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)) {