runtime: mark activeModules nosplit/nowritebarrier

The activeModules function is called by the cgo pointer checking code,
which is called by the write barrier (when GODEBUG=cgocheck=2), and as
such must be nosplit/nowritebarrier.

Fixes #21306

Change-Id: I57f2124f14de7f3872b2de9532abab15df95d45a
Reviewed-on: https://go-review.googlesource.com/53352Reviewed-by: Austin Clements <austin@google.com>

misc/cgo/errors/ptr.go

@@ -343,6 +343,14 @@ var ptrTests = []ptrTest{
 		body:    `var b C.char; p := &b; C.f((*C.u)(unsafe.Pointer(&p)))`,
 		fail:    false,
 	},
+	{
+		// Issue #21306.
+		name:    "preempt-during-call",
+		c:       `void f() {}`,
+		imports: []string{"runtime", "sync"},
+		body:    `var wg sync.WaitGroup; wg.Add(100); for i := 0; i < 100; i++ { go func(i int) { for j := 0; j < 100; j++ { C.f(); runtime.GOMAXPROCS(i) }; wg.Done() }(i) }; wg.Wait()`,
+		fail:    false,
+	},
 }
 
 func main() {

src/runtime/symtab.go

@@ -409,6 +409,11 @@ var modulesSlice unsafe.Pointer // see activeModules
 //
 // A module is active once its gcdatamask and gcbssmask have been
 // assembled and it is usable by the GC.
+//
+// This is nosplit/nowritebarrier because it is called by the
+// cgo pointer checking code.
+//go:nosplit
+//go:nowritebarrier
 func activeModules() []*moduledata {
 	p := (*[]*moduledata)(atomic.Loadp(unsafe.Pointer(&modulesSlice)))
 	if p == nil {