md: stop using for_each_mddev in md_notify_reboot

Just do a simple list_for_each_entry_safe on all_mddevs, and only grab a
reference when we drop the lock.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/md.c

@@ -9585,11 +9585,13 @@ EXPORT_SYMBOL_GPL(rdev_clear_badblocks);
 static int md_notify_reboot(struct notifier_block *this,
 			    unsigned long code, void *x)
 {
-	struct list_head *tmp;
-	struct mddev *mddev;
+	struct mddev *mddev, *n;
 	int need_delay = 0;
 
-	for_each_mddev(mddev, tmp) {
+	spin_lock(&all_mddevs_lock);
+	list_for_each_entry_safe(mddev, n, &all_mddevs, all_mddevs) {
+		mddev_get(mddev);
+		spin_unlock(&all_mddevs_lock);
 		if (mddev_trylock(mddev)) {
 			if (mddev->pers)
 				__md_stop_writes(mddev);
@@ -9598,7 +9600,11 @@ static int md_notify_reboot(struct notifier_block *this,
 			mddev_unlock(mddev);
 		}
 		need_delay = 1;
+		mddev_put(mddev);
+		spin_lock(&all_mddevs_lock);
 	}
+	spin_unlock(&all_mddevs_lock);
+
 	/*
 	 * certain more exotic SCSI devices are known to be
 	 * volatile wrt too early system reboots. While the