linux-kernel - Re: Need lockdep help

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Message-ID: <4755B508.9020806@gmail.com>
Date:	Tue, 04 Dec 2007 21:14:00 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Alan Stern <stern@...land.harvard.edu>
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

Alan Stern wrote, On 12/04/2007 08:28 PM:

> On Tue, 4 Dec 2007, Jarek Poplawski wrote:

...

> 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.


Sure! I probably missed your point... Lockdep always names reported locks,
so I meant 'hypothetical' only trying to explain lockdep with some other,
unknown or unnamed bugs.

So, depending on the code, above example with A & B could be a real bug
(even if very improbable but logically justified) or a false alarm (eg.
when we know both threads could never work at the same).

Jarek P.
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