[patch] Fix for 'info threads' crashes if zombie threads exist

[Jan Kratochvil <lace at jankratochvil dot net>]

Hi,

https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=182116

Steps to Reproduce:
1.Start print-threads within GDB
2.Set a breakpoint in __pthread_unwind
3.Run the program
4.When the program stops at the breakpoint, run `info threads'


Patch disables handling of the TD_DEATH notification from __nptl_death_event()
and tries to keep the thread in TD_THR_ZOMBIE state as long as possible before
its LWP ceases to be. While expecting regressions so far no such was found,
statistically it improves some testcases (+24 passes,-14 fails), no negatives.
('longjmp' part not resolved; not reproducible for GDB-CVS)

Thread is now debuggable through pthread_exit(3) and the thread terminating
code.



Regards,
Jan Kratochvil

Draft patch (for GDB-CVS) proposal for handling+tracing behing pthread_exit(3)

Patch disables handling of the TD_DEATH notification from __nptl_death_event()
and tries to keep the thread in TD_THR_ZOMBIE state as long as possible before
its LWP ceases to be. While expecting regressions so far no such was found,
statistically it improves some testcases (+24 passes,-14 fails), no negatives.
('longjmp' part not resolved; not reproducible for GDB-CVS)


Index: linux-thread-db.c
===================================================================
RCS file: /cvs/src/src/gdb/linux-thread-db.c,v
retrieving revision 1.16
diff -u -p -r1.16 linux-thread-db.c
--- linux-thread-db.c	5 May 2006 22:42:43 -0000	1.16
+++ linux-thread-db.c	19 Jun 2006 16:27:16 -0000
@@ -541,6 +541,7 @@ enable_thread_event_reporting (void)
   td_event_emptyset (&events);
   td_event_addset (&events, TD_CREATE);
 
+#if 0	/* Do not track TD_DEATH to be able to trace pthread_exit(3).  */
 #ifdef HAVE_GNU_LIBC_VERSION_H
   /* FIXME: kettenis/2000-04-23: The event reporting facility is
      broken for TD_DEATH events in glibc 2.1.3, so don't enable it for
@@ -550,6 +551,7 @@ enable_thread_event_reporting (void)
       && (libc_major > 2 || (libc_major == 2 && libc_minor > 1)))
 #endif
     td_event_addset (&events, TD_DEATH);
+#endif
 
   err = td_ta_set_event_p (thread_agent, &events);
   if (err != TD_OK)
@@ -573,6 +575,7 @@ enable_thread_event_reporting (void)
       return;
     }
 
+#if 0	/* Do not track TD_DEATH to be able to trace pthread_exit(3).  */
   /* Set up the thread death event.  */
   err = enable_thread_event (thread_agent, TD_DEATH, &td_death_bp_addr);
   if (err != TD_OK)
@@ -581,6 +584,7 @@ enable_thread_event_reporting (void)
 	       thread_db_err_str (err));
       return;
     }
+#endif
 }
 
 static void
@@ -1006,6 +1010,9 @@ thread_db_fetch_registers (int regno)
     }
 
   thread_info = find_thread_pid (inferior_ptid);
+  if (!thread_info)
+    error (_("Stopped at dead thread %ld; should not happen."),
+	   (long) GET_THREAD (inferior_ptid));
   thread_db_map_id2thr (thread_info, 1);
 
   err = td_thr_getgregs_p (&thread_info->private->th, gregset);
@@ -1127,6 +1134,8 @@ thread_db_thread_alive (ptid_t ptid)
       struct thread_info *thread_info;
       thread_info = find_thread_pid (ptid);
 
+      if (!thread_info)
+	return 0;
       thread_db_map_id2thr (thread_info, 0);
       if (!thread_info->private->th_valid)
 	return 0;
@@ -1145,9 +1154,9 @@ thread_db_thread_alive (ptid_t ptid)
 	  thread_info->private->ti_valid = 1;
 	}
 
-      if (thread_info->private->ti.ti_state == TD_THR_UNKNOWN
-	  || thread_info->private->ti.ti_state == TD_THR_ZOMBIE)
-	return 0;		/* A zombie thread.  */
+      /* Never 0 on TD_THR_ZOMBIE to be able to trace pthread_exit(3).  */
+      if (thread_info->private->ti.ti_state == TD_THR_UNKNOWN)
+	return 0;		/* A disappeared thread.  */
 
       return 1;
     }