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

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Date:	Sun, 17 Jan 2010 09:18:34 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Ming Lei <tom.leiming@...il.com>
Cc:	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,
	Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH] sysfs: Add lockdep annotations for the sysfs active reference

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.

Eric
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