linux-kernel - Re: irda rmmod lockdep trace.

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Message-Id: <20070312.164921.71090866.davem@davemloft.net>
Date:	Mon, 12 Mar 2007 16:49:21 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	samuel@...tiz.org
Cc:	davej@...hat.com, linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: irda rmmod lockdep trace.

From: Samuel Ortiz <samuel@...tiz.org>
Date: Mon, 12 Mar 2007 02:38:43 +0200

> On Sat, Mar 10, 2007 at 07:43:26PM +0200, Samuel Ortiz wrote:
> > Hi Dave,
> > 
> > On Thu, Mar 08, 2007 at 05:54:36PM -0500, Dave Jones wrote:
> > > modprobe irda ; rmmod irda in 2.6.21rc3 gets me the spew below..
> > Well it seems that we call __irias_delete_object() from hashbin_delete(). Then
> > __irias_delete_object() calls itself hashbin_delete() again. We're trying to
> > get the lock recursively.
> Looking at the code more carefully, this seems to be a false positive:
> iriap_cleanup and and __irias_delete_object are taking 2 different locks from
> 2 different hashbin instances. The locks belong to the same lock class but
> they are hierarchically different. We need to tell the validator about it and
> the following patch does that. Comments are welcomed as I'm planning to push
> it to netdev soon:

I would strongly caution against adding any run-time overhead just to
cure a false lockdep warning.  Even adding a new function argument
is too much IMHO.

Make the cost show up for lockdep only, perhaps by putting each
hashbin lock into a seperate locking class?

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