally we should find a way of addressing, but at least lockdep
> >> can tell us about the problems instead of requiring developers to
> >> debug rare strange system deadlocks, that happen when sysfs files are
> >> removed while being written to.
> >
> > The model has hit a possible deadlock in pcmcia, and the lockdep warning
> > comes when I unplug my wlan card from pcmcia slot.
> >
> > Looks like socket->skt_mutex is held in remove path, and it is also
> > grabbed in .stor method.
> 
> 
> Looking a little closer this is simultaneously a legitimate problem
> and also a false positive.
> 
> This is only legitimate if you add/remove a cardbus bridge, plugged into
> another cardbus bridge, which I think is unlikely but physically possible.

Unfortunately, it is not a false positive, as removing a PCMCIA device
racing with "pccardctl eject" seems to trigger this path as well. Patch is
being prepared...

Best,
	Dominik
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