https://git.ruby-lang.org/ruby.git/commit/?id=d2aaf865e3

From d2aaf865e3e5ecfcd10c879ce4263dc3a8f7fccf Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@r...>
Date: Mon, 11 Jan 2021 11:31:52 +0900
Subject: [ruby/etc] Added fallback definition of RUBY_ATOMIC_EXCHANGE

https://github.com/ruby/etc/commit/0c5900bd21

diff --git a/ext/etc/etc.c b/ext/etc/etc.c
index 44273ff..50f566d 100644
--- a/ext/etc/etc.c
+++ b/ext/etc/etc.c
@@ -68,6 +68,15 @@ void rb_deprecate_constant(VALUE mod, const char *name); https://github.com/ruby/ruby/blob/trunk/ext/etc/etc.c#L68
 typedef int rb_atomic_t;
 # define RUBY_ATOMIC_CAS(var, oldval, newval) \
     ((var) == (oldval) ? ((var) = (newval), (oldval)) : (var))
+# define RUBY_ATOMIC_EXCHANGE(var, newval) \
+    atomic_exchange(&var, newval)
+static inline rb_atomic_t
+atomic_exchange(volatile rb_atomic_t *var, rb_atomic_t newval)
+{
+    rb_atomic_t oldval = *var;
+    *var = newval;
+    return oldval;
+}
 #endif
 
 /* call-seq:
-- 
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