https://git.ruby-lang.org/ruby.git/commit/?id=233054a609

From 233054a609a36cc441d26f9064bc3d16ff83b686 Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@r...>
Date: Thu, 7 Jul 2022 00:28:45 +0900
Subject: Cycle `obj_info_buffers_index` atomically

---
 gc.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/gc.c b/gc.c
index 80e7fb596e..f487cb88ed 100644
--- a/gc.c
+++ b/gc.c
@@ -13942,19 +13942,26 @@ rb_raw_obj_info(char *const buff, const size_t buff_size, VALUE obj) https://github.com/ruby/ruby/blob/trunk/gc.c#L13942
 #if RGENGC_OBJ_INFO
 #define OBJ_INFO_BUFFERS_NUM  10
 #define OBJ_INFO_BUFFERS_SIZE 0x100
-static int obj_info_buffers_index = 0;
+static rb_atomic_t obj_info_buffers_index = 0;
 static char obj_info_buffers[OBJ_INFO_BUFFERS_NUM][OBJ_INFO_BUFFERS_SIZE];
 
-static const char *
-obj_info(VALUE obj)
+static char *
+obj_info_next_buffer(void)
 {
-    const int index = obj_info_buffers_index++;
-    char *const buff = &obj_info_buffers[index][0];
+    const rb_atomic_t index = RUBY_ATOMIC_FETCH_ADD(obj_info_buffers_index, 1);
 
-    if (obj_info_buffers_index >= OBJ_INFO_BUFFERS_NUM) {
-	obj_info_buffers_index = 0;
+    rb_atomic_t next_index = (index + 1);
+    if (UNLIKELY(next_index >= OBJ_INFO_BUFFERS_NUM)) {
+        rb_atomic_t reset_index = next_index % OBJ_INFO_BUFFERS_NUM;
+        RUBY_ATOMIC_CAS(obj_info_buffers_index, next_index, reset_index);
     }
+    return obj_info_buffers[index % OBJ_INFO_BUFFERS_NUM];
+}
 
+static const char *
+obj_info(VALUE obj)
+{
+    char *const buff = obj_info_next_buffer();
     return rb_raw_obj_info(buff, OBJ_INFO_BUFFERS_SIZE, obj);
 }
 #else
-- 
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