On Tue, Jul 15, 2025 at 02:06:50PM -0400, Steven Rostedt wrote:
> On Tue, 15 Jul 2025 08:49:32 -0400
> Steven Rostedt <rost...@goodmis.org> wrote:
>
> > > > *
> > > > - * Return: 1 if the the callback was already queued.
> > > > - * 0 if the callback successfully was queued.
> > > > + * Return: 0 if the callback successfully was queued.
> > > > + * UNWIND_ALREADY_PENDING if the the callback was already
> > > > queued.
> > > > + * UNWIND_ALREADY_EXECUTED if the callback was already called
> > > > + * (and will not be called again)
> > > > * Negative if there's an error.
> > > > * @cookie holds the cookie of the first request by any user
> > > > */
> > >
> > > Lots of babbling in the Changelog, but no real elucidation as to why you
> > > need this second return value.
> > >
> > > AFAICT it serves no real purpose; the users of this function should not
> > > care. The only difference is that the unwind reference (your cookie)
> > > becomes a backward reference instead of a forward reference. But why
> > > would anybody care?
> >
> > Older versions of the code required it. I think I can remove it now.
>
> Ah it is still used in the perf code:
>
> perf_callchain() has:
>
> if (defer_user) {
> int ret = deferred_request(event);
> if (!ret)
> local_inc(&event->ctx->nr_no_switch_fast);
> else if (ret < 0)
> defer_user = false;
> }
>
> Where deferred_requests() is as static function that returns the result
> of the unwind request. If it is zero, it means the callback will be
> called, if it is greater than zero it means it has already been called,
> and negative is an error (and use the old method).
>
> It looks like when the callback is called it expects nr_no_switch_fast
> to be incremented and it will decrement it. This is directly from
> Josh's patch and I don't know perf well enough to know if that update
> to nr_no_switch_fast is needed.
>
> If it's not needed, we can just return 0 on success and negative on
> failure. What do you think?
I'm yet again confused. I don't see this code differentiate between 1
and 2 return values (those PENDING and EXECUTED).
Anyway, fundamentally I don't think there is a problem with backward
references as opposed to the normal forward references.
So leave it out for now.
