linux-kernel - Re: [Question] lockdep: Is nested lock handled correctly?

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Message-ID: <20150810114228.GY16853@twins.programming.kicks-ass.net>
Date:	Mon, 10 Aug 2015 13:42:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [Question] lockdep: Is nested lock handled correctly?

On Mon, Aug 10, 2015 at 05:52:47PM +0800, Boqun Feng wrote:
> Hi Peter and Ingo,
> 
> I'm now learning the code of lockdep and find that nested lock may not
> be handled correctly because we fail to take held_lock merging into
> consideration. I come up with an example and hope that could explain my
> concern.
> 
> Please consider this lock/unlock sequence, I also put a patch ading this
> sequence as a test into locking-selftest:
> 
> (lock_X1 and lock_X2 belong to the same lock class X, lock_Y1 belongs to
> another lock class Y)
> 
> spin_lock(&lock_X1);
> spin_lock(&lock_Y1);
> spin_lock_nested_lock(&lock_X2, &lock_X1);
> spin_unlock(&lock_Y1);
> spin_unlock(&lock_X2);
> spin_unlock(&lock_X1);
> 
> 
> This is totally legal in current lockdep rules, right? 

Yuck, I'd say no. That's quite horrible.

Why would you ever want to do that?
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