Re: Questionable breakpoint stepping code

Michael Snyder
2007-11-26T19:01:47Z

On Fri, 2007-11-23 at 16:56 +0300, Vladimir Prus wrote:
> The infrun.c:handle_inferiour_event function has
> this code block:
>
> if (thread_hop_needed)
> {
> ........
> remove_status = remove_breakpoints ();
> /* Did we fail to remove breakpoints? If so, try
> to set the PC past the bp. (There's at least
> one situation in which we can fail to remove
> the bp's: On HP-UX's that use ttrace, we can't
> change the address space of a vforking child
> process until the child exits (well, okay, not
> then either :-) or execs. */
> if (remove_status != 0)
> {
> /* FIXME! This is obviously non-portable! */
> write_pc_pid (stop_pc + 4, ecs->ptid);
> /* We need to restart all the threads now,
> * unles we're running in scheduler-locked mode.
> * Use currently_stepping to determine whether to
> * step or continue.
> */
> /* FIXME MVS: is there any reason not to call resume()? */
> if (scheduler_mode == schedlock_on)
> target_resume (ecs->ptid,
> currently_stepping (ecs), TARGET_SIGNAL_0);
> else
> target_resume (RESUME_ALL,
> currently_stepping (ecs), TARGET_SIGNAL_0);
> prepare_to_wait (ecs);
> return;
> }
>
> The code is a bit scary -- specifically I sure don't want GDB to mess
> with PC values like this on x86, if removing breakpoints fails in any way.
> The essential bits of this code are present as of revision 1.1 of infrun.c
> (added in 1999).
>
> So:
> 1. Anybody knows if this code is still needed for modern HPUX?
> 2. Can we have it wrapped in #ifdef, and if so, which one?
>
> - Volodya

Hi Volodya,

I think it's my code. It's not really related specifically
to HPUX, that comment was there in the previous iteration and
I just kept it.

The several state variables with "thread_hop" as part of their
names are related to single-stepping in the presence of thread-
specific breakpoints. They are meant to solve the problem of
what to do if you are doing a step, and you hit a thread-specific
breakpoint, but with the wrong thread.

You need to do a kind of special single-step to get past that
particular breakpoint, then return to the single-stepping
infrun state.

As for the scheduler-locking code, that pertains to a
different but not wholly unrelated functionality (set
scheduler-locking), which affects which threads can run
at which times.

As for your last question, no, I don't believe we approve
of ifdefs...

Cheers,
Michael