From 554755be08fba31c74f66b82a485e5513205af84 Mon Sep 17 00:00:00 2001
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Date: Wed, 30 May 2018 16:03:50 +0900
Subject: [PATCH] printk: drop in_nmi check from printk_safe_flush_on_panic()
Git-commit: 554755be08fba31c74f66b82a485e5513205af84
Patch-mainline: v4.18-rc1
References: bsc#1112170

Drop the in_nmi() check from printk_safe_flush_on_panic()
and attempt to re-init (IOW unlock) locked logbuf spinlock
from panic CPU regardless of its context.

Otherwise, theoretically, we can deadlock on logbuf trying to flush
per-CPU buffers:

  a) Panic CPU is running in non-NMI context
  b) Panic CPU sends out shutdown IPI via reboot vector
  c) Panic CPU fails to stop all remote CPUs
  d) Panic CPU sends out shutdown IPI via NMI vector
     One of the CPUs that we bring down via NMI vector can hold
     logbuf spin lock (theoretically).

Link: http://lkml.kernel.org/r/20180530070350.10131-1-sergey.senozhatsky@gmail.com
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

---
 kernel/printk/printk_safe.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 449d67edfa4b..d7d091309054 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -281,7 +281,7 @@ void printk_safe_flush_on_panic(void)
 	 * Make sure that we could access the main ring buffer.
 	 * Do not risk a double release when more CPUs are up.
 	 */
-	if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) {
+	if (raw_spin_is_locked(&logbuf_lock)) {
 		if (num_online_cpus() > 1)
 			return;
 
-- 
2.13.7