runtime: eliminate write barriers from dropg

Currently this contains no write barriers because it's writing nil
pointers, but with the hybrid barrier, even these will produce write
barriers. However, since these are *gs and *ms, they don't need write
barriers, so we can simply eliminate them.

Updates #17503.

Change-Id: Ib188a60492c5cfb352814bf9b2bcb2941fb7d6c0
Reviewed-on: https://go-review.googlesource.com/31570
Reviewed-by: Rick Hudson <rlh@golang.org>

diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index c77229b925ab6ba86bc446e9865e7766ad1524be..eb2532f3c3eade70d40df9b98ec6629da2a5a3fa 100644 (file)
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -2175,8 +2175,8 @@ top:
 func dropg() {
        _g_ := getg()
 
-       _g_.m.curg.m = nil
-       _g_.m.curg = nil
+       setMNoWB(&_g_.m.curg.m, nil)
+       setGNoWB(&_g_.m.curg, nil)
 }
 
 func parkunlock_c(gp *g, lock unsafe.Pointer) bool {

diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index 683156daf1f841d9b5941afb6dc867539b4a5f01..49f6e6f64983277abaa2927ef44bfc38884c21b7 100644 (file)
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -205,6 +205,14 @@ func (gp *guintptr) cas(old, new guintptr) bool {
        return atomic.Casuintptr((*uintptr)(unsafe.Pointer(gp)), uintptr(old), uintptr(new))
 }
 
+// setGNoWB performs *gp = new without a write barrier.
+// For times when it's impractical to use a guintptr.
+//go:nosplit
+//go:nowritebarrier
+func setGNoWB(gp **g, new *g) {
+       (*guintptr)(unsafe.Pointer(gp)).set(new)
+}
+
 type puintptr uintptr
 
 //go:nosplit
@@ -221,6 +229,14 @@ func (mp muintptr) ptr() *m { return (*m)(unsafe.Pointer(mp)) }
 //go:nosplit
 func (mp *muintptr) set(m *m) { *mp = muintptr(unsafe.Pointer(m)) }
 
+// setMNoWB performs *mp = new without a write barrier.
+// For times when it's impractical to use an muintptr.
+//go:nosplit
+//go:nowritebarrier
+func setMNoWB(mp **m, new *m) {
+       (*muintptr)(unsafe.Pointer(mp)).set(new)
+}
+
 type gobuf struct {
        // The offsets of sp, pc, and g are known to (hard-coded in) libmach.
        sp   uintptr