md: fix soft lockup in status_resync

status_resync() will calculate 'curr_resync - recovery_active' to show
user a progress bar like following:

[============>........]  resync = 61.4%

'curr_resync' and 'recovery_active' is updated in md_do_sync(), and
status_resync() can read them concurrently, hence it's possible that
'curr_resync - recovery_active' can overflow to a huge number. In this
case status_resync() will be stuck in the loop to print a large amount
of '=', which will end up soft lockup.

Fix the problem by setting 'resync' to MD_RESYNC_ACTIVE in this case,
this way resync in progress will be reported to user.
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230310073855.1337560-3-yukuai1@huaweicloud.com

drivers/md/md.c

@@ -8032,16 +8032,16 @@ static int status_resync(struct seq_file *seq, struct mddev *mddev)
 	} else if (resync > max_sectors) {
 		resync = max_sectors;
 	} else {
-		resync -= atomic_read(&mddev->recovery_active);
-		if (resync < MD_RESYNC_ACTIVE) {
-			/*
-			 * Resync has started, but the subtraction has
-			 * yielded one of the special values. Force it
-			 * to active to ensure the status reports an
-			 * active resync.
-			 */
+		res = atomic_read(&mddev->recovery_active);
+		/*
+		 * Resync has started, but the subtraction has overflowed or
+		 * yielded one of the special values. Force it to active to
+		 * ensure the status reports an active resync.
+		 */
+		if (resync < res || resync - res < MD_RESYNC_ACTIVE)
 			resync = MD_RESYNC_ACTIVE;
-		}
+		else
+			resync -= res;
 	}
 
 	if (resync == MD_RESYNC_NONE) {