Re: single-stepping and internal breakpoints on a multi-threaded program

[Michael Snyder <Michael dot Snyder at access-company dot com>]

On Fri, 2007-04-06 at 15:12 -0700, Jim Blandy wrote:
> Emi SUZUKI <emi-suzuki@tjsys.co.jp> writes:
> > The attached program makes two sub threads, and we do single-stepping
> > on one of them.  
> > When the target get stopped by a internal breakpoint hit (ex. thread
> > creation, thread death, shared library loading) during singe-stepping,
> > GDB unconditionally resume all the thread and it result in running
> > over the range of single-stepping.  
> 
> I'm able to reproduce this problem on my FC6 x86 dual core system.
> Thanks very much for the clear reproduction instructions.
> 
> > I saw that the context of single-stepping thread is preserved by
> > context_switch called in handle_inferior_event, but GDB doesn't go
> > back to that context till the thread is trapped in another reason
> > (in the above case, the death of the thread).  
> >
> > I think it should either stop immediately and prompt the user that the
> > single-stepping has been cancelled, or goes back to the preserved
> > context somehow and continue single-stepping.  
> > How do you think of it? 
> 
> So, if I'm understanding correctly, we single-step the 'counter'
> thread, which proceeds normally until the 'no_counter' thread hits its
> TD_CREATE event, to tell GDB about its existence.  Then, GDB fails to
> resume single-stepping the 'counter' thread, letting it run until it
> exits and reports its TD_DEATH event.
> 
> The fact that the inferior uses breakpoints to report thread creation
> and thread death is supposed to be an internal detail of the thread
> library --- or, more properly, of the thread debugging interface.  One
> thread stepping shouldn't be affected by other threads being created
> (although GDB should still print the 'new thread' messages).
> 
> So I think the right behavior would be for GDB to continue
> single-stepping after processing creation and death events for other
> threads.
> 
> I have forgetten exactly how GDB handles multi-threaded single
> stepping; I'm afraid I can't suggest how to do this.  If you're unable
> to fix it yourself, please file a bug report, and include your test
> program.

I'm thinking that gdb just loses the "I'm stepping this thread" 
bit of state info, which is normally handled in "proceed" and 
stored in the infrun-state struct - whereupon it just continues
both threads without stepping.


But that's just a hunch.