runtime: remove GODEBUG=gcrescanstacks=1 mode

Currently, setting GODEBUG=gcrescanstacks=1 enables a debugging mode
where the garbage collector re-scans goroutine stacks during mark
termination. This was introduced in Go 1.8 to debug the hybrid write
barrier, but I don't think we ever used it.

Now it's one of the last sources of mark work during mark termination.
This CL removes it.

Updates #26903. This is preparation for unifying STW GC and concurrent
GC.

Updates #17503.

Change-Id: I6ae04d3738aa9c448e6e206e21857a33ecd12acf
Reviewed-on: https://go-review.googlesource.com/c/134777
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

1 files changed, 4 insertions, 18 deletions

diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index b86b2d012e..07b8f791d4 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -116,11 +116,6 @@ func gcMarkRootPrepare() {
 		// contain pointers to unmarked objects, so on the
 		// second markroot, there's no need to scan stacks.
 		work.nStackRoots = 0
-
-		if debug.gcrescanstacks > 0 {
-			// Scan stacks anyway for debugging.
-			work.nStackRoots = int(atomic.Loaduintptr(&allglen))
-		}
 	}
 
 	work.markrootNext = 0
@@ -138,19 +133,10 @@ func gcMarkRootCheck() {
 	lock(&allglock)
 	// Check that stacks have been scanned.
 	var gp *g
-	if gcphase == _GCmarktermination && debug.gcrescanstacks > 0 {
-		for i := 0; i < len(allgs); i++ {
-			gp = allgs[i]
-			if !(gp.gcscandone && gp.gcscanvalid) && readgstatus(gp) != _Gdead {
-				goto fail
-			}
-		}
-	} else {
-		for i := 0; i < work.nStackRoots; i++ {
-			gp = allgs[i]
-			if !gp.gcscandone {
-				goto fail
-			}
+	for i := 0; i < work.nStackRoots; i++ {
+		gp = allgs[i]
+		if !gp.gcscandone {
+			goto fail
 		}
 	}
 	unlock(&allglock)