netdev - Re: [LOCKDEP BUG][2.6.36-rc1] xt_info

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Message-ID: <1281988895.1926.1945.camel@laptop>
Date:	Mon, 16 Aug 2010 22:01:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, netdev@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Patrick McHardy <kaber@...sh.net>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [LOCKDEP BUG][2.6.36-rc1] xt_info_wrlock?

On Mon, 2010-08-16 at 21:35 +0200, Eric Dumazet wrote:
> Le lundi 16 août 2010 à 21:16 +0200, Peter Zijlstra a écrit :
> 
> > Adding lockdep_off() is just plain wrong, if you cannot describe the
> > locking there's a fair chance its wrong anyway.
> > 
> 
> I see.
> 
> I described the fine locking after Steven comment, adding a long
> Changelog.
> 
> http://patchwork.ozlabs.org/patch/61827/
> 
> If someone thinks this locking is buggy, please speak now ;)

Urgh,.. I think it might be correct, but wtf! Wasn't this originally RCU
code, why not go back to using RCU now that we have
synchronize_rcu_expedited()?

As to the original issue, why not keep that bh stuff disabled for
CONFIG_PROVE_LOCKING instead, that will at least let you keep lock
coverage, adding lockdep_off() will hide any cycles that would involve
this lock (even though its currently a leaf lock, you never know what
creative things the future brings).

This fancy open coded lock looks like utter fail for -rt though.. please
use RCU if at all possible.

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