System V Reboot is Funny
The system reboot process can be quite involved. Especially on some old systems which adopt the older System-V Init system, e.g. sysvinit-2.85.
Under the older sysvinit release, both reboot and poweroff are just a
symlink pointing to halt (/sbin/halt).
The whole process looks like the following:
// reboot.h
#define BMAGIC_REBOOT 0x01234567
#define init_reboot(magic) reboot(0xfee1dead, 672274793, magic)
// @file: sysvinit-2.85/src/halt.c
# reboot // it's symlink pointing to /sbin/halt
main @sysvinit-2.85/src/halt.c
|
`chdir("/")
`get_runlevel // e.g. get runlevel 4.
`do_shutdown(do_reboot ? "-r" : "-h", tm)
`open("/halt", O_RDWR|O_CREAT, 0644)
`execve("/sbin/shutdown", ["shutdown", "-r", "now"], [/* 19 vars */]);
|
v
`main @src/shutdown.c
| /* "-r", Automatic reboot */
`down_level[0] = '6';
`shutdown
|
`execve("/sbin/init", ["/sbin/init", "6"], [/* 19 vars */]);
|
v
main @src/init.c
| /* Open the FIFO /dev/initctl and write a command asking runlevel 6. */
`telinit
|
+------------+
|
v
/* (pid 1) init got a change runlevel request through the
* init's control FIFO. */
fifo_new_level
read_inittab --\ // change to runlevel 6 by executing "l6:6:wait:/etc/rc.d/rc 6"
setproctitle |
|
/etc/rc.d/rc 6 <---/ // export runlevel 6; First, run KILL scripts, /etc/rc$runlevel.d/K??
| // then, run the START scripts. /etc/rc$runlevel.d/S??;
v
K25dhcpd -> ../init.d/dhcpd
K40xinetd -> ../init.d/xinetd
K50crond -> ../init.d/crond
K55krb5kdc -> ../init.d/krb5kdc
K77rpcbind -> ../init.d/rpcbind
K78syslog -> ../init.d/syslog
K80network -> ../init.d/network
S00killall -> ../init.d/killall
S01reboot -> ../init.d/halt // This is a really workhourse! It is doing a lot,
| // but finally, it forks /sbin/reboot. (Hi reboot!)
v
"/sbin/reboot -i -d" // Note that now the runlevel is 6.
|
`sync
`sleep(2)
`ifdown
`init_reboot(BMAGIC_REBOOT) @sysvinit-2.85/src/halt.c
| (Glibc syscall wrapper)
`reboot @glibc-2.17-c758a686/sysdeps/unix/sysv/linux/reboot.c
|
`INLINE_SYSCALL (reboot, 3, (int) 0xfee1dead, 672274793, howto)
| (syscall)
_________ v (Kernel Mode) _________________________
system_call_fastpath
|
`sys_reboot @kernel/sys.c
|
`kernel_restart
|
`kernel_restart_prepare
| |
| `blocking_notifier_call_chain
| | `notifier_call_chain
| | `...
| `system_state = SYSTEM_RESTART
| `usermodehelper_disable
| `device_shutdown
| `syscore_shutdown
|
`kmsg_dump(KMSG_DUMP_RESTART)
`machine_restart
|
`native_machine_restart
`machine_shutdown
`__machine_emergency_restart(0)
|
`native_machine_emergency_restart
|
`acpi_reboot
|
`acpi_reset