man/man9/kern_reboot.9

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.\" 	Mike Pritchard.  All rights reserved.
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.\" Copyright (c) 1994 Christopher G. Demetriou
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.Dd November 23, 2023
.Dt REBOOT 9
.Os
.Sh NAME
.Nm kern_reboot ,
.Nm shutdown_nice
.Nd reboot, halt, or power off the system
.Sh SYNOPSIS
.In sys/types.h
.In sys/systm.h
.In sys/reboot.h
.Vt extern int rebooting;
.Ft void
.Fn kern_reboot "int howto"
.Ft void
.Fn shutdown_nice "int howto"
.In sys/eventhandler.h
.Fn EVENTHANDLER_REGISTER "shutdown_pre_sync" "shutdown_fn" "private" "priority"
.Fn EVENTHANDLER_REGISTER "shutdown_post_sync" "shutdown_fn" "private" "priority"
.Fn EVENTHANDLER_REGISTER "shutdown_final" "shutdown_fn" "private" "priority"
.Sh DESCRIPTION
The
.Fn kern_reboot
function handles final system shutdown, and either halts, reboots,
or powers down the system.
The exact action to be taken is determined by the flags passed in
.Fa howto .
.Pp
The relevant flags are:
.Bl -tag -compact -offset indent -width "RB_POWERCYCLE"
.It Dv RB_HALT
Halt the system in-place rather than restarting.
.It Dv RB_POWEROFF
Power down the system rather than restarting.
.It Dv RB_POWERCYCLE
Request a power-cycle in addition to restarting.
.It Dv RB_NOSYNC
Do not sync filesystems during shutdown.
.It Dv RB_DUMP
Dump kernel memory during shutdown.
.El
.Pp
The
.Fa howto
field, and its full list of flags are described in additional detail by
.Xr reboot 2 .
.Pp
.Fn kern_reboot
performs the following actions:
.Bl -enum -offset indent
.It
Set the
.Va rebooting
variable to
.Dv 1 ,
indicating that the reboot process has begun and cannot be stopped.
.It
Unless the
.Dv RB_NOSYNC
flag is set in
.Fa howto ,
sync and unmount the system's disks by calling
.Xr vfs_unmountall 9 .
.It
If rebooting after a panic
.Po
.Dv RB_DUMP
is set in
.Fa howto ,
but
.Dv RB_HALT
is not set
.Pc ,
initiate a system crash dump via
.Fn doadump .
.It
Print a message indicating that the system is about to be halted
or rebooted, and a report of the total system uptime.
.It
Execute all registered shutdown hooks.
See
.Sx SHUTDOWN HOOKS
below.
.It
As a last resort, if none of the shutdown hooks handled the reboot, call the
machine-dependent
.Fn cpu_reset
function.
In the unlikely case that this is not supported,
.Fn kern_reboot
will loop forever at the end of the function.
This requires a manual reset of the system.
.El
.Pp
.Fn kern_reboot
may be called from a typical kernel execution context, when the system is
running normally.
It may also be called as the final step of a kernel panic, or from the kernel
debugger.
Therefore, the code in this function is subject to restrictions described by
the
.Sx EXECUTION CONTEXT
section of the
.Xr panic 9
man page.
.Pp
The
.Fn shutdown_nice
function is the intended path for performing a clean reboot or shutdown when
the system is operating under normal conditions.
Calling this function will send a signal to the
.Xr init 8
process, instructing it to perform a shutdown.
When
.Xr init 8
has cleanly terminated its children, it will perform the
.Xr reboot 2
system call, which in turn calls
.Fn kern_reboot .
.Pp
If
.Fn shutdown_nice
is called before the
.Xr init 8
process has been spawned, or if the system has panicked or otherwise halted,
.Fn kern_reboot
will be called directly.
.Sh SHUTDOWN HOOKS
The system defines three separate
.Xr EVENTHANDLER 9
events, which are invoked successively during the shutdown procedure.
These are
.Va shutdown_pre_sync ,
.Va shutdown_post_sync ,
and
.Va shutdown_final .
They will be executed unconditionally in the listed order.
Handler functions registered to any of these events will receive the value of
.Fa howto
as their second argument, which may be used to decide what action to take.
.Pp
The
.Va shutdown_pre_sync
event is invoked before syncing filesystems to disk.
It enables any action or state transition that must happen before this point to
take place.
.Pp
The
.Va shutdown_post_sync
event is invoked at the point immediately after the filesystem sync has
finished.
It enables, for example, disk drivers to complete the sync by flushing their
cache to disk.
Note that this event still takes place before the optional kernel core dump.
.Pp
The
.Va shutdown_final
event is invoked as the very last step of
.Fn kern_reboot .
Drivers and subsystems such as
.Xr acpi 4
can register handlers to this event that will perform the actual reboot,
power-off, or halt.
.Pp
Notably, the
.Va shutdown_final
event is also the point at which all kernel modules will have their shutdown
.Po
.Dv MOD_SHUTDOWN
.Pc
hooks executed, and when the
.Xr DEVICE_SHUTDOWN 9
method will be executed recursively on all devices.
.Pp
All event handlers, like
.Fn kern_reboot
itself, may be run in either normal shutdown context or a kernel panic or
debugger context.
Handler functions are expected to take care not to trigger recursive panics.
.Sh RETURN VALUES
The
.Fn kern_reboot
function does not return.
.Pp
The
.Fn shutdown_nice
function will usually return to its caller, having initiated the asynchronous
system shutdown.
It will not return when called from a panic or debugger context, or during
early boot.
.Sh EXAMPLES
A hypothetical driver, foo(4), defines a
.Va shutdown_final
event handler that can handle system power-off by writing to a device register,
but it does not handle halt or reset.
.Bd -literal -offset indent
void
foo_poweroff_handler(struct void *arg, int howto)
{
        struct foo_softc *sc = arg;
        uint32_t reg;

        if ((howto & RB_POWEROFF) != 0) {
                reg = FOO_POWEROFF;
                WRITE4(sc, FOO_POWEROFF_REG, reg);
        }
}
.Ed
.Pp
The handler is then registered in the device attach routine:
.Bd -literal -offset indent
int
foo_attach(device_t dev)
{
        struct foo_softc *sc;

        ...

        /* Pass the device's software context as the private arg. */
        EVENTHANDLER_REGISTER(shutdown_final, foo_poweroff_handler, sc,
            SHUTDOWN_PRI_DEFAULT);

        ...
}
.Ed
.Pp
This
.Va shutdown_final
handler uses the
.Dv RB_NOSYNC
flag to detect that a panic or other unusual condition has occurred, and
returns early:
.Bd -literal -offset indent
void
bar_shutdown_final(struct void *arg, int howto)
{

        if ((howto & RB_NOSYNC) != 0)
                return;

        /* Some code that is not panic-safe. */
        ...
}
.Ed
.Sh SEE ALSO
.Xr reboot 2 ,
.Xr init 8 ,
.Xr DEVICE_SHUTDOWN 9 ,
.Xr EVENTHANDLER 9 ,
.Xr module 9 ,
.Xr panic 9 ,
.Xr vfs_unmountall 9