Re: [RFC PATCH v3] locking/atomic: Implement atomic{,64,_long}_{fetch_,}{andnot_or}{,_relaxed,_acquire,_release}()

[Peter Zijlstra <peterz@infradead.org>]

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.