perf: behavior of poll() changed in 3.18

[Vince Weaver]

Hello

Some of my code that used poll() on a perf_event fd broke sometime between 
3.17 and current.  (My perf_event_test validation testsuite caught this
too but I haven't been running it as regularly as I should).

I bisected this to:

	commit 179033b3e064d2cd3f5f9945e76b0a0f0fbf4883
	Author: Jiri Olsa <jolsa@xxxxxxxxxx>
	Date:   Thu Aug 7 11:48:26 2014 -0400

	perf: Add PERF_EVENT_STATE_EXIT state for events with exited task


Before, when polling, my code would get a
	result=0/WIFEXITED()
return when a child with events being polled exited.

Now instead I get a 
	result=1/POLLHUP
return.

Needless to say this broke my code.  Is there a reason this change was 
made?

Vince

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