[PATCH v2] md: do not _put wrong device in md_seq_next

[Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>]

During working on changes proposed by Kuai [1], I determined that
mddev->active is continusly decremented for array marked by MD_CLOSING.
It brought me to md_seq_next() changed by [2]. I determined the regression
here, if mddev_get() fails we updated mddev pointer and as a result we
_put failed device.

I isolated the change in md_seq_next() and tested it- issue is no longer
reproducible but I don't see the root cause in this scenario. The bug
is obvious so I proceed with fixing. I will submit MD_CLOSING patches
separatelly.

Put the device which has been _get with previous md_seq_next() call.
Add guard for inproper mddev_put usage(). It shouldn't be called if
there are less than 1 for mddev->active.

I didn't convert atomic_t to refcount_t because refcount warns when 0 is
achieved which is likely to happen for mddev->active.

[1] https://lore.kernel.org/linux-raid/028a21df-4397-80aa-c2a5-7c754560f595@gmail.com/T/#m6a534677d9654a4236623661c10646d45419ee1b
[2] https://bugzilla.kernel.org/show_bug.cgi?id=217798

Fixes: 12a6caf27324 ("md: only delete entries from all_mddevs when the disk is freed")
Cc: Yu Kuai <yukuai3@huawei.com>
Cc: AceLan Kao <acelan@gmail.com>
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
---
 drivers/md/md.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 0fe7ab6e8ab9..bb654ff62765 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -618,6 +618,12 @@ static void mddev_delayed_delete(struct work_struct *ws);
 
 void mddev_put(struct mddev *mddev)
 {
+	/* Guard for ambiguous put. */
+	if (unlikely(atomic_read(&mddev->active) < 1)) {
+		pr_warn("%s: active refcount is lower than 1\n", mdname(mddev));
+		return;
+	}
+
 	if (!atomic_dec_and_lock(&mddev->active, &all_mddevs_lock))
 		return;
 	if (!mddev->raid_disks && list_empty(&mddev->disks) &&
@@ -8227,19 +8233,16 @@ 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)
+	if (v == (void *)2)
 		return NULL;
 
 	spin_lock(&all_mddevs_lock);
-	if (v == (void*)1) {
+	if (v == (void *)1)
 		tmp = all_mddevs.next;
-	} else {
-		to_put = mddev;
+	else
 		tmp = mddev->all_mddevs.next;
-	}
 
 	for (;;) {
 		if (tmp == &all_mddevs) {
@@ -8250,12 +8253,11 @@ static void *md_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 		next_mddev = list_entry(tmp, struct mddev, all_mddevs);
 		if (mddev_get(next_mddev))
 			break;
-		mddev = next_mddev;
-		tmp = mddev->all_mddevs.next;
+		tmp = next_mddev->all_mddevs.next;
 	}
 	spin_unlock(&all_mddevs_lock);
 
-	if (to_put)
+	if (v != (void *)1)
 		mddev_put(mddev);
 	return next_mddev;
 
-- 
2.26.2