sync/atomic: use consistent first-store-in-progress marker

We need to use the same marker everywhere. My CL to rename the
marker (CL 241661) and the CL to add more uses of the marker
under the old name (CL 241678) weren't coordinated with each other.

Fixes #52612

Change-Id: I97023c0769e518491924ef457fe03bf64a2cefa6
Reviewed-on: https://go-review.googlesource.com/c/go/+/403094
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

src/sync/atomic/value.go

@@ -101,7 +101,7 @@ func (v *Value) Swap(new any) (old any) {
 			// active spin wait to wait for completion; and so that
 			// GC does not see the fake type accidentally.
 			runtime_procPin()
-			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
+			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(&firstStoreInProgress)) {
 				runtime_procUnpin()
 				continue
 			}
@@ -111,7 +111,7 @@ func (v *Value) Swap(new any) (old any) {
 			runtime_procUnpin()
 			return nil
 		}
-		if uintptr(typ) == ^uintptr(0) {
+		if typ == unsafe.Pointer(&firstStoreInProgress) {
 			// First store in progress. Wait.
 			// Since we disable preemption around the first store,
 			// we can wait with active spinning.
@@ -153,7 +153,7 @@ func (v *Value) CompareAndSwap(old, new any) (swapped bool) {
 			// active spin wait to wait for completion; and so that
 			// GC does not see the fake type accidentally.
 			runtime_procPin()
-			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
+			if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(&firstStoreInProgress)) {
 				runtime_procUnpin()
 				continue
 			}
@@ -163,7 +163,7 @@ func (v *Value) CompareAndSwap(old, new any) (swapped bool) {
 			runtime_procUnpin()
 			return true
 		}
-		if uintptr(typ) == ^uintptr(0) {
+		if typ == unsafe.Pointer(&firstStoreInProgress) {
 			// First store in progress. Wait.
 			// Since we disable preemption around the first store,
 			// we can wait with active spinning.

test/fixedbugs/issue52612.go

@@ -0,0 +1,49 @@
+// run
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"sync/atomic"
+	"unsafe"
+)
+
+var one interface{} = 1
+
+type eface struct {
+	typ  unsafe.Pointer
+	data unsafe.Pointer
+}
+
+func f(c chan struct{}) {
+	var x atomic.Value
+
+	go func() {
+		x.Swap(one) // writing using the old marker
+	}()
+	for i := 0; i < 100000; i++ {
+		v := x.Load() // reading using the new marker
+
+		p := (*eface)(unsafe.Pointer(&v)).typ
+		if uintptr(p) == ^uintptr(0) {
+			// We read the old marker, which the new reader
+			// doesn't know is a case where it should retry
+			// instead of returning it.
+			panic("bad typ field")
+		}
+	}
+	c <- struct{}{}
+}
+
+func main() {
+	c := make(chan struct{}, 10)
+	for i := 0; i < 10; i++ {
+		go f(c)
+	}
+	for i := 0; i < 10; i++ {
+		<-c
+	}
+}