The following calltrace is possible now:
 handle_sysrq
   machine_emergency_restart
     mach_reboot_fixups
       pci_get_device
         pci_get_subsys
	   down_read
The patch skips reboot fixup if called from sysrq-B code.
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>