Multithreaded debugging: strange thread switches

[Vladimir Prus <ghost at cs dot msu dot su>]

Hello,
I'm observing strange behaviour when debugging with gdb using a custom stub.
I have two threads. After connecting, I say "next" several times and that 
steps in thread 1. Then I say "thread 2" and "next". Gdb then stops again in 
thread 1, not in thread 2 as I'd expected.

In the remote protocol I see "Hc1" packet after last "next" though I'd expect 
"Hc2", and in infrun.c, function prepare_to_proceed, I see this:


 if (!ptid_equal (wait_ptid, minus_one_ptid)
      && !ptid_equal (inferior_ptid, wait_ptid))
    {
      /* Switched over from WAIT_PID.  */
      CORE_ADDR wait_pc = read_pc_pid (wait_ptid);

      if (wait_pc != read_pc ())
	{
	  /* Switch back to WAIT_PID thread.  */
	  inferior_ptid = wait_ptid;

	  /* FIXME: This stuff came from switch_to_thread() in
	     thread.c (which should probably be a public function).  */
	  flush_cached_frames ();
	  registers_changed ();
	  stop_pc = wait_pc;
	  select_frame (get_current_frame ());
	}

Can somebody explain the reason for this explicit switch back to "wait_ptid"?

Thanks,
Volodya