Re: Backward compatibility for insn probe point

[Roland McGrath <roland@redhat.com>]

Ok, that is on the verge of ringing a bell.

The single-step trap hit resets the single-step bit in the registers
(arch/powerpc/kernel/traps.c:single_step_exception).  That needs to be
turned on again before resuming.  The only place that this happens is
utrace_quiescent->tracehook_enable_single_step.  You should get there via
check_quiescent after each event report when UTRACE_ACTION_STATE_* is set.

It makes sense that ptrace does not see the same problem.  It always stops
after each step trap, so it surely goes into utrace_quiescent to stop;
there it will properly re-enable stepping when it gets resumed.  In the
itrace scenario, you don't stop, so it's only the (apparently broken)
bookkeeping that should ensure you get there.

In a reporting loop, update_action should be keeping UTRACE_ACTION_SINGLESTEP
in its return value, so that check_quiescent see it and calls utrace_quiescent.
You can see if some of that is going wrong.

(This is all entirely different in modern utrace.)


Thanks,
Roland