runtime: catch concurrent stacks more often

If two goroutines are racing on a map, one of them will exit
cleanly, clearing the hashWriting bit, and the other will
likely notice and panic.  If we use XOR instead of OR to
set the bit in the first place, even numbers of racers will
hopefully all see the bit cleared and panic simultaneously,
giving the full set of available stacks.  If a third racer
sneaks in, we are no worse than the current code, and
the generated code should be no more expensive.

In practice, this catches most racing goroutines even in
very tight races.  See the demonstration program posted
on https://github.com/golang/go/issues/26703 for an example.

Fixes #26703

Change-Id: Idad17841a3127c24bd0a659b754734f70e307434
Reviewed-on: https://go-review.googlesource.com/126936
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/map.go

@@ -567,7 +567,7 @@ func mapassign(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
 
 	// Set hashWriting after calling alg.hash, since alg.hash may panic,
 	// in which case we have not actually done a write.
-	h.flags |= hashWriting
+	h.flags ^= hashWriting
 
 	if h.buckets == nil {
 		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -679,7 +679,7 @@ func mapdelete(t *maptype, h *hmap, key unsafe.Pointer) {
 
 	// Set hashWriting after calling alg.hash, since alg.hash may panic,
 	// in which case we have not actually done a write (delete).
-	h.flags |= hashWriting
+	h.flags ^= hashWriting
 
 	bucket := hash & bucketMask(h.B)
 	if h.growing() {
@@ -921,7 +921,7 @@ func mapclear(t *maptype, h *hmap) {
 		throw("concurrent map writes")
 	}
 
-	h.flags |= hashWriting
+	h.flags ^= hashWriting
 
 	h.flags &^= sameSizeGrow
 	h.oldbuckets = nil

src/runtime/map_fast32.go

@@ -103,7 +103,7 @@ func mapassign_fast32(t *maptype, h *hmap, key uint32) unsafe.Pointer {
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapassign.
-	h.flags |= hashWriting
+	h.flags ^= hashWriting
 
 	if h.buckets == nil {
 		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -189,7 +189,7 @@ func mapassign_fast32ptr(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapassign.
-	h.flags |= hashWriting
+	h.flags ^= hashWriting
 
 	if h.buckets == nil {
 		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -276,7 +276,7 @@ func mapdelete_fast32(t *maptype, h *hmap, key uint32) {
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapdelete
-	h.flags |= hashWriting
+	h.flags ^= hashWriting
 
 	bucket := hash & bucketMask(h.B)
 	if h.growing() {

src/runtime/map_fast64.go

@@ -103,7 +103,7 @@ func mapassign_fast64(t *maptype, h *hmap, key uint64) unsafe.Pointer {
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapassign.
-	h.flags |= hashWriting
+	h.flags ^= hashWriting
 
 	if h.buckets == nil {
 		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -189,7 +189,7 @@ func mapassign_fast64ptr(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapassign.
-	h.flags |= hashWriting
+	h.flags ^= hashWriting
 
 	if h.buckets == nil {
 		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -276,7 +276,7 @@ func mapdelete_fast64(t *maptype, h *hmap, key uint64) {
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&key)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapdelete
-	h.flags |= hashWriting
+	h.flags ^= hashWriting
 
 	bucket := hash & bucketMask(h.B)
 	if h.growing() {

src/runtime/map_faststr.go

@@ -202,7 +202,7 @@ func mapassign_faststr(t *maptype, h *hmap, s string) unsafe.Pointer {
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&s)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapassign.
-	h.flags |= hashWriting
+	h.flags ^= hashWriting
 
 	if h.buckets == nil {
 		h.buckets = newobject(t.bucket) // newarray(t.bucket, 1)
@@ -294,7 +294,7 @@ func mapdelete_faststr(t *maptype, h *hmap, ky string) {
 	hash := t.key.alg.hash(noescape(unsafe.Pointer(&ky)), uintptr(h.hash0))
 
 	// Set hashWriting after calling alg.hash for consistency with mapdelete
-	h.flags |= hashWriting
+	h.flags ^= hashWriting
 
 	bucket := hash & bucketMask(h.B)
 	if h.growing() {