perf_counter: uncouple data_head updates from wakeups

Keep data_head up-to-date irrespective of notifications. This fixes
the case where you disable a counter and don't get a notification for
the last few pending events, and it also allows polling usage.

[ Impact: increase precision of perfcounter mmap-ed fields ]

Suggested-by: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <20090505155436.925084300@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/linux/perf_counter.h

@@ -362,9 +362,11 @@ struct perf_mmap_data {
 	atomic_t			head;		/* write position    */
 	atomic_t			events;		/* event limit       */
 
-	atomic_t			wakeup_head;	/* completed head    */
+	atomic_t			done_head;	/* completed head    */
 	atomic_t			lock;		/* concurrent writes */
 
+	atomic_t			wakeup;		/* needs a wakeup    */
+
 	struct perf_counter_mmap_page   *user_page;
 	void 				*data_pages[0];
 };