runtime: add _Gdeadextra status

_Gdeadextra is almost the same as _Gdead but for goroutines attached to
extra Ms. The primary difference is that it can be transitioned into a
_Gscan status, unlike _Gdead. (Why not just use _Gdead? For safety,
mostly. There's exactly one case where we're going to want to transition
_Gdead to _Gscan|_Gdead, and it's for extra Ms. It's also a bit weird to
call this state dead when it can still have a syscalling P attached to
it.)

This status is used in a follow-up change that changes entersyscall and
exitsyscall.

Change-Id: I169a4c8617aa3dc329574b829203f56c86b58169
Reviewed-on: https://go-review.googlesource.com/c/go/+/646197
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/heapdump.go

@@ -415,7 +415,7 @@ func dumpgs() {
 		default:
 			print("runtime: unexpected G.status ", hex(status), "\n")
 			throw("dumpgs in STW - bad status")
-		case _Gdead:
+		case _Gdead, _Gdeadextra:
 			// ok
 		case _Grunnable,
 			_Gsyscall,

src/runtime/mgcmark.go

@@ -911,7 +911,7 @@ func scanstack(gp *g, gcw *gcWork) int64 {
 	default:
 		print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
 		throw("mark - bad status")
-	case _Gdead:
+	case _Gdead, _Gdeadextra:
 		return 0
 	case _Grunning:
 		print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")

src/runtime/mprof.go

@@ -1454,7 +1454,7 @@ func goroutineProfileWithLabelsConcurrent(p []profilerecord.StackRecord, labels
 		// were collecting the profile. But probably better to return a
 		// truncated profile than to crash the whole process.
 		//
-		// For instance, needm moves a goroutine out of the _Gdead state and so
+		// For instance, needm moves a goroutine out of the _Gdeadextra state and so
 		// might be able to change the goroutine count without interacting with
 		// the scheduler. For code like that, the race windows are small and the
 		// combination of features is uncommon, so it's hard to be (and remain)
@@ -1480,7 +1480,7 @@ func tryRecordGoroutineProfileWB(gp1 *g) {
 // in the current goroutine profile: either that it should not be profiled, or
 // that a snapshot of its call stack and labels are now in the profile.
 func tryRecordGoroutineProfile(gp1 *g, pcbuf []uintptr, yield func()) {
-	if readgstatus(gp1) == _Gdead {
+	if status := readgstatus(gp1); status == _Gdead || status == _Gdeadextra {
 		// Dead goroutines should not appear in the profile. Goroutines that
 		// start while profile collection is active will get goroutineProfiled
 		// set to goroutineProfileSatisfied before transitioning out of _Gdead,
@@ -1570,7 +1570,16 @@ func goroutineProfileWithLabelsSync(p []profilerecord.StackRecord, labels []unsa
 	isOK := func(gp1 *g) bool {
 		// Checking isSystemGoroutine here makes GoroutineProfile
 		// consistent with both NumGoroutine and Stack.
-		return gp1 != gp && readgstatus(gp1) != _Gdead && !isSystemGoroutine(gp1, false)
+		if gp1 == gp {
+			return false
+		}
+		if status := readgstatus(gp1); status == _Gdead || status == _Gdeadextra {
+			return false
+		}
+		if isSystemGoroutine(gp1, false) {
+			return false
+		}
+		return true
 	}
 
 	pcbuf := makeProfStack() // see saveg() for explanation

src/runtime/preempt.go

@@ -134,7 +134,7 @@ func suspendG(gp *g) suspendGState {
 			dumpgstatus(gp)
 			throw("invalid g status")
 
-		case _Gdead:
+		case _Gdead, _Gdeadextra:
 			// Nothing to suspend.
 			//
 			// preemptStop may need to be cleared, but

src/runtime/proc.go

@@ -1223,7 +1223,8 @@ func casfrom_Gscanstatus(gp *g, oldval, newval uint32) {
 		_Gscanrunning,
 		_Gscansyscall,
 		_Gscanleaked,
-		_Gscanpreempted:
+		_Gscanpreempted,
+		_Gscandeadextra:
 		if newval == oldval&^_Gscan {
 			success = gp.atomicstatus.CompareAndSwap(oldval, newval)
 		}
@@ -1244,7 +1245,8 @@ func castogscanstatus(gp *g, oldval, newval uint32) bool {
 		_Grunning,
 		_Gwaiting,
 		_Gleaked,
-		_Gsyscall:
+		_Gsyscall,
+		_Gdeadextra:
 		if newval == oldval|_Gscan {
 			r := gp.atomicstatus.CompareAndSwap(oldval, newval)
 			if r {
@@ -2459,7 +2461,7 @@ func needm(signal bool) {
 	}
 
 	// mp.curg is now a real goroutine.
-	casgstatus(mp.curg, _Gdead, _Gsyscall)
+	casgstatus(mp.curg, _Gdeadextra, _Gsyscall)
 	sched.ngsys.Add(-1)
 	sched.nGsyscallNoP.Add(1)
 
@@ -2515,11 +2517,10 @@ func oneNewExtraM() {
 	gp.syscallpc = gp.sched.pc
 	gp.syscallsp = gp.sched.sp
 	gp.stktopsp = gp.sched.sp
-	// malg returns status as _Gidle. Change to _Gdead before
-	// adding to allg where GC can see it. We use _Gdead to hide
-	// this from tracebacks and stack scans since it isn't a
-	// "real" goroutine until needm grabs it.
-	casgstatus(gp, _Gidle, _Gdead)
+	// malg returns status as _Gidle. Change to _Gdeadextra before
+	// adding to allg where GC can see it. _Gdeadextra hides this
+	// from traceback and stack scans.
+	casgstatus(gp, _Gidle, _Gdeadextra)
 	gp.m = mp
 	mp.curg = gp
 	mp.isextra = true
@@ -2593,8 +2594,8 @@ func dropm() {
 		trace = traceAcquire()
 	}
 
-	// Return mp.curg to dead state.
-	casgstatus(mp.curg, _Gsyscall, _Gdead)
+	// Return mp.curg to _Gdeadextra state.
+	casgstatus(mp.curg, _Gsyscall, _Gdeadextra)
 	mp.curg.preemptStop = false
 	sched.ngsys.Add(1)
 	sched.nGsyscallNoP.Add(-1)

src/runtime/runtime-gdb.py

@@ -46,11 +46,13 @@ G_MORIBUND_UNUSED = read_runtime_const("'runtime._Gmoribund_unused'", 5)
 G_DEAD = read_runtime_const("'runtime._Gdead'", 6)
 G_ENQUEUE_UNUSED = read_runtime_const("'runtime._Genqueue_unused'", 7)
 G_COPYSTACK = read_runtime_const("'runtime._Gcopystack'", 8)
+G_DEADEXTRA = read_runtime_const("'runtime._Gdeadextra'", 10)
 G_SCAN = read_runtime_const("'runtime._Gscan'", 0x1000)
 G_SCANRUNNABLE = G_SCAN+G_RUNNABLE
 G_SCANRUNNING = G_SCAN+G_RUNNING
 G_SCANSYSCALL = G_SCAN+G_SYSCALL
 G_SCANWAITING = G_SCAN+G_WAITING
+G_SCANEXTRA = G_SCAN+G_DEADEXTRA
 
 sts = {
     G_IDLE: 'idle',
@@ -62,11 +64,13 @@ sts = {
     G_DEAD: 'dead',
     G_ENQUEUE_UNUSED: 'enqueue',
     G_COPYSTACK: 'copystack',
+    G_DEADEXTRA: 'extra',
     G_SCAN: 'scan',
     G_SCANRUNNABLE: 'runnable+s',
     G_SCANRUNNING: 'running+s',
     G_SCANSYSCALL: 'syscall+s',
     G_SCANWAITING: 'waiting+s',
+    G_SCANEXTRA: 'extra+s',
 }
 
 
@@ -524,7 +528,7 @@ class GoroutinesCmd(gdb.Command):
 		# args = gdb.string_to_argv(arg)
 		vp = gdb.lookup_type('void').pointer()
 		for ptr in SliceValue(gdb.parse_and_eval("'runtime.allgs'")):
-			if ptr['atomicstatus']['value'] == G_DEAD:
+			if ptr['atomicstatus']['value'] in [G_DEAD, G_DEADEXTRA]:
 				continue
 			s = ' '
 			if ptr['m']:
@@ -549,7 +553,7 @@ def find_goroutine(goid):
 	"""
 	vp = gdb.lookup_type('void').pointer()
 	for ptr in SliceValue(gdb.parse_and_eval("'runtime.allgs'")):
-		if ptr['atomicstatus']['value'] == G_DEAD:
+		if ptr['atomicstatus']['value'] in [G_DEAD, G_DEADEXTRA]:
 			continue
 		if ptr['goid'] == goid:
 			break

src/runtime/runtime2.go

@@ -90,6 +90,10 @@ const (
 	// _Gleaked represents a leaked goroutine caught by the GC.
 	_Gleaked // 10
 
+	// _Gdeadextra is a _Gdead goroutine that's attached to an extra M
+	// used for cgo callbacks.
+	_Gdeadextra // 11
+
 	// _Gscan combined with one of the above states other than
 	// _Grunning indicates that GC is scanning the stack. The
 	// goroutine is not executing user code and the stack is owned
@@ -108,6 +112,7 @@ const (
 	_Gscanwaiting   = _Gscan + _Gwaiting   // 0x1004
 	_Gscanpreempted = _Gscan + _Gpreempted // 0x1009
 	_Gscanleaked    = _Gscan + _Gleaked    // 0x100a
+	_Gscandeadextra = _Gscan + _Gdeadextra // 0x100b
 )
 
 const (

src/runtime/synctest.go

@@ -52,7 +52,7 @@ func (bubble *synctestBubble) changegstatus(gp *g, oldval, newval uint32) {
 	totalDelta := 0
 	wasRunning := true
 	switch oldval {
-	case _Gdead:
+	case _Gdead, _Gdeadextra:
 		wasRunning = false
 		totalDelta++
 	case _Gwaiting:
@@ -62,7 +62,7 @@ func (bubble *synctestBubble) changegstatus(gp *g, oldval, newval uint32) {
 	}
 	isRunning := true
 	switch newval {
-	case _Gdead:
+	case _Gdead, _Gdeadextra:
 		isRunning = false
 		totalDelta--
 		if gp == bubble.main {
@@ -86,7 +86,7 @@ func (bubble *synctestBubble) changegstatus(gp *g, oldval, newval uint32) {
 			bubble.running++
 		} else {
 			bubble.running--
-			if raceenabled && newval != _Gdead {
+			if raceenabled && newval != _Gdead && newval != _Gdeadextra {
 				// Record that this goroutine parking happens before
 				// any subsequent Wait.
 				racereleasemergeg(gp, bubble.raceaddr())

src/runtime/traceback.go

@@ -1208,6 +1208,7 @@ var gStatusStrings = [...]string{
 	_Gcopystack: "copystack",
 	_Gleaked:    "leaked",
 	_Gpreempted: "preempted",
+	_Gdeadextra: "waiting for cgo callback",
 }
 
 func goroutineheader(gp *g) {
@@ -1295,7 +1296,16 @@ func tracebacksomeothers(me *g, showf func(*g) bool) {
 	// against concurrent creation of new Gs, but even with allglock we may
 	// miss Gs created after this loop.
 	forEachGRace(func(gp *g) {
-		if gp == me || gp == curgp || readgstatus(gp) == _Gdead || !showf(gp) || (isSystemGoroutine(gp, false) && level < 2) {
+		if gp == me || gp == curgp {
+			return
+		}
+		if status := readgstatus(gp); status == _Gdead || status == _Gdeadextra {
+			return
+		}
+		if !showf(gp) {
+			return
+		}
+		if isSystemGoroutine(gp, false) && level < 2 {
 			return
 		}
 		print("\n")

src/runtime/tracestatus.go

@@ -134,7 +134,7 @@ func goStatusToTraceGoStatus(status uint32, wr waitReason) tracev2.GoStatus {
 		if status == _Gwaiting && wr.isWaitingForSuspendG() {
 			tgs = tracev2.GoRunning
 		}
-	case _Gdead:
+	case _Gdead, _Gdeadextra:
 		throw("tried to trace dead goroutine")
 	default:
 		throw("tried to trace goroutine with invalid or unsupported status")