Re: Multithreaded debugging: strange thread switches

Daniel Jacobowitz
2006-01-24T15:02:13Z

On Tue, Jan 24, 2006 at 05:41:44PM +0300, Vladimir Prus wrote:
> > Well, GDB doesn't have any command that it expects to have that
> > behavior. If you want GDB to do that, I recommend adding such an
> > interface. From what I can see, you can get the same effect by "single
> > step all threads in sync, repeatedly, until this thread reaches the
> > next source line", which is "vCont;s",
>
> Generally speaking if 2 threads execute 10 steps each, they no longer are at
> the same time, because instructions in thread 1 can take one cycle each,
> while instructions in thread can take two cycles each. I'd prefer this
> synchronization logic to be inside my remote part, which is specifialized for
> that task.

I think that adds weight to my argument that the existing GDB commands
don't correspond to this action :-)

There's a documented (but never implemented, as far as I know) 'i'
packet to step a single cycle. And the vCont mechanism is extensible
for new continue operations (deliberately). So you could make GDB do
exactly what you want in at least two ways:

1. vCont;TID:s until at the next line
query time
vCont;TID:0;i until times match (where 0 means "stopped" and "i"
means "one cycle")

2. vCont;i until at the next line

I'd recommend #2 unless you have some reason not to do that.

> > or possibly "Hc-1" "s" - not
> > completely sure about the last one, the documentation suggests that's
> > right, but I don't know any stub that handles either of these
> > correctly.
> >
> > > I see. But after we've single-stepped over breakpoint, will we switch
> > > back to the thread where "next" was issued?
> >
> > At the moment, no. This is definitely a bug but it's a pretty nasty
> > infrun limitation; really that whole subsystem needs some love and
> > attention.
>
> Is that bug filed in the bug tracker? Should I file it?

I don't know; you might as well. I think the case described above
might actually work, but there are definitely some gotchas in this
area.

> But if I'm looking at Changelog entry, how can I guess the subject of email
> that discusses this change?

You have the date; in practice, it never takes me more than a couple of
minutes.

Please, take further discussion of the ChangeLog convention to a more
appropriate forum - it's the GNU standard.

--
Daniel Jacobowitz
CodeSourcery