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

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Message-ID: <20150810134924.GB4606@fixme-laptop.cn.ibm.com>
Date:	Mon, 10 Aug 2015 21:49:24 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [Question] lockdep: Is nested lock handled correctly?

Hi Peter,

On Mon, Aug 10, 2015 at 01:42:28PM +0200, Peter Zijlstra wrote:
> 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);

Sorry for the typo here.. should be spin_lock_nest_lock().

> > 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.
> 

I admit that I didn't find this is horrible at first, but now I agree
with you, this is not a rational locking order. Thank you.

> Why would you ever want to do that?

Though I don't want to have a locking order like that either, we can't
stop others from using that order(maybe a good design review will) and
lockdep yells something -unrelated- in such an order.

I think we can either let lockdep complain if some one uses this
locking order or clean up current code a little bit to tolarent this.

If you really think we should do something about it, I can write the
patch and add test cases.

Thank you anyway.

Regards,
Boqun
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