linux-kernel - Re: Lock dependency based tree report in perf lock

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Date:	Mon, 01 Feb 2010 09:25:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Hitoshi Mitake <mitake@....info.waseda.ac.jp>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Lock dependency based tree report in perf lock

On Sat, 2010-01-30 at 19:57 +0100, Frederic Weisbecker wrote:
> On Sat, Jan 30, 2010 at 09:46:28AM +0100, Peter Zijlstra wrote:
> > On Sat, 2010-01-30 at 00:17 +0100, Frederic Weisbecker wrote:
> > > 
> > > 
> > > Anyway, that's just an idea, not trivial I must admit.
> > 
> > lockdep actually collects all this information, so writing it out isn't
> > too hard.
> 
> 
> 
> Lockdep collects the theorical dependencies but not the practical
> scenarios.
> 
> Say B and C depend on A, you'll get:
> 
>    A
>   / \
>  B   C
> 
> But nothing can tell you that if A is taken, B and C will always
> be taken. You may have different scenarios based on this dependency,
> which is not something that lockdep logs, right?

Right. But we keep track of the full held lock stack, which is what was
requested.

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