runtime: copy stack before adjusting

Currently copystack adjusts pointers in the old stack and then copies
the adjusted stack to the new stack. In addition to being generally
confusing, this is going to make concurrent stack shrinking harder.

Switch this around so that we first copy the stack and then adjust
pointers on the new stack (never writing to the old stack).

This reprises CL 15996, but takes a different and simpler approach. CL
15996 still walked the old stack while adjusting pointers on the new
stack. In this CL, we adjust auxiliary structures before walking the
stack, so we can just walk the new stack.

For #12967.

Change-Id: I94fa86f823ba9ee478e73b2ba509eed3361c43df
Reviewed-on: https://go-review.googlesource.com/20033Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/stack.go

@@ -748,29 +748,27 @@ func copystack(gp *g, newsize uintptr) {
 		print("copystack gp=", gp, " [", hex(old.lo), " ", hex(old.hi-used), " ", hex(old.hi), "]/", gp.stackAlloc, " -> [", hex(new.lo), " ", hex(new.hi-used), " ", hex(new.hi), "]/", newsize, "\n")
 	}
 
-	// Disallow sigprof scans of this stack and block if there's
-	// one in progress.
-	gcLockStackBarriers(gp)
-
-	// adjust pointers in the to-be-copied frames
+	// Compute adjustment.
 	var adjinfo adjustinfo
 	adjinfo.old = old
 	adjinfo.delta = new.hi - old.hi
-	gentraceback(^uintptr(0), ^uintptr(0), 0, gp, 0, nil, 0x7fffffff, adjustframe, noescape(unsafe.Pointer(&adjinfo)), 0)
 
-	// adjust other miscellaneous things that have pointers into stacks.
+	// copy the stack to the new location
+	memmove(unsafe.Pointer(new.hi-used), unsafe.Pointer(old.hi-used), used)
+
+	// Disallow sigprof scans of this stack and block if there's
+	// one in progress.
+	gcLockStackBarriers(gp)
+
+	// Adjust structures that have pointers into stacks. We have
+	// to do most of these before we traceback the new stack
+	// because gentraceback uses them.
 	adjustctxt(gp, &adjinfo)
 	adjustdefers(gp, &adjinfo)
 	adjustpanics(gp, &adjinfo)
 	adjustsudogs(gp, &adjinfo)
 	adjuststkbar(gp, &adjinfo)
 
-	// copy the stack to the new location
-	if stackPoisonCopy != 0 {
-		fillstack(new, 0xfb)
-	}
-	memmove(unsafe.Pointer(new.hi-used), unsafe.Pointer(old.hi-used), used)
-
 	// copy old stack barriers to new stack barrier array
 	newstkbar = newstkbar[:len(gp.stkbar)]
 	copy(newstkbar, gp.stkbar)
@@ -784,6 +782,9 @@ func copystack(gp *g, newsize uintptr) {
 	gp.stkbar = newstkbar
 	gp.stktopsp += adjinfo.delta
 
+	// Adjust pointers in the new stack.
+	gentraceback(^uintptr(0), ^uintptr(0), 0, gp, 0, nil, 0x7fffffff, adjustframe, noescape(unsafe.Pointer(&adjinfo)), 0)
+
 	gcUnlockStackBarriers(gp)
 
 	// free old stack