kernel/reboot: Add SYS_OFF_MODE_RESTART_PREPARE mode

Add SYS_OFF_MODE_RESTART_PREPARE callbacks to be invoked before
a system restart.

Suggested-by: Dmitry Osipenko <[e-mail]>
Reviewed-by: Dmitry Osipenko <[e-mail]>
Signed-off-by: Kai-Heng Feng <[e-mail]>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <[e-mail]>

include/linux/reboot.h

···
 	SYS_OFF_MODE_POWER_OFF,
 
 	/**
+	 * @SYS_OFF_MODE_RESTART_PREPARE:
+	 *
+	 * Handlers prepare system to be restarted. Handlers are
+	 * allowed to sleep.
+	 */
+	SYS_OFF_MODE_RESTART_PREPARE,
+
+	/**
 	 * @SYS_OFF_MODE_RESTART:
 	 *
 	 * Handlers restart system. Handlers are disallowed to sleep.

kernel/reboot.c

···
 	set_cpus_allowed_ptr(current, cpumask_of(cpu));
 }
 
+/*
+ *	Notifier list for kernel code which wants to be called
+ *	to prepare system for restart.
+ */
+static BLOCKING_NOTIFIER_HEAD(restart_prep_handler_list);
+
+static void do_kernel_restart_prepare(void)
+{
+	blocking_notifier_call_chain(&restart_prep_handler_list, 0, NULL);
+}
+
 /**
  *	kernel_restart - reboot the system
  *	@cmd: pointer to buffer containing command to execute for restart
···
 void kernel_restart(char *cmd)
 {
 	kernel_restart_prepare(cmd);
+	do_kernel_restart_prepare();
 	migrate_to_reboot_cpu();
 	syscore_shutdown();
 	if (!cmd)
···
 
 	case SYS_OFF_MODE_POWER_OFF:
 		handler->list = &power_off_handler_list;
+		break;
+
+	case SYS_OFF_MODE_RESTART_PREPARE:
+		handler->list = &restart_prep_handler_list;
+		handler->blocking = true;
 		break;
 
 	case SYS_OFF_MODE_RESTART: