linux-kernel - [PATCH printk v1 02/18] printk: Add NMI check to down_trylock_console

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Message-Id: <20230302195618.156940-3-john.ogness@linutronix.de>
Date:   Thu,  2 Mar 2023 21:02:02 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: [PATCH printk v1 02/18] printk: Add NMI check to down_trylock_console_sem()

The printk path is NMI safe because it only adds content to the
buffer and then triggers the delayed output via irq_work. If the
console is flushed or unblanked (on panic) from NMI then it can
deadlock in down_trylock_console_sem() because the semaphore is not
NMI safe.

Avoid try-locking the console from NMI and assume it failed.

Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
 kernel/printk/printk.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 40c5f4170ac7..84af038292d9 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -318,6 +318,10 @@ static int __down_trylock_console_sem(unsigned long ip)
 	int lock_failed;
 	unsigned long flags;
 
+	/* Semaphores are not NMI-safe. */
+	if (in_nmi())
+		return 1;
+
 	/*
 	 * Here and in __up_console_sem() we need to be in safe mode,
 	 * because spindump/WARN/etc from under console ->lock will
-- 
2.30.2