ocfs2: export heartbeat thread pid via configfs

The patch allows the ocfs2 heartbeat thread to prioritize I/O which may
help cut down on spurious fencing. Most of this will be in the tools -
we can have a pid configfs attribute and let userspace (ocfs2_hb_ctl)
calls the ioprio_set syscall after starting heartbeat, but only cfq
scheduler supports I/O priorities now.
Signed-off-by: Zhen Wei <zwei@novell.com>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>

fs/ocfs2/cluster/heartbeat.c

@@ -1447,6 +1447,15 @@ out:
 	return ret;
 }
 
+static ssize_t o2hb_region_pid_read(struct o2hb_region *reg,
+                                      char *page)
+{
+	if (!reg->hr_task)
+		return 0;
+
+	return sprintf(page, "%u\n", reg->hr_task->pid);
+}
+
 struct o2hb_region_attribute {
 	struct configfs_attribute attr;
 	ssize_t (*show)(struct o2hb_region *, char *);
@@ -1485,11 +1494,19 @@ static struct o2hb_region_attribute o2hb_region_attr_dev = {
 	.store	= o2hb_region_dev_write,
 };
 
+static struct o2hb_region_attribute o2hb_region_attr_pid = {
+       .attr   = { .ca_owner = THIS_MODULE,
+                   .ca_name = "pid",
+                   .ca_mode = S_IRUGO | S_IRUSR },
+       .show   = o2hb_region_pid_read,
+};
+
 static struct configfs_attribute *o2hb_region_attrs[] = {
 	&o2hb_region_attr_block_bytes.attr,
 	&o2hb_region_attr_start_block.attr,
 	&o2hb_region_attr_blocks.attr,
 	&o2hb_region_attr_dev.attr,
+	&o2hb_region_attr_pid.attr,
 	NULL,
 };