[PATCH v3 3/4] printk/nmi: Warn when some message has been lost in NMI context

[Petr Mladek]

We could not resize the temporary buffer in NMI context. Let's warn
if a message is lost.

This is rather theoretical. printk() should not be used in NMI.
The only sensible use is when we want to print backtrace from all
CPUs. The current buffer should be enough for this purpose.

Signed-off-by: Petr Mladek <pmladek at suse.com>
---
 kernel/printk/nmi.c    |  5 ++++-
 kernel/printk/printk.c | 10 ++++++++++
 kernel/printk/printk.h | 11 +++++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index 29a498586465..5465230b75ec 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))
 
@@ -63,8 +64,10 @@ 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 88641c74163d..a6bb0b0838f3 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1669,6 +1669,7 @@ 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;
@@ -1719,6 +1720,15 @@ 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.
diff --git a/kernel/printk/printk.h b/kernel/printk/printk.h
index dc97c1c4d53b..7f778b609492 100644
--- a/kernel/printk/printk.h
+++ b/kernel/printk/printk.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 */
-- 
1.8.5.6