[PATCH 0/3] UTRACE_DETACH fixes

[Oleg Nesterov]

On 09/03, Roland McGrath wrote:
>
> > > So, it is technically kosher enough to use UTRACE_INTERRUPT without
> > > UTRACE_EVENT(QUIESCE) set, if you really know the situation and are sure
> > > about all those caveats above.
> >
> > How? see below.
>
> I mentioned the examples.

I misunderstood the "UTRACE_INTERRUPT without UTRACE_EVENT(QUIESCE)" above
as if you mean it is possible to get UTRACE_SIGNAL_REPORT without QUIESCE.

> > > But, it's only UTRACE_EVENT(QUIESCE) that
> > > gives you any expectation of an "extra" callback for "no event" from either
> > > UTRACE_REPORT or UTRACE_INTERRUPT.
> >
> > Yes, this is clear too.
>
> Apparently not. ;-)

Perhaps ;) At least I certainly missed your point.

> QUIESCE is the only event type for "no event".  If you only asked for
> signal events, then you only get a callback when there is an actual signal
> event.  If you want an extra callback before dequeuing any signal, then
> that is what QUIESCE is for.

OK. This means ugdb should set QUIESCE, just to ensure its ->report_signal()
will be called.

> > Once again, I am not asking to change utrace now, but could you explain
> > what is wrong with the patch below?
>
> That gives an extra unrequested report_signal callback to every engine that
> is only interested in some signal event.  Consider a "crash-catcher"
> engine.  It would only track UTRACE_EVENT(SIGNAL_CORE) (and perhaps CLONE
> for its bookkeeping).  This engine never asked for a callback before each
> and every attempt to dequeue any signal,

I see your point (I hope).

> which is what you would give it
> with your change.

No ;) My change was wrong. event == 0 in this case, and then later
utrace_get_signal() checks

	if ((want & (event | UTRACE_EVENT(QUIESCE))) == 0)
		continue;



OK, please forget. I must admit, this still looks a bit, well, unnatural
to me. May be it makes sense to add

	_UTRACE_EVENT_SIGNAL_REPORT,
	_UTRACE_EVENT_SIGNAL_HANDLER,

into utrace_events. Then ugdb could ask for UTRACE_SIGNAL_REPORT and
avoid the unnecessary ->report_quiesce() calls.

But at least I can see the logic now, thanks.



Roland, could you also comment this patch?

	https://www.redhat.com/archives/utrace-devel/2010-August/msg00108.html

Again, this looks like a bug to me, but I won't insist if it is not.

Oleg.