Hi,
We found a problem when using SCSI device.

Setting the read-only attribute to scsi disk by calling "ioctl(fd, BLKROSET, &ro)"
only block the first write operation.

In detail:
1: set ioctl(fd, BLKROSET, &ro)
2: dd to the block device(as /dev/sdc), write failed
3: dd to the block device again, write successed

Above operation can be reproduced by following script:

#! /bin/bash
DEV=$1
[[ "$DEV" =~ ^/dev/sd ]] || echo "Need a scsi device to reproduce"
blockdev --setro $DEV
echo $DEV getro is [ $(blockdev --getro $DEV) ]
dd if=/dev/zero of=$DEV count=1
echo $DEV getro is [ $(blockdev --getro $1) ]
dd if=/dev/zero of=$DEV count=1

Environment:
  - Kernel: ALL
  We reproduced this problem in following kernel version: 4.15 4.10, 3.10.
  And by reference source, we think the problem exist in all versions.
  - Driver: SCSI
  it exists only when using SCSI disks, and xen-blk，virtio-blk do not have such a 
  problem

Here is a test result:
# uname -r
4.15.0+

# ./reproduce.sh /dev/sda
/dev/sda getro is [ 1 ]
dd: writing to ‘/dev/sda’: Operation not permitted
1+0 records in
0+0 records out
0 bytes (0 B) copied, 0.000258806 s, 0.0 kB/s
/dev/sda getro is [ 0 ]
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.000321595 s, 1.6 MB/s

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