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SubjectRe: [PATCH] locking/atomics: Clean up the atomic.h maze of #defines
On Sat, May 05, 2018 at 11:04:03AM +0200, Ingo Molnar wrote:
>
> * Peter Zijlstra <peterz@infradead.org> wrote:
>
> > > So we could do the following simplification on top of that:
> > >
> > > #ifndef atomic_fetch_dec_relaxed
> > > # ifndef atomic_fetch_dec
> > > # define atomic_fetch_dec(v) atomic_fetch_sub(1, (v))
> > > # define atomic_fetch_dec_relaxed(v) atomic_fetch_sub_relaxed(1, (v))
> > > # define atomic_fetch_dec_acquire(v) atomic_fetch_sub_acquire(1, (v))
> > > # define atomic_fetch_dec_release(v) atomic_fetch_sub_release(1, (v))
> > > # else
> > > # define atomic_fetch_dec_relaxed atomic_fetch_dec
> > > # define atomic_fetch_dec_acquire atomic_fetch_dec
> > > # define atomic_fetch_dec_release atomic_fetch_dec
> > > # endif
> > > #else
> > > # ifndef atomic_fetch_dec
> > > # define atomic_fetch_dec(...) __atomic_op_fence(atomic_fetch_dec, __VA_ARGS__)
> > > # define atomic_fetch_dec_acquire(...) __atomic_op_acquire(atomic_fetch_dec, __VA_ARGS__)
> > > # define atomic_fetch_dec_release(...) __atomic_op_release(atomic_fetch_dec, __VA_ARGS__)
> > > # endif
> > > #endif
> >
> > This would disallow an architecture to override just fetch_dec_release for
> > instance.
>
> Couldn't such a crazy arch just define _all_ the 3 APIs in this group?
> That's really a small price and makes the place pay the complexity
> price that does the weirdness...

I would expect the pattern where it can do all 'release' and/or all
'acquire' variants special but cannot use the __atomic_op_*() wrappery.

> > I don't think there currently is any architecture that does that, but the
> > intent was to allow it to override anything and only provide defaults where it
> > does not.
>
> I'd argue that if a new arch only defines one of these APIs that's probably a bug.
> If they absolutely want to do it, they still can - by defining all 3 APIs.
>
> So there's no loss in arch flexibility.

Ideally we'd generate the whole mess.. and then allowing these extra few
overrides is not a problem at all.

> > None of this takes away the giant trainwreck that is the annotated atomic stuff
> > though.
> >
> > And I seriously hate this one:
> >
> > ba1c9f83f633 ("locking/atomic/x86: Un-macro-ify atomic ops implementation")
> >
> > and will likely undo that the moment I need to change anything there.
>
> If it makes the code more readable then I don't object - the problem was that the
> instrumentation indirection made all that code much harder to follow.

Thing is, it is all the exact same loop, and bitrot mandates they drift
over time. When I cleaned up all the architectures I found plenty cases
where there were spurious differences between things.


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