linux-kernel - Re: [RFC][PATCH 7/7] lockdep: spin_lock_nest

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Date:	Wed, 8 Oct 2008 11:27:08 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	David Miller <davem@...emloft.net>, hugh@...itas.com,
	mingo@...e.hu, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, davej@...hat.com, srostedt@...hat.com
Subject: Re: [RFC][PATCH 7/7] lockdep: spin_lock_nest_lock()

On Mon, Aug 04, 2008 at 12:31:22PM -0700, Linus Torvalds wrote:
> 
> 
> On Mon, 4 Aug 2008, Jeremy Fitzhardinge wrote:
> > 
> > OK.  I don't actually need to do this, but I was asking for completeness.  But
> > to clarify, you only need to do the reverse unlock if you do it after
> > unlocking the outer lock?  If you're still holding the outer lock, you can
> > unlock in any order?
> 
> Release order should always be totally irrelevant, whether you hold outer 
> locks or not. Only the order of _getting_ locks matter.

Technically, you are 100% correct.

> 
> And yes, if there is an outer lock, even the order of getting locks is 
> irrelevant, as long as anybody who gets more than one inner lock always 
> holds the outer one.

But I need to disagree on a programming practice style.  Unlocking locks
in a non nested order is just bad programming practice. Unless there is
a good reason to do so, one should never release locks in a non reverse
order they were taken.

This can be a source of bugs, where people might notice an outer lock
being released and think the inner locks were too.

Lately the kernel has been going through a lot of clean ups that have
been making the kernel a much more maintainable beast. I feel we should
enforce the rule of unlocking order (again, unless there is a good
reason not to). Not for a technical reason, but just for a more
maintainable one.

-- Steve

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