Re: [RFA] Fix watchpoints when stepping over a breakpoint

Daniel Jacobowitz
2002-04-06T17:49:08Z

On Sat, Apr 06, 2002 at 08:17:25PM +0300, Eli Zaretskii wrote:
> > > If this is limited to stepping, can we check whether we are stepping
> > > instead of (or in addition to) the test for whether to ignore
> > > breakpoints?
> >
> > Well, I set the ignore breakpoints flag in the caller only if we are
> > stepping.
>
> Isn't it better to make bpstat_stop_status test for stepping directly?

The test I'm using is 'trap_expected' - well:
if (stop_signal == TARGET_SIGNAL_TRAP && trap_expected
&& through_sigtramp_breakpoint == NULL)

It's static to infrun.c, and it describes the precise circumstances in
which we want this behavior. We can ignore stop_signal (since we only
call bpstat_stop_status if we got a trap), but I have no idea how
through_sigtramp_breakpoint works. I'd rather calculate this at the
call site than duplicating logic I don't understand very well in
breakpoint.c (which has no business knowing about the internals of
infrun).

> > No. In my original message I made a comment about shlib_event
> > breakpoints being a problem. Other breakpoints would to. This is all
> > because of the "watchpoint after instr, breakpoint before" thing - we
> > would still have to deal with this, or we'd just keep hitting the same
> > breakpoint over and over if there was a watchpoint on the next
> > instruction.
>
> Sounds like Andrew was right: the decrement-PC logic is screwed.

I agree. It's a rat's nest.

> > But something more fundamental is wrong, because we never
> > stop -at all-. I remember something involving initializing the
> > watchpoint registers...
>
> I thought that part was solved, but perhaps I'm confused.

So did I, but it does not appear to be the case. Even access
watchpoints die. At a guess, in the good case:

insert_watchpoint (addr=8049574, len=4, type=data-read/write):
CONTROL (DR7): 000f0101 STATUS (DR6): ffff4ff1
DR0: addr=0x08049574, ref.count=1 DR1: addr=0x00000000, ref.count=0
DR2: addr=0x00000000, ref.count=0 DR3: addr=0x00000000, ref.count=0

In the bad case:

insert_watchpoint (addr=8049574, len=4, type=data-read/write):
CONTROL (DR7): 000f0101 STATUS (DR6): ffff4ff0
DR0: addr=0x08049574, ref.count=2 DR1: addr=0x00000000, ref.count=0
DR2: addr=0x00000000, ref.count=0 DR3: addr=0x00000000, ref.count=0

insert_watchpoint (addr=8049574, len=4, type=data-read/write):
CONTROL (DR7): 000f0101 STATUS (DR6): ffff4ff0
DR0: addr=0x08049574, ref.count=3 DR1: addr=0x00000000, ref.count=0
DR2: addr=0x00000000, ref.count=0 DR3: addr=0x00000000, ref.count=0

The watchpoint is not removed after program normal exit, and the refcount escalates.

--
Daniel Jacobowitz Carnegie Mellon University
MontaVista Software Debian GNU/Linux Developer