Messages in this thread | | | Date | Thu, 5 Jan 2006 14:17:28 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [patch 00/21] mutex subsystem, -V14 |
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On Thu, 5 Jan 2006, Ingo Molnar wrote: > > [ long details removed ] > > to sum it up: atomic_dec/inc_return() alone is not enough to implement > critical sections, on a number of architectures. atomic_xchg() seems to > have similar problems too.
Yes.
> the patch below adds the smp_mb() barriers to the generic headers, which > should now fulfill all the ordering requirements, on every architecture. > It only relies on one property of the atomic primitives: that they wont > get reordered with respect to themselves, so an atomic_inc_ret() and an > atomic_dec_ret() cannot switch place. > > Can you see any hole in this reasoning?
No. The alternative is to just make the ordering requirements for "atomic_dec_return()" and "atomic_xchg()" be absolute. Say that they have to be full memory barriers, and push the problem into the low-level architecture.
I _think_ your patch is the right approach, because most architectures are likely to do their own fast-paths for mutexes, and as such the generic ones are more of a template for how to do it, and hopefilly aren't that performance critical.
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