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Re: RFA: fix for thread events in thread-db.c

Joel Brobecker wrote:
Ok to commit?

-- Jeff J.

2003-04-23 Jeff Johnston <jjohnstn@redhat.com>

	* thread-db.c (check_event): For create/death event breakpoints,
	loop through all messages to ensure that we read the message
	corresponding to the breakpoint we are at.



Jeff, this patch has been approved but there is something I don't get.
Is the loop variable really necessary? It doesn't seem to ever be
changed except when you set it right before looping. And the exit from
the "do ... while" loop seems to be done via a "return".

Thanks.


I made it a loop so one can turn on and off the looping externally.
This makes sense if events other than the TD_CREATE event are supported.
For such events, you can use the other libthread_db "get message" interface which allows
you to specify the thread you are interested in and then there is no reason
to loop.  I have, for example, made this work for the TD_DEATH event.

The comment explains this; i.e. the loop is not always necessary.
I could have just as easily made it a for (;;) loop but this is functionally
equivalent and makes it sort of obvious how to change it in the future.


Index: thread-db.c
===================================================================
RCS file: /cvs/src/src/gdb/thread-db.c,v
retrieving revision 1.30
diff -u -r1.30 thread-db.c
--- thread-db.c	17 Apr 2003 17:30:02 -0000	1.30
+++ thread-db.c	23 Apr 2003 19:37:47 -0000
@@ -775,64 +775,73 @@
  td_thrinfo_t ti;
  td_err_e err;
  CORE_ADDR stop_pc;
+  int loop = 0;

  /* Bail out early if we're not at a thread event breakpoint.  */
  stop_pc = read_pc_pid (ptid) - DECR_PC_AFTER_BREAK;
  if (stop_pc != td_create_bp_addr && stop_pc != td_death_bp_addr)
    return;

- err = td_ta_event_getmsg_p (thread_agent, &msg);
- if (err != TD_OK)
+ /* If we are at a create breakpoint, we do not know what new lwp
+ was created and cannot specifically locate the event message for it.
+ We have to call td_ta_event_getmsg() to get
+ the latest message. Since we have no way of correlating whether
+ the event message we get back corresponds to our breakpoint, we must + loop and read all event messages, processing them appropriately.
+ This guarantees we will process the correct message before continuing + from the breakpoint. +
+ Currently, death events are not enabled. If they are enabled,
+ the death event can use the td_thr_event_getmsg() interface to
+ get the message specifically for that lwp and avoid looping
+ below. */
+
+ loop = 1;
+
+ do
{
- if (err == TD_NOMSG)
- return;
+ err = td_ta_event_getmsg_p (thread_agent, &msg);
+ if (err != TD_OK)
+ {
+ if (err == TD_NOMSG)
+ return;

-      error ("Cannot get thread event message: %s", thread_db_err_str (err));
-    }
+	  error ("Cannot get thread event message: %s",
+		 thread_db_err_str (err));
+	}

-  err = td_thr_get_info_p (msg.th_p, &ti);
-  if (err != TD_OK)
-    error ("check_event: cannot get thread info: %s",
-	   thread_db_err_str (err));
+      err = td_thr_get_info_p (msg.th_p, &ti);
+      if (err != TD_OK)
+	error ("Cannot get thread info: %s", thread_db_err_str (err));

-  ptid = BUILD_THREAD (ti.ti_tid, GET_PID (ptid));
+      ptid = BUILD_THREAD (ti.ti_tid, GET_PID (ptid));

-  switch (msg.event)
-    {
-    case TD_CREATE:
-#if 0
-      /* FIXME: kettenis/2000-08-26: Since we use td_ta_event_getmsg,
-         there is no guarantee that the breakpoint will match the
-         event.  Should we use td_thr_event_getmsg instead?  */
-
-      if (stop_pc != td_create_bp_addr)
-	error ("Thread creation event doesn't match breakpoint.");
-#endif
-
-      /* We may already know about this thread, for instance when the
-         user has issued the `info threads' command before the SIGTRAP
-         for hitting the thread creation breakpoint was reported.  */
-      if (!in_thread_list (ptid))
-	attach_thread (ptid, msg.th_p, &ti, 1);
-      return;
-
-    case TD_DEATH:
-#if 0
-      /* FIXME: See TD_CREATE.  */
-
-      if (stop_pc != td_death_bp_addr)
-	error ("Thread death event doesn't match breakpoint.");
-#endif
+      switch (msg.event)
+	{
+	case TD_CREATE:
+
+	  /* We may already know about this thread, for instance when the
+	     user has issued the `info threads' command before the SIGTRAP
+	     for hitting the thread creation breakpoint was reported.  */
+	  if (!in_thread_list (ptid))
+	    attach_thread (ptid, msg.th_p, &ti, 1);
+
+	  break;
+
+	case TD_DEATH:
+
+	  if (!in_thread_list (ptid))
+	    error ("Spurious thread death event.");

-      if (!in_thread_list (ptid))
-	error ("Spurious thread death event.");
+	  detach_thread (ptid, 1);

-      detach_thread (ptid, 1);
-      return;
+	  break;

-    default:
-      error ("Spurious thread event.");
+	default:
+	  error ("Spurious thread event.");
+	}
    }
+  while (loop);
}

static ptid_t




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