md: Fix md_seq_ops() regressions

Commit cf1b6d4 ("md: simplify md_seq_ops") introduce following
regressions:

1) If list all_mddevs is emptly, personalities and unused devices won't
   be showed to user anymore.
2) If seq_file buffer overflowed from md_seq_show(), then md_seq_start()
   will be called again, hence personalities will be showed to user
   again.
3) If seq_file buffer overflowed from md_seq_stop(), seq_read_iter()
   doesn't handle this, hence unused devices won't be showed to user.

Fix above problems by printing personalities and unused devices in
md_seq_show().

Fixes: cf1b6d4 ("md: simplify md_seq_ops")
Cc: stable@vger.kernel.org # v6.7+
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240109133957.2975272-1-yukuai1@huaweicloud.com

Author: YuKuai-huawei

drivers/md/md.c

@@ -8135,6 +8135,19 @@ static void status_unused(struct seq_file *seq)
 	seq_printf(seq, "\n");
 }
 
+static void status_personalities(struct seq_file *seq)
+{
+	struct md_personality *pers;
+
+	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");
+}
+
 static int status_resync(struct seq_file *seq, struct mddev *mddev)
 {
 	sector_t max_sectors, resync, res;
@@ -8276,20 +8289,10 @@ 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 md_personality *pers;
-
-	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);
 
-	return seq_list_start(&all_mddevs, *pos);
+	return seq_list_start_head(&all_mddevs, *pos);
 }
 
 static void *md_seq_next(struct seq_file *seq, void *v, loff_t *pos)
@@ -8300,16 +8303,23 @@ static void *md_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 static void md_seq_stop(struct seq_file *seq, void *v)
 	__releases(&all_mddevs_lock)
 {
-	status_unused(seq);
 	spin_unlock(&all_mddevs_lock);
 }
 
 static int md_seq_show(struct seq_file *seq, void *v)
 {
-	struct mddev *mddev = list_entry(v, struct mddev, all_mddevs);
+	struct mddev *mddev;
 	sector_t sectors;
 	struct md_rdev *rdev;
 
+	if (v == &all_mddevs) {
+		status_personalities(seq);
+		if (list_empty(&all_mddevs))
+			status_unused(seq);
+		return 0;
+	}
+
+	mddev = list_entry(v, struct mddev, all_mddevs);
 	if (!mddev_get(mddev))
 		return 0;
 
@@ -8385,6 +8395,10 @@ static int md_seq_show(struct seq_file *seq, void *v)
 	}
 	spin_unlock(&mddev->lock);
 	spin_lock(&all_mddevs_lock);
+
+	if (mddev == list_last_entry(&all_mddevs, struct mddev, all_mddevs))
+		status_unused(seq);
+
 	if (atomic_dec_and_test(&mddev->active))
 		__mddev_put(mddev);