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>

Author: aclements

src/runtime/extern.go

@@ -50,11 +50,6 @@ It is a comma-separated list of name=val pairs setting these named variables:
 	gcshrinkstackoff: setting gcshrinkstackoff=1 disables moving goroutines
 	onto smaller stacks. In this mode, a goroutine's stack can only grow.
 
-	gcrescanstacks: setting gcrescanstacks=1 enables stack
-	re-scanning during the STW mark termination phase. This is
-	helpful for debugging if objects are being prematurely
-	garbage collected.
-
 	gcstoptheworld: setting gcstoptheworld=1 disables concurrent garbage collection,
 	making every garbage collection a stop-the-world event. Setting gcstoptheworld=2
 	also disables concurrent sweeping after the garbage collection finishes.

src/runtime/mgc.go

@@ -1914,7 +1914,7 @@ func gcMark(start_time int64) {
 		//
 		// TODO(austin): Move STW marking out of
 		// mark termination and eliminate this code path.
-		if debug.gcstoptheworld == 0 && debug.gcrescanstacks == 0 {
+		if debug.gcstoptheworld == 0 {
 			print("runtime: full=", hex(work.full), " nDataRoots=", work.nDataRoots, " nBSSRoots=", work.nBSSRoots, " nSpanRoots=", work.nSpanRoots, " nStackRoots=", work.nStackRoots, "\n")
 			panic("non-empty mark queue after concurrent mark")
 		}

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)

src/runtime/runtime1.go

@@ -305,7 +305,6 @@ var debug struct {
 	gccheckmark        int32
 	gcpacertrace       int32
 	gcshrinkstackoff   int32
-	gcrescanstacks     int32
 	gcstoptheworld     int32
 	gctrace            int32
 	invalidptr         int32
@@ -323,7 +322,6 @@ var dbgvars = []dbgVar{
 	{"gccheckmark", &debug.gccheckmark},
 	{"gcpacertrace", &debug.gcpacertrace},
 	{"gcshrinkstackoff", &debug.gcshrinkstackoff},
-	{"gcrescanstacks", &debug.gcrescanstacks},
 	{"gcstoptheworld", &debug.gcstoptheworld},
 	{"gctrace", &debug.gctrace},
 	{"invalidptr", &debug.invalidptr},