perf: Fix PERF_FORMAT_GROUP scale info

As Corey reported, the total_enabled and total_running times
could occasionally be 0, even though there were events counted.

It turns out this is because we record the times before reading
the counter while the latter updates the times.

This patch corrects that.

While looking at this code I found that there is a lot of
locking iffyness around, the following patches correct most of
that.
Reported-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: <20091120212508.685559857@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_event.c

@@ -1784,30 +1784,15 @@ u64 perf_event_read_value(struct perf_event *event)
 }
 EXPORT_SYMBOL_GPL(perf_event_read_value);
 
-static int perf_event_read_entry(struct perf_event *event,
-				   u64 read_format, char __user *buf)
-{
-	int n = 0, count = 0;
-	u64 values[2];
-
-	values[n++] = perf_event_read_value(event);
-	if (read_format & PERF_FORMAT_ID)
-		values[n++] = primary_event_id(event);
-
-	count = n * sizeof(u64);
-
-	if (copy_to_user(buf, values, count))
-		return -EFAULT;
-
-	return count;
-}
-
 static int perf_event_read_group(struct perf_event *event,
 				   u64 read_format, char __user *buf)
 {
 	struct perf_event *leader = event->group_leader, *sub;
-	int n = 0, size = 0, err = -EFAULT;
-	u64 values[3];
+	int n = 0, size = 0, ret = 0;
+	u64 values[5];
+	u64 count;
+
+	count = perf_event_read_value(leader);
 
 	values[n++] = 1 + leader->nr_siblings;
 	if (read_format & PERF_FORMAT_TOTAL_TIME_ENABLED) {
@@ -1818,28 +1803,33 @@ static int perf_event_read_group(struct perf_event *event,
 		values[n++] = leader->total_time_running +
 			atomic64_read(&leader->child_total_time_running);
 	}
+	values[n++] = count;
+	if (read_format & PERF_FORMAT_ID)
+		values[n++] = primary_event_id(leader);
 
 	size = n * sizeof(u64);
 
 	if (copy_to_user(buf, values, size))
 		return -EFAULT;
 
-	err = perf_event_read_entry(leader, read_format, buf + size);
-	if (err < 0)
-		return err;
-
-	size += err;
+	ret += size;
 
 	list_for_each_entry(sub, &leader->sibling_list, group_entry) {
-		err = perf_event_read_entry(sub, read_format,
-				buf + size);
-		if (err < 0)
-			return err;
+		n = 0;
 
-		size += err;
+		values[n++] = perf_event_read_value(sub);
+		if (read_format & PERF_FORMAT_ID)
+			values[n++] = primary_event_id(sub);
+
+		size = n * sizeof(u64);
+
+		if (copy_to_user(buf + size, values, size))
+			return -EFAULT;
+
+		ret += size;
 	}
 
-	return size;
+	return ret;
 }
 
 static int perf_event_read_one(struct perf_event *event,