[PATCH 5.12 058/677] md: split mddev_find

[Greg Kroah-Hartman]

From: Christoph Hellwig <hch@xxxxxx>

commit 65aa97c4d2bfd76677c211b9d03ef05a98c6d68e upstream.

Split mddev_find into a simple mddev_find that just finds an existing
mddev by the unit number, and a more complicated mddev_find that deals
with find or allocating a mddev.

This turns out to fix this bug reported by Zhao Heming.

----------------------------- snip ------------------------------
commit d3374825ce57 ("md: make devices disappear when they are no longer
needed.") introduced protection between mddev creating & removing. The
md_open shouldn't create mddev when all_mddevs list doesn't contain
mddev. With currently code logic, there will be very easy to trigger
soft lockup in non-preempt env.

---
 drivers/md/md.c |   24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -736,6 +736,22 @@ EXPORT_SYMBOL_GPL(mddev_init);
 
 static struct mddev *mddev_find(dev_t unit)
 {
+	struct mddev *mddev;
+
+	if (MAJOR(unit) != MD_MAJOR)
+		unit &= ~((1 << MdpMinorShift) - 1);
+
+	spin_lock(&all_mddevs_lock);
+	mddev = mddev_find_locked(unit);
+	if (mddev)
+		mddev_get(mddev);
+	spin_unlock(&all_mddevs_lock);
+
+	return mddev;
+}
+
+static struct mddev *mddev_find_or_alloc(dev_t unit)
+{
 	struct mddev *mddev, *new = NULL;
 
 	if (unit && MAJOR(unit) != MD_MAJOR)
@@ -5644,7 +5660,7 @@ static int md_alloc(dev_t dev, char *nam
 	 * writing to /sys/module/md_mod/parameters/new_array.
 	 */
 	static DEFINE_MUTEX(disks_mutex);
-	struct mddev *mddev = mddev_find(dev);
+	struct mddev *mddev = mddev_find_or_alloc(dev);
 	struct gendisk *disk;
 	int partitioned;
 	int shift;
@@ -6524,11 +6540,9 @@ static void autorun_devices(int part)
 
 		md_probe(dev);
 		mddev = mddev_find(dev);
-		if (!mddev || !mddev->gendisk) {
-			if (mddev)
-				mddev_put(mddev);
+		if (!mddev)
 			break;
-		}
+
 		if (mddev_lock(mddev))
 			pr_warn("md: %s locked, cannot run\n", mdname(mddev));
 		else if (mddev->raid_disks || mddev->major_version