From 772c7ae7e140ebd3896fc7786aae8b43ab3128c3 Mon Sep 17 00:00:00 2001
From: qiulaidongfeng <2645477756@qq.com>
Date: Sat, 12 Aug 2023 18:06:08 +0800
Subject: [PATCH] sync: document why copyChecker checks the condition twice

Fixes #40924
Change-Id: I249a278be1ec3c67088819af4456e6c393431724
---
 src/sync/cond.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/sync/cond.go b/src/sync/cond.go
index cc927adf58..7ef3188a1e 100644
--- a/src/sync/cond.go
+++ b/src/sync/cond.go
@@ -96,6 +96,10 @@ func (c *Cond) Broadcast() {
 type copyChecker uintptr
 
 func (c *copyChecker) check() {
+	// Check if c has been copied in three steps:
+	// 1. The first comparison is the fast-path. If c has been initialized and not copied, this will return immediately. Otherwise, c is either not initialized, or has been copied.
+	// 2. Ensure c is initialized. If the CAS succeeds, we're done. If it fails, c was either initialized concurrently and we simply lost the race, or c has been copied.
+	// 3. Do step 1 again. Now that c is definitely initialized, if this fails, c was copied.
 	if uintptr(*c) != uintptr(unsafe.Pointer(c)) &&
 		!atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) &&
 		uintptr(*c) != uintptr(unsafe.Pointer(c)) {