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

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Message-ID: <48ECD9C8.4000700@redhat.com>
Date:	Wed, 08 Oct 2008 12:03:20 -0400
From:	Steven Rostedt <srostedt@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	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
Subject: Re: [RFC][PATCH 7/7] lockdep: spin_lock_nest_lock()

Linus Torvalds wrote:
> On Wed, 8 Oct 2008, Steven Rostedt wrote:
>   
>>> 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.
>>     
>
> No it is not.
>
> Unlocking locks in non-nested order can sometimes be very much the rigth 
> thing to do, and thinking otherwise is (a) naive and (b) can generate 
> totally unnecessary and pointless bugs.
>
> The thing is, sometimes you have to do it, and imposing totally made-up 
> rules ("unlocks have to nest") just confuses everybody.
>
> The FACT is, that unlocks do not have to nest cleanly. That's a rock solid 
> *FACT*. The locking order matters, and the unlocking order does not.
>
> And if you cannot accept that as a fact, and you then say "unlock order 
> should matter just to keep things nice and clean", then you end up being 
> screwed and/or confused when you can't hold to the unlock order.
>
> There are many perfectly valid reasons not to unlock in reverse order. 
> Don't create make-believe rules that break those reasons for no gain.
>   

Unfortunately, you cut out my comment that I stated "unless there is a 
good reason not to",
which the below example is a good reason ;-)
> For example:
>  - let's say that you have a singly-linked list of objects.
>  - you need to lock all objects, do something, and then unlock all 
>    objects.
>  - the *only* sane way to do that is to just traverse the list twice.
>  - that means that you will unlock the objects in the same order you 
>    locked them, _not_ in reverse ("nested") order.
>  - if you force a rule of "unlocks must be nested", then
>
> 	YOU ARE A F*CKING MORON.
>
> It's that simple. Don't do made-up rules that have no technical reason for 
> them. 
>
> Lock ordering matters. Unlock ordering does not. It really is that simple. 
> Don't confuse the issue by claiming anything else.
>   

Yes, I totally agree that there are good reasons not to unlock in 
reverse order, and the example
you gave happens to be one of them.

I just find that seeing something like:

    lock(A);
    lock(B);

    [do something]

    unlock(A);
    unlock(B);

just seems to be sloppy.

I wont harp on this, it only came up in conversation in which someone 
pointed out your
post.

-- Steve

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