Re: Why does gdb use its own thread ids internally rather than the ?tid from the underlying thread implementation?

[Mark Kettenis <mark.kettenis@xs4all.nl>]

> From: Michael Snyder <msnyder@specifix.com>
> Date: Mon, 03 Mar 2008 12:37:58 -0800
> 
> On Mon, 2008-03-03 at 21:19 +0100, Mark Kettenis wrote:
> > > From: "Michael Snyder" <msnyder@specifix.com>
> > > Date: Mon, 03 Mar 2008 11:57:08 -0800
> > > 
> > > On Mon, 2008-03-03 at 10:38 -0800, Martin Fouts wrote:
> [...]
> > > > Or suggestions about how to accommodate M:N without zombie queues?
> > > 
> > > Have you looked at the linux and solaris implementations?
> > > They both have M:N thread models.
> > 
> > Linux doesn't.  And even Solaris uses a 1:1 model by default nowadays.
> > And I really doubt the code for M:N ever worked properly in GDB.
> 
> Really?  It was never perfect, but I always thought of M:N on
> solaris as "mostly working"...
> 
> > 
> > I'm afraid that implementing GDB support for an M:N threading model is
> > a largely unsolved problem.
> > 
> > Given the fact that GDB doesn't even have support for kernel-level
> > threads (or LWP's) on NetBSD, I'd start with getting that working.
> > After that, it might be possible by implementing an additional stratum
> > on top of that, that does the LWP to user-level thread ID translation,
> > and adds in the threads that are not bound to an LWP.
> 
> Out of curiosity, how different are the thread models 
> between netbsd and freebsd?  I've noticed that there
> seem to be at least two different models for debugging
> threads on freebsd, but I'm not up to speed on it.

The code we have in GDB is for the old FreeBSD 4 userlevel threads
library, which you could call N:1 threading.  As far as I know FreeBSD
tried to do both M:N and 1:1 in FreeBSD 5, but I think M:N was more or
less abandoned because they couldn't get it to work properly.

I'm not sure what the NetBSD folks are up to these days.  Given the
history of FreeBSD, Linux and Solaris, I'd advise them to stick to
1:1, or at least get that working first before trying M:N.

OpenBSD has the beginnings of a 1:1 model, but it is not enabled by
default.  The official threads library on OpenBSD is based on the same
code as the old FreeBSD userlevel threads library (N:1).  This one is
supported by the same code in GDB.