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

[Jim Blandy <jimb at codesourcery dot com>]

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.