From: Eric Wong <normalperson@...>
Date: 2018-05-18T18:10:35+00:00
Subject: [ruby-core:87175] Re: [Ruby trunk Feature#14767] [PATCH] gc.c: use monotonic counters for objspace_malloc_increase

nobu@ruby-lang.org wrote:
> ```C
>     size_t add = ATOMIC_SIZE_EXCHANGE(mc->add, 0);
>     size_t sub = ATOMIC_SIZE_EXCHANGE(mc->sub, 0);
> ```
> 
> Is this combination of two atomic operations atomic?

No, though I don't think it matters too much.
If we really care about atomicity, we can use pointers to
monoctr and swap those, instead.

	struct monoctr *active;
	struct monoctr a;
	struct monoctr b;

ATOMIC_PTR_EXCHANGE will keep ->active pointing to &a or &b
But I think I'll redo the approach to this patch entirely next
week...

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