perf: PERF_FLAG_FD_OUTPUT has been broken since 2.6.35

[Vince Weaver]

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.zijlstra@xxxxxxxxx>
		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?

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.

Vince
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