[RFC][PATCHv4 4/6] printk: report lost messages in printk safe/nmi contexts

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

Account lost messages in pritk-safe and printk-safe-nmi
contexts and report those numbers during printk_safe_flush().

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 kernel/printk/internal.h    | 17 ---------------
 kernel/printk/printk.c      | 10 ---------
 kernel/printk/printk_safe.c | 50 +++++++++++++++++++++++++++++++++++++++++++--
 3 files changed, 48 insertions(+), 29 deletions(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 87c784b..384542d 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -16,23 +16,6 @@
  */
 #include <linux/percpu.h>
 
-#ifdef CONFIG_PRINTK_NMI
-
-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 int get_nmi_message_lost(void)
-{
-	return 0;
-}
-
-#endif /* CONFIG_PRINTK_NMI */
-
 #ifdef CONFIG_PRINTK
 
 #define PRINTK_SAFE_CONTEXT_MASK	0x7fffffff
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1878b2d..4675b8d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1795,7 +1795,6 @@ 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;
@@ -1846,15 +1845,6 @@ 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_safe.c b/kernel/printk/printk_safe.c
index af74d9c..721b091 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -52,10 +52,11 @@ struct printk_safe_seq_buf {
 
 static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
 static DEFINE_PER_CPU(int, printk_safe_context);
+static atomic_t safe_message_lost;
 
 #ifdef CONFIG_PRINTK_NMI
 static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
-atomic_t nmi_message_lost;
+static atomic_t nmi_message_lost;
 #endif
 
 static int printk_safe_log_store(struct printk_safe_seq_buf *s,
@@ -122,6 +123,41 @@ static void printk_safe_flush_seq_line(struct printk_safe_seq_buf *s,
 	printk_safe_flush_line(buf, (end - start) + 1);
 }
 
+static void report_message_lost(atomic_t *num_lost, char *fmt)
+{
+	int lost = atomic_xchg(num_lost, 0);
+
+	if (lost) {
+		char msg[56];
+
+		scnprintf(msg, sizeof(msg), fmt, lost);
+
+		printk_safe_flush_line(msg, strlen(msg));
+	}
+}
+
+#ifdef CONFIG_PRINTK_NMI
+
+static void report_nmi_message_lost(void)
+{
+	report_message_lost(&nmi_message_lost,
+			"Lost %d message(s) from NMI context!");
+}
+
+#else
+
+static void report_nmi_message_lost(void)
+{
+}
+
+#endif /* CONFIG_PRINTK_NMI */
+
+static void report_safe_message_lost(void)
+{
+	report_message_lost(&safe_message_lost,
+			"Lost %d message(s) from printk-safe context!");
+}
+
 /*
  * Flush data from the associated per-CPU buffer. The function
  * can be called either via IRQ work or independently.
@@ -147,6 +183,9 @@ static void __printk_safe_flush(struct irq_work *work)
 
 	i = 0;
 more:
+	report_nmi_message_lost();
+	report_safe_message_lost();
+
 	len = atomic_read(&s->len);
 
 	/*
@@ -289,8 +328,15 @@ static int vprintk_safe_nmi(const char *fmt, va_list args)
 static int vprintk_safe(const char *fmt, va_list args)
 {
 	struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq);
+	int add;
 
-	return printk_safe_log_store(s, fmt, args);
+	add = printk_safe_log_store(s, fmt, args);
+	if (add == -E2BIG) {
+		atomic_inc(&safe_message_lost);
+		add = 0;
+	}
+
+	return add;
 }
 
 /* Can be preempted by NMI. */
-- 
2.10.1.502.g6598894