linux-kernel - Re: hrtimers: simplify lockdep handling

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Date:	Wed, 25 Jun 2008 15:46:37 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Oleg Nesterov <oleg@...sign.ru>
cc:	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: hrtimers: simplify lockdep handling

Hi Oleg,

I'm currently porting -rt to 26-rc7 and I came across this change:

Commit:     8e60e05fdc7344415fa69a3883b11f65db967b47

With the

-	double_spin_lock(&new_base->lock, &old_base->lock,
-			 smp_processor_id() < cpu);
+	spin_lock(&new_base->lock);
+	spin_lock_nested(&old_base->lock, SINGLE_DEPTH_NESTING);

What's the reason that this is possible?  Is it because the migration
happens only on CPU hotplugging and that the CPU hotplugging code has
locks that would prevent a reversal of the lock taking?

I'm not arguing that the code is incorrect, but this looks like a subtlety
that can bite us later.

In other words, we really need comments around this code to explain to
casual viewers why this code is not deadlock prone. The change log here
and for 0d180406f2914aea3a78ddb880e2fe9ac78a9372 does not explain why the
straight forward taking of the locks is OK.

Thanks,

-- Steve
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