Re: [PATCH] Fix AIX thread NULL assertion failure during fork

[Ulrich Weigand <Ulrich.Weigand@de.ibm.com>]

Aditya Kamath1 <Aditya.Kamath1@ibm.com> wrote:

>Assume we have set detach_on_fork = on and set follow-fork-mode child.
>In AIX, on a fork () event we set our status and return our parent ptid from rs6000-aix-nat.c..
>Once the object file of the new_inferior or child process is loaded we call pd_enable () to
>set our thread target and sync our threadlists. In our sync_threadlists we have pbuf having
>our pthread library threads and gbuf having our GDB threads known to GDB core that have been
>registered. 

>While I cannot say with 100% surety that from where GDB core got this ptid and why it did not
>update to ptid_t (pid, 0, tid) , my observation post debugging is that GDB core would have got
>ptid_t(pid, 0, 0) from the rs6000-aix-nat.c file, inside the wait () where we did inform GDB
>by setting a status that this a child process belonging a parent process on a fork event.
>GDB could not change this ptid it got during the fork event, even though we changed it later
>via sync_threadlists () from aix-thread.c for the threaded event.

So here's the piece I do not fully understand: Yes, wait () inside rs6000-aix-nat.c
will set the ourstatus child_ptid to a non-threaded ptid.  But that's for a newly
created child process that should not *be* threaded at this time, right?

So how is it possible that in between wait () setting the child_ptid and infrun.c
using it to switch to the child, the child is becoming multi-threaded?  Where is
the sync_threadlists () call that makes this happen?

I'm aware that the wait () in aix-thread.c (which is the caller of the rs6000-aix-nat.c
one) does perform a pd_enable / sync_threadlists, but only on the *parent*, not
on the child.  That should happen only later.

I think we should understand better how this could have happened.

If there is a good reason why the child can already be multi-threaded, then
one option to fix this would be to switch ourstatus->child_ptid to the multi-
threaded version in the *aix-thread.c* version of wait (), just like it
switches the returned ptid.

Bye,
Ulrich