runtime: fix data race in GC

dvyukov · dvyukov · commit b3a3afc9b788 · 2014-01-15T19:38:08.000+04:00
Fixes #5139. Update #7065. R=golang-codereviews, bradfitz, minux.ma CC=golang-codereviews https://golang.org/cl/52090045
diff --git a/src/pkg/runtime/mgc0.c b/src/pkg/runtime/mgc0.c
@@ -1956,6 +1956,8 @@ runtime·memorydump(void)
 void
 runtime·gchelper(void)
 {
+	int32 nproc;
+
 	gchelperstart();
 
 	// parallel mark for over gc roots
@@ -1972,7 +1974,8 @@ 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);
 }
 
