Currently the scsi_debug driver wraps every queued command
with the host_lock and every mid-level completion callback
(apart from delay=0) with a spinlock.

Attached is a patch against Linus's tree that also applies
to lk 3.15.1 . It attempts to address some of these issues.

ChangeLog
   - 'host_lock' option added that simply drops that lock
     when host_lock=0 (which is the default)
   - allow delay=-1 [delay=1 (jiffy) is still the default]
     that uses a tasklet to schedule the response quickly
   - for completions (when delay!=0) the callback to the
     mid-level is un-(spin)locked
   - completions are counted; can be viewed with
       cat /proc/scsi/scsi_debug/<host_num>
   - delay_override removed from TEST UNIT READY.
     This makes 'sg_turs -n 1m -t /dev/bsg/<hctl>' a more
     realistic test of command overhead. I get about 100k
     IOPS on my laptop.

This patch has been lightly tested. Perhaps someone could
throw a scsi-mq test at it.

Comments welcome.

Doug Gilbert