pNFS SCSI troubleshooting

SB validate failed with error -52

You tried to mount your LUN on another host and

[root@gfs-a24c-01 /]# mount /exports/scsi_pnfs_lun_path
mount: mount /dev/sda on /exports/scsi_pnfs_lun_path failed: Invalid exchange
[root@gfs-a24c-01 /]# dmesg | tail
[159317.348025] TECH PREVIEW: NFSv4 Block Layout Driver may not be fully supported.
Please review provided documentation for limitations.
[159317.348033] TECH PREVIEW: NFSv4 SCSI Layout Driver may not be fully supported.
Please review provided documentation for limitations.
[160301.652274] nfsd: last server has exited, flushing export cache
[160358.465235] sd 4:0:0:0: reservation conflict
[160358.465253] sd 4:0:0:0: [sda] tag#0 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_OK
[160358.465259] sd 4:0:0:0: [sda] tag#0 CDB: Read(10) 28 00 00 00 00 00 00 00 01 00
[160358.465263] blk_update_request: critical nexus error, dev sda, sector 0
[160358.466084] XFS (sda): SB validate failed with error -52.
#define EBADE       52  /* Invalid exchange */

The problem might be a persistent reservation on the device. Query and clear that reservation, most easily done from the original initiator that created it.