perf: race with automatic rdpmc() disabling

[Vince Weaver]

Hello

I've been trying to track this issue down for a few days and haven't been 
able to isolate it.  So maybe someone who understands low-level perf mmap
reference counting can help here.

As you might recall, 7911d3f7af14a614617e38245fedf98a724e46a9
introduced automatic disabling of userspace rdpmc when no perf_events 
were running.

I've run into a problem with PAPI when using rdpmc.  If you have PAPI
measuring events in multiple pthread threads, sometimes (but not always)
the program will GPF because CR4/rdpmc gets turned off while events are 
still active.

I've been trying to put together a reproducible test case but haven't been 
able to manage.  I have a PAPI test that will show the problem about 
50% of the time but I can't seem to isolate the problem.

Any ideas?

If you really want to try to reproduce it, get the current git version of 
PAPI
	git clone https://bitbucket.org/icl/papi.git
edit src/components/perf_event/perf_event.c 
	so that #define PERF_USE_RDPMC 1
in src run ./configure , make
then run the ./ctests/zero_pthreads test a few times.  It will GPF and I'm
relatively (though not entirely) sure it's not a PAPI issue.  
The problem does go away if you set /sys/devices/cpu/rdpmc to 2

Vince