# We want these probes to fire as deterministically as possible so that
      # their outputs can easily be predicted and compared. Unfortunately this
      # is complicated by multiple facts, including
      #     (1) the timer might be so fast that the probes don't have time to
      #         fire and print something,
      #     (2) the kprobe might print something after it is re-enabled, but
      #         before the kretprobe is re-enabled,
      #     (3) disabling a kretprobe won't stop an already running handler.
      #
      # To get around these issues, we use a simple state machine. The states
      # are as follow:
      #
      #     0 = cond disabled
      #     1 = cond enabled, but kprobe && kretprobe not yet enabled
      #     2 = cond enabled, kprobe enabled, kretprobe not yet enabled
      #     3 = cond enabled, kprobe && kretprobe enabled, nothing printed yet
      #     4 = cond enabled, 'hit' printed but not 'rethit'
      #     5 = cond enabled, 'rethit' printed

      global state = 1
      global toggles = 0

      probe kernel.function("vfs_read").call if (state > 0) {
         if (state == 3) {
            println("hit")
            state++;
         } else if (state == 1) {
            state++;
         }
      }

      probe kernel.function("vfs_read").return if (state > 0) {
         # ensure that nothing changed during the vfs_read body
         if (state != @entry(state))
            next
         if (state == 4) {
            println("rethit")
            state++;
         } else if (state == 2) {
            state++;
         }
      }

      probe begin, end, error, kernel.function("*@workqueue.c"),  process.begin, process.end, kernel.trace("*"), process("echo").function("*"),  netfilter.pf("NFPROTO_IPV4").hook("NF_INET_LOCAL_IN")?,  netfilter.pf("NFPROTO_IPV4").hook("NF_INET_LOCAL_OUT")?,  perf.sw.cpu_clock.sample(1000000)?, timer.profile.tick? {
         if (state != 0 && state != 5)
            next # give probes more time to move through the states
         toggles++
         if (toggles > 5000)
            exit()
         else {
            println("toggling")
            state = !state
         }
      }

      probe timer.s(360) {
         println("timed out")
         exit()
      }