[release-branch.go1.2] runtime: fix data race in GC

««« CL 52090045 / 302bdb5b08b1
runtime: fix data race in GC
Fixes #5139.
Update #7065.

R=golang-codereviews, bradfitz, minux.ma
CC=golang-codereviews
https://golang.org/cl/52090045
»»»

LGTM=r
R=golang-codereviews, r
CC=golang-dev
https://golang.org/cl/69790043

src/pkg/runtime/mgc0.c

@@ -1798,6 +1798,8 @@ runtime·memorydump(void)
 void
 runtime·gchelper(void)
 {
+	int32 nproc;
+
 	gchelperstart();
 
 	// parallel mark for over gc roots
@@ -1814,7 +1816,8 @@ runtime·gchelper(void)
 
 	runtime·parfordo(work.sweepfor);
 	bufferList[m->helpgc].busy = 0;
-	if(runtime·xadd(&work.ndone, +1) == work.nproc-1)
+	nproc = work.nproc;  // work.nproc can change right after we increment work.ndone
+	if(runtime·xadd(&work.ndone, +1) == nproc-1)
 		runtime·notewakeup(&work.alldone);
 }