// Most clients should use the [runtime/trace] package or the [testing] package's
// -test.trace flag instead of calling StartTrace directly.
func StartTrace() error {
	if traceEnabled() || traceShuttingDown() {
		return errorString("tracing is already enabled")
	}
	// Block until cleanup of the last trace is done.
	semacquire(&traceShutdownSema)
	semrelease(&traceShutdownSema)

// and alloc/free events are also enabled.
//
//go:nosplit
func traceAllocFreeEnabled() bool {
	return trace.enabledWithAllocFree
}

// traceShuttingDown returns true if the trace is currently shutting down.
func traceShuttingDown() bool {
	return trace.shutdown.Load()
}

// traceLocker represents an M writing trace events. While a traceLocker value