runtime: always lock OS thread in debugcall

Right now debuggers like Delve rely on the new goroutine created to run
a debugcall function to run on the same thread it started on, up until
it hits itself with a SIGINT as part of the debugcall protocol.

That's all well and good, except debugCallWrap1 isn't particularly
careful about not growing the stack. For example, if the new goroutine
happens to have a stale preempt flag, then it's possible a stack growth
will cause a roundtrip into the scheduler, possibly causing the
goroutine to switch to another thread.

Previous attempts to just be more careful around debugCallWrap1 were
helpful, but insufficient. This change takes everything a step further
and always locks the debug call goroutine and the new goroutine it
creates to the OS thread.

For golang#61732.

Change-Id: I038f3a4df30072833e27e6a5a1ec01806a32891f
Reviewed-on: https://go-review.googlesource.com/c/go/+/515637
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alessandro Arzilli <alessandro.arzilli@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/runtime/debugcall.go

@@ -102,11 +102,18 @@ func debugCallCheck(pc uintptr) string {
 //
 //go:nosplit
 func debugCallWrap(dispatch uintptr) {
-	var lockedm bool
 	var lockedExt uint32
 	callerpc := getcallerpc()
 	gp := getg()
 
+	// Lock ourselves to the OS thread.
+	//
+	// Debuggers rely on us running on the same thread until we get to
+	// dispatch the function they asked as to.
+	//
+	// We're going to transfer this to the new G we just created.
+	lockOSThread()
+
 	// Create a new goroutine to execute the call on. Run this on
 	// the system stack to avoid growing our stack.
 	systemstack(func() {
@@ -121,27 +128,22 @@ func debugCallWrap(dispatch uintptr) {
 		}
 		newg.param = unsafe.Pointer(args)
 
-		// If the current G is locked, then transfer that
-		// locked-ness to the new goroutine.
-		if gp.lockedm != 0 {
-			// Save lock state to restore later.
-			mp := gp.m
-			if mp != gp.lockedm.ptr() {
-				throw("inconsistent lockedm")
-			}
-
-			lockedm = true
-			lockedExt = mp.lockedExt
-
-			// Transfer external lock count to internal so
-			// it can't be unlocked from the debug call.
-			mp.lockedInt++
-			mp.lockedExt = 0
-
-			mp.lockedg.set(newg)
-			newg.lockedm.set(mp)
-			gp.lockedm = 0
+		// Transfer locked-ness to the new goroutine.
+		// Save lock state to restore later.
+		mp := gp.m
+		if mp != gp.lockedm.ptr() {
+			throw("inconsistent lockedm")
 		}
+		// Save the external lock count and clear it so
+		// that it can't be unlocked from the debug call.
+		// Note: we already locked internally to the thread,
+		// so if we were locked before we're still locked now.
+		lockedExt = mp.lockedExt
+		mp.lockedExt = 0
+
+		mp.lockedg.set(newg)
+		newg.lockedm.set(mp)
+		gp.lockedm = 0
 
 		// Mark the calling goroutine as being at an async
 		// safe-point, since it has a few conservative frames
@@ -177,13 +179,13 @@ func debugCallWrap(dispatch uintptr) {
 	// We'll resume here when the call returns.
 
 	// Restore locked state.
-	if lockedm {
-		mp := gp.m
-		mp.lockedExt = lockedExt
-		mp.lockedInt--
-		mp.lockedg.set(gp)
-		gp.lockedm.set(mp)
-	}
+	mp := gp.m
+	mp.lockedExt = lockedExt
+	mp.lockedg.set(gp)
+	gp.lockedm.set(mp)
+
+	// Undo the lockOSThread we did earlier.
+	unlockOSThread()
 
 	gp.asyncSafePoint = false
 }