[Issue 16352] dead-lock in std.allocator.free_list unittest

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16352

--- Comment #9 from safety0ff.bugz <safety0ff.bugz at gmail.com> ---
(In reply to Rainer Schuetze from comment #8)
> 
> I agree. The actual pattern to use depends on the hardware, but x86 usually
> uses a modification counter modified in lock step.

I'm just going to slap core.internal.spinlock on it for now.
Somebody else can improve it later.
I just don't want the autotester choking on unrelated changes.

There's also the issue on x86_64 that we can't use the upper bits (because
ParentAllocator could be GCAllocator,) and not all x86_64 machines have
cmpxchg16b.

AFAIK shared free lists aren't very good for high contention regardless.

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