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

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Date:   Sat, 5 May 2018 10:47:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.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

On Sat, May 05, 2018 at 10:11:00AM +0200, Ingo Molnar wrote:

> Before:
> 
>  #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 /* atomic_fetch_dec */
>  #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 /* atomic_fetch_dec */
> 
>  #else /* atomic_fetch_dec_relaxed */
> 
>  #ifndef atomic_fetch_dec_acquire
>  #define atomic_fetch_dec_acquire(...)					\
> 	__atomic_op_acquire(atomic_fetch_dec, __VA_ARGS__)
>  #endif
> 
>  #ifndef atomic_fetch_dec_release
>  #define atomic_fetch_dec_release(...)					\
> 	__atomic_op_release(atomic_fetch_dec, __VA_ARGS__)
>  #endif
> 
>  #ifndef atomic_fetch_dec
>  #define atomic_fetch_dec(...)						\
> 	__atomic_op_fence(atomic_fetch_dec, __VA_ARGS__)
>  #endif
>  #endif /* atomic_fetch_dec_relaxed */
> 
> After:
> 
>  #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_acquire
>  #  define atomic_fetch_dec_acquire(...)	__atomic_op_acquire(atomic_fetch_dec, __VA_ARGS__)
>  # endif
>  # ifndef atomic_fetch_dec_release
>  #  define atomic_fetch_dec_release(...)	__atomic_op_release(atomic_fetch_dec, __VA_ARGS__)
>  # endif
>  # ifndef atomic_fetch_dec
>  #  define atomic_fetch_dec(...)		__atomic_op_fence(atomic_fetch_dec, __VA_ARGS__)
>  # endif
>  #endif
> 
> The new variant is readable at a glance, and the hierarchy of defines is very 
> obvious as well.

It wraps and looks hideous in my normal setup. And I do detest that indent
after # thing.

> And I think we could do even better - there's absolutely no reason why _every_ 
> operation has to be made conditional on a finegrained level - they are overriden 
> in API groups. In fact allowing individual override is arguably a fragility.
> 
> 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.

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.

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.

So no, don't like.