Hi,

Currently, loop device has only one global lock:
loop_ctl_mutex.

This becomes hot in scenarios where many loop devices are used.

Scale it by introducing per-device lock: lo_mutex that protects the
fields in struct loop_device. Keep loop_ctl_mutex to protect global
data such as loop_index_idr, loop_lookup, loop_add.

Lock ordering: loop_ctl_mutex > lo_mutex.

Reviewed-by: Petr Vorel <pvorel@suse.cz>

Kind regards,
Petr