Messages in this thread | | | Date | Thu, 29 Jul 2021 13:15:07 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH v3] locking/atomic: Implement atomic{,64,_long}_{fetch_,}{andnot_or}{,_relaxed,_acquire,_release}() |
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On Thu, Jul 29, 2021 at 10:55:52AM +0100, Will Deacon wrote:
> Overall, I'm not thrilled to bits by extending the atomics API with > operations that cannot be implemented efficiently on any (?) architectures > and are only used by the qspinlock slowpath on machines with more than 16K > CPUs.
My rationale for proposing this primitive is similar to the existence of other composite atomic ops from the Misc (and refcount) class (as per atomic_t.txt). They're common/performance sensitive operations that, on LL/SC platforms, can be better implemented than a cmpxchg() loop.
Specifically here, it can be used to implement short xchg() in an architecturally neutral way, but more importantly it provides fwd progress on LL/SC, while most LL/SC based cmpxchg() implementations are arguably broken there.
People seem to really struggle to implement that sanely.
It's such a shame we can't have the compiler generate sane composite atomics for us..
> I also think we're lacking documentation justifying when you would use this > new primitive over e.g. a sub-word WRITE_ONCE() on architectures that > support those, especially for the non-returning variants.
Given the sub-word ordering 'fun', this might come in handy somewhere :-) But yes, it's existence is more of a completeness/symmetry argument than anything else.
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