s390/vtime: correct idle time calculation

Use the ACCESS_ONCE macro for both accesses to idle->sequence in the
loops to calculate the idle time. If only one access uses the macro,
the compiler is free to cache the value for the second access which
can cause endless loops.

Cc: stable@vger.kernel.org # 3.6+
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/smp.c

@@ -920,7 +920,7 @@ static ssize_t show_idle_count(struct device *dev,
 		idle_count = ACCESS_ONCE(idle->idle_count);
 		if (ACCESS_ONCE(idle->clock_idle_enter))
 			idle_count++;
-	} while ((sequence & 1) || (idle->sequence != sequence));
+	} while ((sequence & 1) || (ACCESS_ONCE(idle->sequence) != sequence));
 	return sprintf(buf, "%llu\n", idle_count);
 }
 static DEVICE_ATTR(idle_count, 0444, show_idle_count, NULL);
@@ -938,7 +938,7 @@ static ssize_t show_idle_time(struct device *dev,
 		idle_time = ACCESS_ONCE(idle->idle_time);
 		idle_enter = ACCESS_ONCE(idle->clock_idle_enter);
 		idle_exit = ACCESS_ONCE(idle->clock_idle_exit);
-	} while ((sequence & 1) || (idle->sequence != sequence));
+	} while ((sequence & 1) || (ACCESS_ONCE(idle->sequence) != sequence));
 	idle_time += idle_enter ? ((idle_exit ? : now) - idle_enter) : 0;
 	return sprintf(buf, "%llu\n", idle_time >> 12);
 }

arch/s390/kernel/vtime.c

@@ -191,7 +191,7 @@ cputime64_t s390_get_idle_time(int cpu)
 		sequence = ACCESS_ONCE(idle->sequence);
 		idle_enter = ACCESS_ONCE(idle->clock_idle_enter);
 		idle_exit = ACCESS_ONCE(idle->clock_idle_exit);
-	} while ((sequence & 1) || (idle->sequence != sequence));
+	} while ((sequence & 1) || (ACCESS_ONCE(idle->sequence) != sequence));
 	return idle_enter ? ((idle_exit ?: now) - idle_enter) : 0;
 }