Re: [rfc][2/2] Signal delivery + software single-step is broken

["Ulrich Weigand" <uweigand at de dot ibm dot com>]

Pedro Alves wrote:
> On Wednesday 19 January 2011 09:42:49, Ulrich Weigand wrote:
> > The patch below changes linux_nat_wait_1 to actually look at the stepping
> > status of the thread directly, instead of relying on the "step" flag.
> > This means the "currently_stepping" routine has to be exported from
> > infrun.c so it can be called here.
> 
> I'm not objecting, but I'm curious on whether you've thought about how
> the same problem would be solved in gdbserver/linux-low.c, which
> can't call currently_stepping, since it's running in a different process?

Good point, however this test already must use information only available
in GDB itself, namely whether or not the signal is to be passed to the
inferior or intercepted by GDB:

     if (!lp->step
          && inf->control.stop_soon == NO_STOP_QUIETLY
          && signal_stop_state (signo) == 0
          && signal_print_state (signo) == 0
          && signal_pass_state (signo) == 1)

(The "stop_soon" state likewise is GDB private data.)

Presumably because of this, gdbserver today does not appear to be
implementing this particular optimization at all, but always reports
all signals back to GDB to decide what to do with them.

> One way to do it would be to do:
> 
>  QPassSignals:
>  vCont;c
>  QPassSignals:foo;bar
> 
> but that is a lot of extra roundtrips, and not really (inferior) threadsafe in 
> non-stop mode.

I agree that this would probably be the way to go about it.  I'm not sure
thread safety is really a concern here, given that we're talking about an
optimization.  If the implementation is conservative in the right direction,
the worst thing that could happen is that a signal is reported that might
have gotten short-circuited ..

Similarly, the number of roundtrips could probably be reduced by only
sending a QPassSignals when the list of interesting signal changes.
For example, once we start single-stepping, we'd once send the 
  QPassSignals:
and then not send and further QPassSignals until we go back to letting
the inferior continue freely.  (And even then, we might further delay
sending of the QPassSignals until such time as we notice we're actually
seeing many to-be-passed signals being delivered to GDB, at which point
switching on the optimization is actually worthwhile.)

> It sounds like we'd need to tweak the target resume interface to be
> able to say "continue, but I'm interested in signals and everything else",
> or, "I'm telling you to continue, but you're really single-stepping",
> like a new vCont;cs or some such?

I'm not so sure I like this, as it introduces somewhat less well-
defined semantics: what does "*really* single-stepping" mean, other
than in terms of doing whatever it is GDB does now ...

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU Toolchain for Linux on System z and Cell BE
  Ulrich.Weigand@de.ibm.com