loop: set GENHD_FL_NO_PART_SCAN after blkdev_reread_part()

Commit 0da03cab87
("loop: Fix deadlock when calling blkdev_reread_part()") moves
blkdev_reread_part() out of the loop_ctl_mutex. However,
GENHD_FL_NO_PART_SCAN is set before __blkdev_reread_part(). As a result,
__blkdev_reread_part() will fail the check of GENHD_FL_NO_PART_SCAN and
will not rescan the loop device to delete all partitions.

Below are steps to reproduce the issue:

step1 # dd if=/dev/zero of=tmp.raw bs=1M count=100
step2 # losetup -P /dev/loop0 tmp.raw
step3 # parted /dev/loop0 mklabel gpt
step4 # parted -a none -s /dev/loop0 mkpart primary 64s 1
step5 # losetup -d /dev/loop0

Step5 will not be able to delete /dev/loop0p1 (introduced by step4) and
there is below kernel warning message:

[  464.414043] __loop_clr_fd: partition scan of loop0 failed (rc=-22)

This patch sets GENHD_FL_NO_PART_SCAN after blkdev_reread_part().

Fixes: 0da03cab87 ("loop: Fix deadlock when calling blkdev_reread_part()")
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -1089,16 +1089,12 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
 		kobject_uevent(&disk_to_dev(bdev->bd_disk)->kobj, KOBJ_CHANGE);
 	}
 	mapping_set_gfp_mask(filp->f_mapping, gfp);
-	lo->lo_state = Lo_unbound;
 	/* This is safe: open() is still holding a reference. */
 	module_put(THIS_MODULE);
 	blk_mq_unfreeze_queue(lo->lo_queue);
 
 	partscan = lo->lo_flags & LO_FLAGS_PARTSCAN && bdev;
 	lo_number = lo->lo_number;
-	lo->lo_flags = 0;
-	if (!part_shift)
-		lo->lo_disk->flags |= GENHD_FL_NO_PART_SCAN;
 	loop_unprepare_queue(lo);
 out_unlock:
 	mutex_unlock(&loop_ctl_mutex);
@@ -1121,6 +1117,23 @@ out_unlock:
 		/* Device is gone, no point in returning error */
 		err = 0;
 	}
+
+	/*
+	 * lo->lo_state is set to Lo_unbound here after above partscan has
+	 * finished.
+	 *
+	 * There cannot be anybody else entering __loop_clr_fd() as
+	 * lo->lo_backing_file is already cleared and Lo_rundown state
+	 * protects us from all the other places trying to change the 'lo'
+	 * device.
+	 */
+	mutex_lock(&loop_ctl_mutex);
+	lo->lo_flags = 0;
+	if (!part_shift)
+		lo->lo_disk->flags |= GENHD_FL_NO_PART_SCAN;
+	lo->lo_state = Lo_unbound;
+	mutex_unlock(&loop_ctl_mutex);
+
 	/*
 	 * Need not hold loop_ctl_mutex to fput backing file.
 	 * Calling fput holding loop_ctl_mutex triggers a circular