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From: John Ogness <john.ogness@linutronix.de>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
    Steven Rostedt <rostedt@goodmis.org>,
    Juri Lelli <juri.lelli@redhat.com>,
    linux-kernel@vger.kernel.org
Subject: [PATCH linux-next] printk: nbcon: Fix nbcon_cpu_emergency_flush()
 when preemptible
Date: Wed, 05 Jun 2024 11:57:34 +0206
Message-ID: <87zfrzvhsp.fsf@jogness.linutronix.de>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain

nbcon_cpu_emergency_flush() can be called in a preemptible
context. In that case the CPU is not in an emergency state.
However, in order to see that the CPU is not in an emergency
state (accessing the per-cpu variable), preemption must be
disabled.

Disable preemption when checking the CPU state.

Reported-by: Juri Lelli <juri.lelli@redhat.com>
Closes: https://lore.kernel.org/aqkcpca4vgadxc3yzcu74xwq3grslj5m43f3eb5fcs23yo2gy4@gcsnqcts5tos
Fixes: 46a1379208b7 ("printk: nbcon: Implement emergency sections")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/printk/nbcon.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 19f0db6945e4..39eb369880d9 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1443,8 +1443,19 @@ void nbcon_cpu_emergency_exit(void)
  */
 void nbcon_cpu_emergency_flush(void)
 {
+	bool is_emergency;
+
+	/*
+	 * If this context is not an emergency context, preemption might be
+	 * enabled. To be sure, disable preemption when checking if this is
+	 * an emergency context.
+	 */
+	preempt_disable();
+	is_emergency = (*nbcon_get_cpu_emergency_nesting() != 0);
+	preempt_enable();
+
 	/* The explicit flush is needed only in the emergency context. */
-	if (*(nbcon_get_cpu_emergency_nesting()) == 0)
+	if (!is_emergency)
 		return;
 
 	nbcon_atomic_flush_pending();
-- 
2.30.2