On Thu, Oct 15, 2015 at 08:30:50AM -0700, Bart Van Assche wrote:

Can you explain why this functionality has been added to the sd driver 
instead of the SCSI core ? Aren't persistent reservations a concept that 
applies to all SCSI device types ?

In theory it does.  In practice it's only used by disks/arrays and tapes,
and as tapes don't have a struct block_device this isn't going to be useful
for them.