src/internal/godebug/godebug_test.go | 31 +++++++++++++++++++++++++++++++
 src/runtime/runtime.go               | 13 +++++++++----

diff --git a/src/internal/godebug/godebug_test.go b/src/internal/godebug/godebug_test.go
index 8e46283adab3267a9f7bf671791748c03eecaf00..65dd256e8e7d29890b6375313a7a71bebdaed76c 100644
--- a/src/internal/godebug/godebug_test.go
+++ b/src/internal/godebug/godebug_test.go
@@ -7,6 +7,7 @@
 import (
 	"fmt"
 	. "internal/godebug"
+	"internal/race"
 	"internal/testenv"
 	"os"
 	"os/exec"
@@ -68,6 +69,36 @@ 	}
 	if count := m[0].Value.Uint64(); count != 3 {
 		t.Fatalf("NonDefault value = %d, want 3", count)
 	}
+}
+
+// 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) {
diff --git a/src/runtime/runtime.go b/src/runtime/runtime.go
index 0822d0e8054e7b53f6347f8bcdbb1228dd82d80a..15119cf5dfc730d24757650e45c0ac16578e7b9b 100644
--- a/src/runtime/runtime.go
+++ b/src/runtime/runtime.go
@@ -101,12 +101,17 @@ 		newInc := godebugNewIncNonDefault.Load()
 		if newInc == nil {
 			return
 		}
-		// If other goroutines are racing here, no big deal. One will win,
-		// and all the inc functions will be using the same underlying
-		// *godebug.Setting.
 		inc = new(func())
 		*inc = (*newInc)(g.name)
-		g.inc.Store(inc)
+		if raceenabled {
+			racereleasemerge(unsafe.Pointer(&g.inc))
+		}
+		if !g.inc.CompareAndSwap(nil, inc) {
+			inc = g.inc.Load()
+		}
+	}
+	if raceenabled {
+		raceacquire(unsafe.Pointer(&g.inc))
 	}
 	(*inc)()
 }