Messages in this thread | | | From | Nick Piggin <> | Subject | Re: [RFC][PATCH 7/7] lockdep: spin_lock_nest_lock() | Date | Thu, 9 Oct 2008 02:52:14 +1100 |
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On Thursday 09 October 2008 02:27, Steven Rostedt wrote: > 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.
An outer one might be more likely to be contended, so you might want to release it asap.
Other times, you have lock A and lock B held (like scheduler rqs). You can say unlock(A); unlock(B); or if (A < B) unlock(B); unlock(A); if (A > B) unlock (B);
> 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.
I don't really think it would make things more maintainable, FWIW.
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