runtime: emit the correct P status from a safepoint in the v2 tracer

The GoSyscallBegin event is a signal for both the P and the G to enter a
syscall state for the trace parser. (Ps can't have their own event
because it's too hard to model. As soon as the P enters _Psyscall it can
get stolen out of it.) But there's a window in time between when that
event is emitted and when the P enters _Psyscall where the P's status
can get emitted. In this window the tracer will emit the wrong status:
Running instead of Syscall. Really any call into the tracer could emit a
status event for the P, but in this particular case it's when running a
safepoint function that explicitly emits an event for the P's status.

The fix is straightforward. The source-of-truth on syscall status is the
G's status, so the function that emits the P's status just needs to
check the status of any G attached to it. If it's in _Gsyscall, then the
tracer should emit a Syscall status for the P if it's in _Prunning.

Fixes #64318.

Change-Id: I3b0fb0d41ff578e62810b04fa5a3ef73e2929b0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/546025
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/trace2status.go

@@ -82,8 +82,17 @@ func (w traceWriter) writeProcStatusForP(pp *p, inSTW bool) traceWriter {
 			// in _Pgcstop, but we model it as running in the tracer.
 			status = traceProcRunning
 		}
-	case _Prunning, _Psyscall:
+	case _Prunning:
 		status = traceProcRunning
+		// There's a short window wherein the goroutine may have entered _Gsyscall
+		// but it still owns the P (it's not in _Psyscall yet). The goroutine entering
+		// _Gsyscall is the tracer's signal that the P its bound to is also in a syscall,
+		// so we need to emit a status that matches. See #64318.
+		if w.mp.p.ptr() == pp && readgstatus(w.mp.curg)&^_Gscan == _Gsyscall {
+			status = traceProcSyscall
+		}
+	case _Psyscall:
+		status = traceProcSyscall
 	default:
 		throw("attempt to trace invalid or unsupported P status")
 	}