runtime: do not treat asmcgocall as a topofstack on g0

This was added in 2018 to fix a runtime crash during unwind
during a unhandled-panic-induced crash.
(See https://golang.org/cl/90895 and #23576.)
Clearly we cannot unwind past this function, and the change
did stop the unwind. But it's not a top-of-stack function, and
the real issue is that SP is changed.

The new SPWRITE bit takes care of this instead, so we can drop
it from the topofstack function.

At this point the topofstack function is only checking the
TOPFRAME bit, so we can inline that into the one call site.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: I856552298032770e48e06c95a20823a1dbd5e38c
Reviewed-on: https://go-review.googlesource.com/c/go/+/288805
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 1 insertions, 12 deletions

diff --git a/src/runtime/traceback.go b/src/runtime/traceback.go
index dbc2bddf42..7321790b78 100644
--- a/src/runtime/traceback.go
+++ b/src/runtime/traceback.go
@@ -209,7 +209,7 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 			}
 		}
 		var flr funcInfo
-		if topofstack(f, gp.m != nil && gp == gp.m.g0) {
+		if flag&funcFlag_TOPFRAME != 0 {
 			// This function marks the top of the stack. Stop the traceback.
 			frame.lr = 0
 			flr = funcInfo{}
@@ -1000,17 +1000,6 @@ 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 ||
-		// asmcgocall is TOS on the system stack because it
-		// switches to the system stack, but in this case we
-		// can come back to the regular stack and still want
-		// to be able to unwind through the call that appeared
-		// on the regular stack.
-		(g0 && f.funcID == funcID_asmcgocall)
-}
-
 // isSystemGoroutine reports whether the goroutine g must be omitted
 // in stack dumps and deadlock detector. This is any goroutine that
 // starts at a runtime.* entry point, except for runtime.main,