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 <dmitry.osipenko@collabora.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

include/linux/reboot.h

@@ -105,6 +105,14 @@ enum sys_off_mode {
 	 */
 	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:
 	 *

kernel/reboot.c

@@ -243,6 +243,17 @@ void migrate_to_reboot_cpu(void)
 	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
@@ -254,6 +265,7 @@ void migrate_to_reboot_cpu(void)
 void kernel_restart(char *cmd)
 {
 	kernel_restart_prepare(cmd);
+	do_kernel_restart_prepare();
 	migrate_to_reboot_cpu();
 	syscore_shutdown();
 	if (!cmd)
@@ -396,6 +408,11 @@ register_sys_off_handler(enum sys_off_mode mode,
 		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:
 		handler->list = &restart_handler_list;
 		break;