From 52f6a084c0e6c3cd9207ec5d1d95904fb089a1aa Mon Sep 17 00:00:00 2001
From: John Ogness <john.ogness@linutronix.de>
Date: Mon, 11 Sep 2023 15:53:39 +0000
Subject: [PATCH] rcu: Mark emergency sections in rcu stalls

BugLink: https://bugs.launchpad.net/bugs/2060704

Mark emergency sections wherever multiple lines of
rcu stall information are generated. In an emergency
section the CPU will not perform console output for the
printk() calls. Instead, a flushing of the console
output is triggered when exiting the emergency section.
This allows the full message block to be stored as
quickly as possible in the ringbuffer.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Kevin Becker <kevin.becker@canonical.com>
---
 kernel/rcu/tree_exp.h   | 7 +++++++
 kernel/rcu/tree_stall.h | 9 +++++++++
 2 files changed, 16 insertions(+)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 8107f818455d..b17130b7e522 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -7,6 +7,7 @@
  * Authors: Paul E. McKenney <paulmck@linux.ibm.com>
  */
 
+#include <linux/console.h>
 #include <linux/lockdep.h>
 
 static void rcu_exp_handler(void *unused);
@@ -636,6 +637,9 @@ static void synchronize_rcu_expedited_wait(void)
 			return;
 		if (rcu_stall_is_suppressed())
 			continue;
+
+		nbcon_cpu_emergency_enter();
+
 		j = jiffies;
 		rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_EXP, (void *)(j - jiffies_start));
 		trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
@@ -689,6 +693,9 @@ static void synchronize_rcu_expedited_wait(void)
 			rcu_exp_print_detail_task_stall_rnp(rnp);
 		}
 		jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
+
+		nbcon_cpu_emergency_exit();
+
 		panic_on_rcu_stall();
 	}
 }
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index b5ec62b2d850..2d51a1816c10 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -7,6 +7,7 @@
  * Author: Paul E. McKenney <paulmck@linux.ibm.com>
  */
 
+#include <linux/console.h>
 #include <linux/kvm_para.h>
 #include <linux/rcu_notifier.h>
 
@@ -605,6 +606,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 	if (rcu_stall_is_suppressed())
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	/*
 	 * OK, time to rat on our buddy...
 	 * See Documentation/RCU/stallwarn.rst for info on how to debug
@@ -656,6 +659,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
 	rcu_check_gp_kthread_expired_fqs_timer();
 	rcu_check_gp_kthread_starvation();
 
+	nbcon_cpu_emergency_exit();
+
 	panic_on_rcu_stall();
 
 	rcu_force_quiescent_state();  /* Kick them all. */
@@ -676,6 +681,8 @@ static void print_cpu_stall(unsigned long gps)
 	if (rcu_stall_is_suppressed())
 		return;
 
+	nbcon_cpu_emergency_enter();
+
 	/*
 	 * OK, time to rat on ourselves...
 	 * See Documentation/RCU/stallwarn.rst for info on how to debug
@@ -704,6 +711,8 @@ static void print_cpu_stall(unsigned long gps)
 			   jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 
+	nbcon_cpu_emergency_exit();
+
 	panic_on_rcu_stall();
 
 	/*