From 427934b8714ec130b068d1c9d88f25b24aaede32 Mon Sep 17 00:00:00 2001
From: Petr Mladek <pmladek@suse.com>
Date: Fri, 20 May 2016 17:00:39 -0700
Subject: [PATCH] printk/nmi: increase the size of NMI buffer and make it
 configurable

Testing has shown that the backtrace sometimes does not fit into the 4kB
temporary buffer that is used in NMI context.  The warnings are gone
when I double the temporary buffer size.

This patch doubles the buffer size and makes it configurable.

Note that this problem existed even in the x86-specific implementation
that was added by the commit a9edc8809328 ("x86/nmi: Perform a safe NMI
stack trace on all CPUs").  Nobody noticed it because it did not print
any warnings.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jiri Kosina <jkosina@suse.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: David Miller <davem@davemloft.net>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 init/Kconfig        | 22 ++++++++++++++++++++++
 kernel/printk/nmi.c |  3 ++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/init/Kconfig b/init/Kconfig
index bccc1d607be5..a9c4aefd5436 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -862,6 +862,28 @@ config LOG_CPU_MAX_BUF_SHIFT
 		     13 =>   8 KB for each CPU
 		     12 =>   4 KB for each CPU
 
+config NMI_LOG_BUF_SHIFT
+	int "Temporary per-CPU NMI log buffer size (12 => 4KB, 13 => 8KB)"
+	range 10 21
+	default 13
+	depends on PRINTK_NMI
+	help
+	  Select the size of a per-CPU buffer where NMI messages are temporary
+	  stored. They are copied to the main log buffer in a safe context
+	  to avoid a deadlock. The value defines the size as a power of 2.
+
+	  NMI messages are rare and limited. The largest one is when
+	  a backtrace is printed. It usually fits into 4KB. Select
+	  8KB if you want to be on the safe side.
+
+	  Examples:
+		     17 => 128 KB for each CPU
+		     16 =>  64 KB for each CPU
+		     15 =>  32 KB for each CPU
+		     14 =>  16 KB for each CPU
+		     13 =>   8 KB for each CPU
+		     12 =>   4 KB for each CPU
+
 #
 # Architectures with an unreliable sched_clock() should select this:
 #
diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index 572f94922230..bf08557d7e3d 100644
--- a/kernel/printk/nmi.c
+++ b/kernel/printk/nmi.c
@@ -41,7 +41,8 @@ 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))
+#define NMI_LOG_BUF_LEN ((1 << CONFIG_NMI_LOG_BUF_SHIFT) -		\
+			 sizeof(atomic_t) - sizeof(struct irq_work))
 
 struct nmi_seq_buf {
 	atomic_t		len;	/* length of written data */