linux-kernel - Re: Need lockdep help

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Message-ID: <Pine.LNX.4.44L0.0712041424270.13516-100000@iolanthe.rowland.org>
Date:	Tue, 4 Dec 2007 14:28:31 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Jarek Poplawski <jarkao2@...il.com>
cc:	Jarek Poplawski <jarkao2@...pl>, Ingo Molnar <mingo@...e.hu>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Need lockdep help

On Tue, 4 Dec 2007, Jarek Poplawski wrote:

> Alan Stern wrote, On 12/04/2007 04:17 PM:
> ...
> 
> > Furthermore, in this case deadlock isn't really impossible -- it could 
> > occur if there were a bug somewhere else in the kernel.  So lockdep was 
> > correct to warn that deadlock might occur.
> 
> 
> Alan, if the scenario was like you described at the beginning, there was
> no deadlock possible, unless some errors in the notifier.

Or errors in the notifier's caller.

> These #1-#3
> threads were only helpful to guess what lockdep could 'think', but I
> guess notifier doesn't use 2 rivaling threads for a wake, so, lockdep
> probably needed additional information. And you really can't consider
> any hypothetical kernel bugs here because then each lock is vulnerable.

But you have to consider hypothetical kernel bugs.  That's exactly what 
lockdep is for -- to warn you about possible deadlocks that could be 
caused by bugs.

As a simple example, if thread #1 does "lock(A); lock(B)" and thread 
#2 does "lock(B); lock(A)" then there's a possible bug.  Lockdep should 
warn about you, and it does -- even if those two threads can never run 
at the same time.

If lockdep warned about deadlocks only when they actually happened, it 
wouldn't be nearly so useful.

Alan Stern

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