On Thu, Jan 08, 2015 at 10:22:37AM -0500, Vince Weaver wrote:
> Hello,
>
> I was working on improving the manpage by working out how the
> perf_event_open() PERF_FLAG_FD_OUTPUT flag works.
>
> It turns out it doesn't.
>
> In kernel/events/core.c when opening a file this is done:
>
> if (output_event) {
> err = perf_event_set_output(event, output_event);
> if (err)
> goto err_context;
> }
>
> and perf_event_set_output() does:
>
> if (output_event->cpu == -1 && output_event->ctx !=
> event->ctx) {
> goto out;
> }
>
> But event->ctx is always NULL; it does not get set to ctx until
> after the call to perf_event_set_output().
>
> It looks like this was broken back in 2.6.35 days with the change:
>
> commit ac9721f3f54b27a16c7e1afb2481e7ee95a70318
> Author: Peter Zijlstra <a.p.zijls...@chello.nl>
> Date: Thu May 27 12:54:41 2010 +0200
>
> perf_events: Fix races and clean up perf_event and
> perf_mmap_data interaction
>
> So is this worth fixing seeing as apparently no one uses this feature?
I think there's a fair argument for removing it, Ingo, Acme?
> As an aside, added error reporting is *really* needed as I had to sprinkle
> all of events/core.c with printk() calls to even begin to understand why
> I was getting the EINVAL result.
Yes, let me see if I can find someone to work on that :/
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