Re: [RFC][PATCH 7/7] lockdep: spin_lock_nest_lock()

Steven Rostedt
2008-10-08T15:27:20Z

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