btrfs: use mapping shared locking for reading super block

There's no need to exclusively lock the mapping, shared locking is enough
to protect from a concurrent set block size operation (BLKBSZSET ioctl).

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/volumes.c

@@ -1369,9 +1369,9 @@ struct btrfs_super_block *btrfs_read_disk_super(struct block_device *bdev,
 				      (bytenr + BTRFS_SUPER_INFO_SIZE) >> PAGE_SHIFT);
 	}
 
-	filemap_invalidate_lock(mapping);
+	filemap_invalidate_lock_shared(mapping);
 	page = read_cache_page_gfp(mapping, bytenr >> PAGE_SHIFT, GFP_NOFS);
-	filemap_invalidate_unlock(mapping);
+	filemap_invalidate_unlock_shared(mapping);
 	if (IS_ERR(page))
 		return ERR_CAST(page);
 

fs/btrfs/zoned.c

@@ -131,10 +131,10 @@ static int sb_write_pointer(struct block_device *bdev, struct blk_zone *zones,
 			u64 bytenr = ALIGN_DOWN(zone_end, BTRFS_SUPER_INFO_SIZE) -
 						BTRFS_SUPER_INFO_SIZE;
 
-			filemap_invalidate_lock(mapping);
+			filemap_invalidate_lock_shared(mapping);
 			page[i] = read_cache_page_gfp(mapping,
 					bytenr >> PAGE_SHIFT, GFP_NOFS);
-			filemap_invalidate_unlock(mapping);
+			filemap_invalidate_unlock_shared(mapping);
 			if (IS_ERR(page[i])) {
 				if (i == 1)
 					btrfs_release_disk_super(super[0]);