[RFC][PATCHv6 07/12] printk: register syscore notifier

[Sergey Senozhatsky]

We need to switch to emergency printk mode in kernel_kexec(). One
kernel_kexec() branch calls kernel_restart_prepare(), which updates
`system_state', however, the other one, when user requested to
->preserve_context, does not and we are lacking the information
in printk about kexec being executed. Register a syscore notifier
so printk will be notified by syscore_suspend().

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
---
 kernel/printk/printk.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index a427cee2aa00..372c71f69e8e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -49,6 +49,7 @@
 #include <linux/sched/debug.h>
 #include <linux/sched/task_stack.h>
 #include <linux/kthread.h>
+#include <linux/syscore_ops.h>
 
 #include <linux/uaccess.h>
 #include <asm/sections.h>
@@ -2914,6 +2915,22 @@ static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = {
 	.flags = IRQ_WORK_LAZY,
 };
 
+static int printk_syscore_suspend(void)
+{
+	printk_emergency_begin();
+	return 0;
+}
+
+static void printk_syscore_resume(void)
+{
+	printk_emergency_end();
+}
+
+static struct syscore_ops printk_syscore_ops = {
+	.suspend = printk_syscore_suspend,
+	.resume = printk_syscore_resume,
+};
+
 static int printk_kthread_func(void *data)
 {
 	while (1) {
@@ -2961,6 +2978,8 @@ static int __init init_printk_kthread(void)
 	if (!alloc_cpumask_var(&printk_offload_cpus, GFP_KERNEL))
 		return -ENOMEM;
 
+	register_syscore_ops(&printk_syscore_ops);
+
 	thread = kthread_run(printk_kthread_func, NULL, "printk");
 	if (IS_ERR(thread)) {
 		pr_err("printk: unable to create printing thread\n");
-- 
2.15.1