linux-kernel - Re: [PATCH] locking/atomics: Clean up the atomic.h maze of #defines

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Date:   Sat, 5 May 2018 11:04:03 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        aryabinin@...tuozzo.com, boqun.feng@...il.com,
        catalin.marinas@....com, dvyukov@...gle.com, will.deacon@....com
Subject: Re: [PATCH] locking/atomics: Clean up the atomic.h maze of #defines


* Peter Zijlstra <peterz@...radead.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 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.

> 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.

Thanks,

	Ingo