perf_counter: Fix throttling lock-up

Throttling logic is broken and we can lock up with too small
hw sampling intervals.

Make the throttling code more robust: disable counters even
if we already disabled them.

( Also clean up whitespace damage i noticed while reading
  various pieces of code related to throttling. )

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
LKML-Reference: <new-submission>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/perf_counter.c

@@ -2822,13 +2822,22 @@ int perf_counter_overflow(struct perf_counter *counter,
 
 	if (!throttle) {
 		counter->hw.interrupts++;
-	} else if (counter->hw.interrupts != MAX_INTERRUPTS) {
+	} else {
+		if (counter->hw.interrupts != MAX_INTERRUPTS) {
 			counter->hw.interrupts++;
 			if (HZ*counter->hw.interrupts > (u64)sysctl_perf_counter_limit) {
 				counter->hw.interrupts = MAX_INTERRUPTS;
 				perf_log_throttle(counter, 0);
 				ret = 1;
 			}
+		} else {
+			/*
+			 * Keep re-disabling counters even though on the previous
+			 * pass we disabled it - just in case we raced with a
+			 * sched-in and the counter got enabled again:
+			 */
+			ret = 1;
+		}
 	}
 
 	/*