Cell multi-arch broken (Re: [PATCH 2/2] GNU/Linux: Stop using libthread_db/td_ta_thr_iter)

["Ulrich Weigand" <uweigand at de dot ibm dot com>]

Pedro Alves wrote:

> gdb/ChangeLog:
> 2015-02-08  Pedro Alves  <palves@redhat.com>

> @@ -1719,6 +1696,9 @@ find_new_threads_once (struct thread_db_info *info, int iteration,
>    data.info = info;
>    data.new_threads = 0;
>  
> +  /* See comment in thread_db_update_thread_list.  */
> +  gdb_assert (!target_has_execution || thread_db_use_events ());
> +


Now that I've had the occasion to run some tests on Cell/B.E. again,
I've tried to verify combined multi-arch debugging still works, but
noticed that it is currently completely broken; GDB will immediately
run into the assertion you've added above.

In fact, it is so broken that the test suite assumes we're not even
on a Cell/B.E. (since it can't debug the trivial test program), and
silently skips all Cell tests, so I didn't notice in the daily build
reports ...

The reason why we're running into the abort is that the multi-arch
debugging logic attempts to resolve a thread-local variable from
inside the frame unwinders (which is probably not done elsewhere).
This uncovers a code path where the above assertion is wrong:

In thread_db_get_thread_local_address, we have:

  /* If we have not discovered any threads yet, check now.  */
  if (!have_threads (ptid))
    thread_db_find_new_threads_1 (ptid);

Now, note that thread_db_get_thread_local_address is one of the few
remaining routines that always uses the thread DB, even if we do
not use thread events.  However, thread_db_find_new_threads_1 now
assumes it gets only ever called when using thread events, which
ultimately leads to the assert.

As a quick fix, the patch appended below makes it work again;
but this may be a bit overkill since thread_db_update_thread_list
no longer realized that we're only interested in process ptid,
and updates all inferiors.  (Maybe the to_update_thread_list
target callback should get a ptid argument?)

Any suggestions on how best to fix this?

Bye,
Ulrich

P.S. With this fix, the Cell multi-arch tests now run again.
As probably to be expected, this uncovers a series of other
problems where code has bit-rotten in the meantime.  I'll
have other patches to fix this again ...



Index: binutils-gdb/gdb/linux-thread-db.c
===================================================================
--- binutils-gdb.orig/gdb/linux-thread-db.c
+++ binutils-gdb/gdb/linux-thread-db.c
@@ -1853,7 +1853,7 @@ thread_db_get_thread_local_address (stru
 
   /* If we have not discovered any threads yet, check now.  */
   if (!have_threads (ptid))
-    thread_db_find_new_threads_1 (ptid);
+    thread_db_update_thread_list (ops);
 
   /* Find the matching thread.  */
   thread_info = find_thread_ptid (ptid);


-- 
  Dr. Ulrich Weigand
  GNU/Linux compilers and toolchain
  Ulrich.Weigand@de.ibm.com