Multiple Threads Cannot Access the Same RAMdisk Created by modprobe

I have created RAMdisks of 60GB using modprove brd rd_size=62914560 on CentOS 7.5. Checking the results, fdisk -l /dev/ram* shows 16 ram block devices of 60GB size (/dev/ram0, /dev/ram1, ..., /dev/ram15). I want to run 16 jobs (threads) with random accesses on one ram block device to check the performance. I run such a workload using FIO tool. However, I get the following error: > cache invalidation of /dev/ram1 failed: Device or resource busy Why this happens? Is there a limitation in the number of jobs (threads) accessing a single ram block device? When I check the block devices with lsblk, the ram block devices are not shown. What is the reason? Thanks