diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go
index 8b061e0..9b269d9 100644
--- a/src/runtime/export_test.go
+++ b/src/runtime/export_test.go
@@ -381,3 +381,17 @@
 	h := *(**hmap)(unsafe.Pointer(&m))
 	return 1 << h.B
 }
+
+func LockOSCounts() (external, internal uint32) {
+	g := getg()
+	if g.m.lockedExt+g.m.lockedInt == 0 {
+		if g.lockedm != 0 {
+			panic("lockedm on non-locked goroutine")
+		}
+	} else {
+		if g.lockedm == 0 {
+			panic("nil lockedm on locked goroutine")
+		}
+	}
+	return g.m.lockedExt, g.m.lockedInt
+}
diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index f8716b1..cb9b1aa 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -1498,7 +1498,7 @@
 	casgstatus(gp, _Gidle, _Gdead)
 	gp.m = mp
 	mp.curg = gp
-	mp.locked = _LockInternal
+	mp.lockedInt++
 	mp.lockedg.set(gp)
 	gp.lockedm.set(mp)
 	gp.goid = int64(atomic.Xadd64(&sched.goidgen, 1))
@@ -2402,11 +2402,11 @@
 	gp.gcscanvalid = true
 	dropg()
 
-	if _g_.m.locked&^_LockExternal != 0 {
-		print("invalid m->locked = ", _g_.m.locked, "\n")
+	if _g_.m.lockedInt != 0 {
+		print("invalid m->lockedInt = ", _g_.m.lockedInt, "\n")
 		throw("internal lockOSThread error")
 	}
-	_g_.m.locked = 0
+	_g_.m.lockedExt = 0
 	gfput(_g_.m.p.ptr(), gp)
 	schedule()
 }
@@ -3172,16 +3172,23 @@
 //go:nosplit
 
 // LockOSThread wires the calling goroutine to its current operating system thread.
-// Until the calling goroutine exits or calls UnlockOSThread, it will always
-// execute in that thread, and no other goroutine can.
+// The calling goroutine will always execute in that thread,
+// and no other goroutine will execute in it,
+// until the calling goroutine exits or has made as many calls to
+// UnlockOSThread as to LockOSThread.
 func LockOSThread() {
-	getg().m.locked |= _LockExternal
+	_g_ := getg()
+	_g_.m.lockedExt++
+	if _g_.m.lockedExt == 0 {
+		_g_.m.lockedExt--
+		panic("LockOSThread nesting overflow")
+	}
 	dolockOSThread()
 }
 
 //go:nosplit
 func lockOSThread() {
-	getg().m.locked += _LockInternal
+	getg().m.lockedInt++
 	dolockOSThread()
 }
 
@@ -3191,7 +3198,7 @@
 //go:nosplit
 func dounlockOSThread() {
 	_g_ := getg()
-	if _g_.m.locked != 0 {
+	if _g_.m.lockedInt != 0 || _g_.m.lockedExt != 0 {
 		return
 	}
 	_g_.m.lockedg = 0
@@ -3200,20 +3207,27 @@
 
 //go:nosplit
 
-// UnlockOSThread unwires the calling goroutine from its fixed operating system thread.
-// If the calling goroutine has not called LockOSThread, UnlockOSThread is a no-op.
+// UnlockOSThread undoes an earlier call to LockOSThread.
+// If this drops the number of active LockOSThread calls on the
+// calling goroutine to zero, it unwires the calling goroutine from
+// its fixed operating system thread.
+// If there are no active LockOSThread calls, this is a no-op.
 func UnlockOSThread() {
-	getg().m.locked &^= _LockExternal
+	_g_ := getg()
+	if _g_.m.lockedExt == 0 {
+		return
+	}
+	_g_.m.lockedExt--
 	dounlockOSThread()
 }
 
 //go:nosplit
 func unlockOSThread() {
 	_g_ := getg()
-	if _g_.m.locked < _LockInternal {
+	if _g_.m.lockedInt == 0 {
 		systemstack(badunlockosthread)
 	}
-	_g_.m.locked -= _LockInternal
+	_g_.m.lockedInt--
 	dounlockOSThread()
 }
 
diff --git a/src/runtime/proc_test.go b/src/runtime/proc_test.go
index 90a6cab..835b548 100644
--- a/src/runtime/proc_test.go
+++ b/src/runtime/proc_test.go
@@ -722,3 +722,27 @@
 func TestStealOrder(t *testing.T) {
 	runtime.RunStealOrderTest()
 }
+
+func TestLockOSThreadNesting(t *testing.T) {
+	go func() {
+		e, i := runtime.LockOSCounts()
+		if e != 0 || i != 0 {
+			t.Errorf("want locked counts 0, 0; got %d, %d", e, i)
+			return
+		}
+		runtime.LockOSThread()
+		runtime.LockOSThread()
+		runtime.UnlockOSThread()
+		e, i = runtime.LockOSCounts()
+		if e != 1 || i != 0 {
+			t.Errorf("want locked counts 1, 0; got %d, %d", e, i)
+			return
+		}
+		runtime.UnlockOSThread()
+		e, i = runtime.LockOSCounts()
+		if e != 0 || i != 0 {
+			t.Errorf("want locked counts 0, 0; got %d, %d", e, i)
+			return
+		}
+	}()
+}
diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index 055fff1..f56876f 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -430,7 +430,8 @@
 	freglo        [16]uint32     // d[i] lsb and f[i]
 	freghi        [16]uint32     // d[i] msb and f[i+16]
 	fflag         uint32         // floating point compare flags
-	locked        uint32         // tracking for lockosthread
+	lockedExt     uint32         // tracking for external LockOSThread
+	lockedInt     uint32         // tracking for internal lockOSThread
 	nextwaitm     uintptr        // next m waiting for lock
 	waitunlockf   unsafe.Pointer // todo go func(*g, unsafe.pointer) bool
 	waitlock      unsafe.Pointer
@@ -576,18 +577,6 @@
 	totaltime      int64 // ∫gomaxprocs dt up to procresizetime
 }
 
-// The m.locked word holds two pieces of state counting active calls to LockOSThread/lockOSThread.
-// The low bit (LockExternal) is a boolean reporting whether any LockOSThread call is active.
-// External locks are not recursive; a second lock is silently ignored.
-// The upper bits of m.locked record the nesting depth of calls to lockOSThread
-// (counting up by LockInternal), popped by unlockOSThread (counting down by LockInternal).
-// Internal locks can be recursive. For instance, a lock for cgo can occur while the main
-// goroutine is holding the lock during the initialization phase.
-const (
-	_LockExternal = 1
-	_LockInternal = 2
-)
-
 // Values for the flags field of a sigTabT.
 const (
 	_SigNotify   = 1 << iota // let signal.Notify have signal, even if from kernel