[RFC] lin-lwp.c prelim changes for new thread model

[Daniel Jacobowitz]

On Mon, Jan 06, 2003 at 03:44:05PM -0800, Michael Snyder wrote:
> Hi folks, 
> 
> The up and coming kernel (2.4.20, I believe?) and the next glibc (2.3.1)
> both bring some drastic changes to linux threads.  The current gdb thread
> debugging code will not handle them as is.
> 
> This is a smallish change that I propose as a preliminary step;
> it'll get things partly working in the new world, without breaking
> them in the old.
> 
> Here's the rationalle.
> 
> In the old/current model, when one thread gets a signal (such as TRAP), 
> we (gdb) have to call kill (SIGSTOP, pid) for every other thread
> (excepting the event thread), and then do a waitpid on each of them.
> 
> In the new model, when one thread gets a signal, we only have to 
> send kill(SIGSTOP, pid) to _one_ thread, and the kernel will then
> propagate the signal to all of them (_including_ the one that has
> already stopped with eg. SIGTRAP).  We must still do a waitpid on
> each and every thread -- however, that now _includes_ the one that
> stopped in the first place (and which we've already done one waitpid on).
> 
> I know, you're thinking "wasn't this supposed to get simpler?"
> 
> The minimal change I propose below is as follows:
> When we send kill(SIGSTOP) to all the threads, we now include
> the event thread, where previously we had made him a special case.
> That way, whether in the new model or the old one, we can now do
> a waitpid on every thread including the event thread.
> 
> What do you think?

To be honest, I don't like this very much.  You're hurting performance
in the current case (whose performance is already quite bad enough,
thank you!).  I don't think that the additional complexity/waiting is
worthwhile.

If we detect CLONE_THREAD (how do we detect CLONE_THREAD?) we can mark
the new LWP as having a pending stop based on that.  Or, Roland has a
kernel patch that's stewing in my mailbox which provides a better way
to handle this entire thing than sending SIGSTOP.  I don't think he
ever tested it, and I know I haven't had time, but let me know if you
want a copy.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer