Well, how about using separate (independent lock) for sysfs_remove_battery()
case? Since we can't safely drop battery->lock in sysfs_remove_battery() before 
power_supply_unregister() call.
Not sure if it should be within struct acpi_battery, perhaps we could
have it as a 'global' battery lock. Anyway, here it is:

I think it should be within struct acpi_battery. There maybe one or more batteries.