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>

diff --git a/src/runtime/map.go b/src/runtime/map.go
index c03e745dc52aded16405d520d0c82174ac388d38..c3fcfbfdbe40579c37a421b8ae1c06d81e0284df 100644 (file)
--- a/src/runtime/map.go
+++ b/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

diff --git a/src/runtime/map_fast32.go b/src/runtime/map_fast32.go
index bf0b23604bb0c496921a355d6c3bf9e540a613a7..671558545a2ba462f3eb69a2988ab9613ef789a7 100644 (file)
--- a/src/runtime/map_fast32.go
+++ b/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() {

diff --git a/src/runtime/map_fast64.go b/src/runtime/map_fast64.go
index 4bde9e2be0726e92c6981cf5e6f58e88c09a3dbe..164a4dd1cef7189b553e033589545f21b4460b81 100644 (file)
--- a/src/runtime/map_fast64.go
+++ b/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() {

diff --git a/src/runtime/map_faststr.go b/src/runtime/map_faststr.go
index 415bbff143ff53bb4250226635a3bea0cee0c3bf..bee62dfb03b98fd30ac1c2b51aa41628035047f8 100644 (file)
--- a/src/runtime/map_faststr.go
+++ b/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() {