md: simplify md_seq_ops

Before this patch, the implementation is hacky and hard to understand:

1) md_seq_start set pos to 1;
2) md_seq_show found pos is 1, then print Personalities;
3) md_seq_next found pos is 1, then it update pos to the first mddev;
4) md_seq_show found pos is not 1 or 2, show mddev;
5) md_seq_next found pos is not 1 or 2, update pos to next mddev;
6) loop 4-5 until the last mddev, then md_seq_next update pos to 2;
7) md_seq_show found pos is 2, then print unused devices;
8) md_seq_next found pos is 2, stop;

This patch remove the magic value and use seq_list_start/next/stop()
directly, and move printing "Personalities" to md_seq_start(),
"unsed devices" to md_seq_stop():

1) md_seq_start print Personalities, and then set pos to first mddev;
2) md_seq_show show mddev;
3) md_seq_next update pos to next mddev;
4) loop 2-3 until the last mddev;
5) md_seq_stop print unsed devices;
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230927061241.1552837-3-yukuai1@huaweicloud.com

drivers/md/md.c

@@ -8213,105 +8213,46 @@ static int status_resync(struct seq_file *seq, struct mddev *mddev)
 }
 
 static void *md_seq_start(struct seq_file *seq, loff_t *pos)
+	__acquires(&all_mddevs_lock)
 {
-	struct list_head *tmp;
-	loff_t l = *pos;
-	struct mddev *mddev;
+	struct md_personality *pers;
 
-	if (l == 0x10000) {
-		++*pos;
-		return (void *)2;
-	}
-	if (l > 0x10000)
-		return NULL;
-	if (!l--)
-		/* header */
-		return (void*)1;
+	seq_puts(seq, "Personalities : ");
+	spin_lock(&pers_lock);
+	list_for_each_entry(pers, &pers_list, list)
+		seq_printf(seq, "[%s] ", pers->name);
+
+	spin_unlock(&pers_lock);
+	seq_puts(seq, "\n");
+	seq->poll_event = atomic_read(&md_event_count);
 
 	spin_lock(&all_mddevs_lock);
-	list_for_each(tmp,&all_mddevs)
-		if (!l--) {
-			mddev = list_entry(tmp, struct mddev, all_mddevs);
-			if (!mddev_get(mddev))
-				continue;
-			spin_unlock(&all_mddevs_lock);
-			return mddev;
-		}
-	spin_unlock(&all_mddevs_lock);
-	if (!l--)
-		return (void*)2;/* tail */
-	return NULL;
+
+	return seq_list_start(&all_mddevs, *pos);
 }
 
 static void *md_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 {
-	struct list_head *tmp;
-	struct mddev *next_mddev, *mddev = v;
-	struct mddev *to_put = NULL;
-
-	++*pos;
-	if (v == (void*)2)
-		return NULL;
-
-	spin_lock(&all_mddevs_lock);
-	if (v == (void*)1) {
-		tmp = all_mddevs.next;
-	} else {
-		to_put = mddev;
-		tmp = mddev->all_mddevs.next;
-	}
-
-	for (;;) {
-		if (tmp == &all_mddevs) {
-			next_mddev = (void*)2;
-			*pos = 0x10000;
-			break;
-		}
-		next_mddev = list_entry(tmp, struct mddev, all_mddevs);
-		if (mddev_get(next_mddev))
-			break;
-		mddev = next_mddev;
-		tmp = mddev->all_mddevs.next;
-	}
-	spin_unlock(&all_mddevs_lock);
-
-	if (to_put)
-		mddev_put(to_put);
-	return next_mddev;
-
+	return seq_list_next(v, &all_mddevs, pos);
 }
 
 static void md_seq_stop(struct seq_file *seq, void *v)
+	__releases(&all_mddevs_lock)
 {
-	struct mddev *mddev = v;
-
-	if (mddev && v != (void*)1 && v != (void*)2)
-		mddev_put(mddev);
+	status_unused(seq);
+	spin_unlock(&all_mddevs_lock);
 }
 
 static int md_seq_show(struct seq_file *seq, void *v)
 {
-	struct mddev *mddev = v;
+	struct mddev *mddev = list_entry(v, struct mddev, all_mddevs);
 	sector_t sectors;
 	struct md_rdev *rdev;
 
-	if (v == (void*)1) {
-		struct md_personality *pers;
-		seq_printf(seq, "Personalities : ");
-		spin_lock(&pers_lock);
-		list_for_each_entry(pers, &pers_list, list)
-			seq_printf(seq, "[%s] ", pers->name);
-
-		spin_unlock(&pers_lock);
-		seq_printf(seq, "\n");
-		seq->poll_event = atomic_read(&md_event_count);
+	if (!mddev_get(mddev))
 		return 0;
-	}
-	if (v == (void*)2) {
-		status_unused(seq);
-		return 0;
-	}
 
+	spin_unlock(&all_mddevs_lock);
 	spin_lock(&mddev->lock);
 	if (mddev->pers || mddev->raid_disks || !list_empty(&mddev->disks)) {
 		seq_printf(seq, "%s : %sactive", mdname(mddev),
@@ -8382,6 +8323,9 @@ static int md_seq_show(struct seq_file *seq, void *v)
 		seq_printf(seq, "\n");
 	}
 	spin_unlock(&mddev->lock);
+	spin_lock(&all_mddevs_lock);
+	if (atomic_dec_and_test(&mddev->active))
+		__mddev_put(mddev);
 
 	return 0;
 }