linux-kernel - Re: lockdep problem conversion semaphore->mutex (dev->sem)

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Message-Id: <1197133910.6353.33.camel@lappy>
Date:	Sat, 08 Dec 2007 18:11:50 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	Remy Bohmer <linux@...mer.net>, Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dave Chinner <dgc@....com>
Subject: Re: lockdep problem conversion semaphore->mutex (dev->sem)


On Sat, 2007-12-08 at 08:53 -0800, Daniel Walker wrote:
> On Sat, 2007-12-08 at 13:16 +0100, Peter Zijlstra wrote:
> > On Sat, 2007-12-08 at 00:02 +0100, Remy Bohmer wrote:
> > > Hello Peter,
> > > 
> > > > > What specifically is wrong with dev->sem ?
> > > >
> > > > Nothing really, other than that they use semaphores to avoid lockdep :-/
> > > >
> > > > I think I know how to annotate this, after Alan Stern explained all the
> > > > use cases, but I haven't come around to implementing it. Hope to do that
> > > > soonish.
> > > 
> > > I was looking for an easy semaphore I could convert to a mutex, and I
> > > ran into one that was widely spread and interesting, and which seemed
> > > quite doable at first sight.
> > > So, I started working on it, but was forgotten this discussion, (until
> > > Daniel made me remember it this afternoon). So, I (stupid me ;-) )
> > > tried to convert dev->sem...
> > > 
> > > After doing the monkey part of the conversion I can boot the kernel
> > > completely on X86 and ARM, and everything works fine, except after
> > > enabling lockdep, lockdep starts complaining...
> > > 
> > > Is this the problem you were pointing at?
> > 
> > Yeah, one of the interesting nestings :-)
> 
> It must be the locking in __driver_attach(), taking dev->parent->sem
> then taking dev->sem .. Assuming those are different structures, why
> does lockdep trigger?

They aren't different, parent is a struct device again.

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