linux-kernel - Re: [Patch 0/2] sysfs: fix s

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Date:	Fri, 5 Feb 2010 10:30:58 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg KH <gregkh@...e.de>, Thomas Gleixner <tglx@...utronix.de>,
	Cong Wang <amwang@...hat.com>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>, Miles Lane <miles.lane@...il.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Larry Finger <Larry.Finger@...inger.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [Patch 0/2] sysfs: fix s_active lockdep warning

On Fri, 5 Feb 2010, Peter Zijlstra wrote:

> Right, so this device stuff is much more complicated than I was led to
> believe ;-)

Haven't I told you all along that tree-structured locking is
complicated?  :-)

> So the device core doesn't know, so how are you guys making sure there
> really are no deadlocks hidden in there somewhere?

In the code I've seen, deadlocks are avoided by always taking the locks
in the same order.  But who knows?  Maybe there _are_ some hidden
deadlocks lurking.  For now we can't rely on lockdep to find them,
though, because it gets sidetracked by all the false positives.

> >  But for now perhaps a compromise is in
> > order.  We could make the switch from semaphores to mutexes while
> > avoiding lockdep issues by assigning the device mutexes to a
> > "don't-verify" class.  Is there such a thing, or could it be added?
> 
> Something like the below might work, but it should go along with a
> checkpatch.pl mod to ensure we don't grow any new users (just don't feel
> like brushing up my perl fu enough to actually make sense of that
> script)

I'll try it out in the next few days, and if it looks good maybe the 
checkpatch maintainers can lend some assistance before it gets 
submitted.

Alan Stern

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