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

[Michael Snyder <msnyder at redhat dot com>]

Daniel Jacobowitz wrote:
> 
> 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.

Well, your feedback is high on the priority list.
 
> If we detect CLONE_THREAD (how do we detect CLONE_THREAD?) 

That I don't know.  Because I don't know, and because CLONE_THREAD
represents an entirely new approach, I was looking for something 
that I could get working quickly on both models, using mostly
the existing code and approach.

> 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.

I'm not as kernel-hackerish as you.  I'll discuss with Roland 
whether he thinks he can get that patch into the kernel in the
near term (and how it will help).