diff --git a/src/runtime/symtab.go b/src/runtime/symtab.go
index d7da255e43..00f802aaa7 100644
--- a/src/runtime/symtab.go
+++ b/src/runtime/symtab.go
@@ -337,7 +337,21 @@ const (
 type funcFlag uint8
 
 const (
+	// TOPFRAME indicates a function that appears at the top of its stack.
+	// The traceback routine stop at such a function and consider that a
+	// successful, complete traversal of the stack.
+	// Examples of TOPFRAME functions include goexit, which appears
+	// at the top of a user goroutine stack, and mstart, which appears
+	// at the top of a system goroutine stack.
 	funcFlag_TOPFRAME funcFlag = 1 << iota
+
+	// SPWRITE indicates a function that writes an arbitrary value to SP
+	// (any write other than adding or subtracting a constant amount).
+	// The traceback routines cannot encode such changes into the
+	// pcsp tables, so the function traceback cannot safely unwind past
+	// SPWRITE functions. Stopping at an SPWRITE function is considered
+	// to be an incomplete unwinding of the stack. In certain contexts
+	// (in particular garbage collector stack scans) that is a fatal error.
 	funcFlag_SPWRITE
 )
 
diff --git a/src/runtime/traceback.go b/src/runtime/traceback.go
index e2bd968919..c89a8913ae 100644
--- a/src/runtime/traceback.go
+++ b/src/runtime/traceback.go
@@ -1003,7 +1003,6 @@ func tracebackHexdump(stk stack, frame *stkframe, bad uintptr) {
 // Does f mark the top of a goroutine stack?
 func topofstack(f funcInfo, g0 bool) bool {
 	return f.flag&funcFlag_TOPFRAME != 0 ||
-		f.funcID == funcID_mcall ||
 		f.funcID == funcID_morestack ||
 		// asmcgocall is TOS on the system stack because it
 		// switches to the system stack, but in this case we