Re: [patch] Fix threads vs. fork following

[Michael Snyder <msnyder@redhat.com>]

Daniel Jacobowitz wrote:
> On Mon, Mar 22, 2004 at 12:12:18PM -0500, Daniel Jacobowitz wrote:
> 
>>On Thu, Mar 11, 2004 at 01:08:40AM +0000, Michael Snyder wrote:
>>
>>>Hey Daniel,
>>>
>>>Got a question concerning the code in 
>>>linux-nat.c::linux_handle_extended_wait.
>>>
>>>You've got a PTRACE_EVENT_FORK event, and now you're going to call 
>>>waitpid.  You pull a pid out of a list of stopped pids, and wait for
>>>it using waitpid.  In your comment, you explain that you don't have to
>>>worry about the pid being a clone, because you didn't ask for pids in
>>>the event mask.
>>>
>>>But how is this affected by threads, especially NPTL threads?
>>>I've got a fairly simple test-case (modified from pthreads.c,
>>>I'll attach it), in which a child thread calls fork -- but gdb
>>>apparently tries to wait on the main thread (or perhaps the most
>>>recent event thread).  Since that's not the thread that called
>>>fork, waitpid returns -1 with "no child".  Gdb reports:
>>>	waiting for new child: No child processes.
>>>
>>>FWIW, I've tried this on both a single-processor and an SMP machine.
> 
> 
> Here's what happened: I was using ptid_get_pid, which gave me the
> _process_ ID rather than the _lwp_ ID.  I've committed this fix for
> HEAD.  Should I fix this on the 6.1 branch also?
> 

I don't see why not.

FYI, I've had feedback from one user who says this patch helped get
him past the point at which he was stuck before -- but he's seeing
a new problem which may or may not be related.  He has one thread
which calls system("/bin/ls"), and system ("cd <...>").  He can now
get past at least the first few such calls under GDB, but eventually
(and non-deterministically), he sees something like:

     Detaching after fork from child process 22087.
     ---Type <return> to continue, or q <return> to quit---

And then for some unknown percent of the times when the
above occurs, it goes more like this:

     Detaching after fork from child process 21502.

     Suspended (tty output)
     [msnyder@reddwarf gdb]$ fg
     /export/msnyder/03r1-2/bin/gdb  ...
     ---Type <return> to continue, or q <return> to quit---
     Detaching after fork from child process 21510.

Any thoughts?