runtime: add race annotations in IncNonDefault

Also use CompareAndSwap to make the code actually less racy.

Added a test which will be meaningful when run under the race
detector (tested it -race with broken fix in runtime, it failed).

Fixes golang#64649

Change-Id: I5972e08901d1adc8ba74858edad7eba91be1b0ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/549796
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/internal/godebug/godebug_test.go

@@ -7,6 +7,7 @@ package godebug_test
 import (
 	"fmt"
 	. "internal/godebug"
+	"internal/race"
 	"internal/testenv"
 	"os"
 	"os/exec"
@@ -70,6 +71,36 @@ func TestMetrics(t *testing.T) {
 	}
 }
 
+// TestPanicNilRace checks for a race in the runtime caused by use of runtime
+// atomics (not visible to usual race detection) to install the counter for
+// non-default panic(nil) semantics.  For #64649.
+func TestPanicNilRace(t *testing.T) {
+	if !race.Enabled {
+		t.Skip("Skipping test intended for use with -race.")
+	}
+	if os.Getenv("GODEBUG") != "panicnil=1" {
+		cmd := testenv.CleanCmdEnv(testenv.Command(t, os.Args[0], "-test.run=^TestPanicNilRace$", "-test.v", "-test.parallel=2", "-test.count=1"))
+		cmd.Env = append(cmd.Env, "GODEBUG=panicnil=1")
+		out, err := cmd.CombinedOutput()
+		t.Logf("output:\n%s", out)
+
+		if err != nil {
+			t.Errorf("Was not expecting a crash")
+		}
+		return
+	}
+
+	test := func(t *testing.T) {
+		t.Parallel()
+		defer func() {
+			recover()
+		}()
+		panic(nil)
+	}
+	t.Run("One", test)
+	t.Run("Two", test)
+}
+
 func TestCmdBisect(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 	out, err := exec.Command("go", "run", "cmd/vendor/golang.org/x/tools/cmd/bisect", "GODEBUG=buggy=1#PATTERN", os.Args[0], "-test.run=^TestBisectTestCase$").CombinedOutput()

src/runtime/runtime.go

@@ -172,7 +172,15 @@ func (g *godebugInc) IncNonDefault() {
 		// *godebug.Setting.
 		inc = new(func())
 		*inc = (*newInc)(g.name)
-		g.inc.Store(inc)
+		if raceenabled {
+			racerelease(unsafe.Pointer(&g.inc))
+		}
+		if !g.inc.CompareAndSwap(nil, inc) {
+			inc = g.inc.Load()
+		}
+	}
+	if raceenabled {
+		raceacquire(unsafe.Pointer(&g.inc))
 	}
 	(*inc)()
 }