[v5,2/4] printk/nmi: warn when some message has been lost in NMI context

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 2de99faedfc1..341bedccc065 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -34,6 +34,12 @@  static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
 	return this_cpu_read(printk_func)(fmt, args);
 }
 
+extern atomic_t nmi_message_lost;
+static inline int get_nmi_message_lost(void)
+{
+	return atomic_xchg(&nmi_message_lost, 0);
+}
+
 #else /* CONFIG_PRINTK_NMI */
 
 static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
@@ -41,4 +47,9 @@  static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
 	return vprintk_default(fmt, args);
 }
 
+static inline int get_nmi_message_lost(void)
+{
+	return 0;
+}
+
 #endif /* CONFIG_PRINTK_NMI */
diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index 303cf0d15e57..572f94922230 100644
--- a/kernel/printk/nmi.c
+++ b/kernel/printk/nmi.c
@@ -39,6 +39,7 @@ 
  */
 DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
 static int printk_nmi_irq_ready;
+atomic_t nmi_message_lost;
 
 #define NMI_LOG_BUF_LEN (4096 - sizeof(atomic_t) - sizeof(struct irq_work))
 
@@ -64,8 +65,10 @@  static int vprintk_nmi(const char *fmt, va_list args)
 again:
 	len = atomic_read(&s->len);
 
-	if (len >= sizeof(s->buffer))
+	if (len >= sizeof(s->buffer)) {
+		atomic_inc(&nmi_message_lost);
 		return 0;
+	}
 
 	/*
 	 * Make sure that all old data have been read before the buffer was
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 71eba0607034..e38579d730f4 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1617,6 +1617,7 @@  asmlinkage int vprintk_emit(int facility, int level,
 	unsigned long flags;
 	int this_cpu;
 	int printed_len = 0;
+	int nmi_message_lost;
 	bool in_sched = false;
 	/* cpu currently holding logbuf_lock in this function */
 	static unsigned int logbuf_cpu = UINT_MAX;
@@ -1667,6 +1668,15 @@  asmlinkage int vprintk_emit(int facility, int level,
 					 strlen(recursion_msg));
 	}
 
+	nmi_message_lost = get_nmi_message_lost();
+	if (unlikely(nmi_message_lost)) {
+		text_len = scnprintf(textbuf, sizeof(textbuf),
+				     "BAD LUCK: lost %d message(s) from NMI context!",
+				     nmi_message_lost);
+		printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0,
+					 NULL, 0, textbuf, text_len);
+	}
+
 	/*
 	 * The printf needs to come first; we need the syslog
 	 * prefix which might be passed-in as a parameter.