perf_counter: update mmap() counter read, take 2
Update the userspace read method. Paul noted that: - userspace cannot observe ->lock & 1 on the same cpu. - we need a barrier() between reading ->lock and ->index to ensure we read them in that prticular order. Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl> Cc: Paul Mackerras <paulus@samba.org> Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com> LKML-Reference: <20090406094517.368446033@chello.nl> Signed-off-by: NIngo Molnar <mingo@elte.hu>
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