linux-kernel - [PATCH v3] printk_legacy_map: use LD_WAIT_CONFIG instead of LD_WAIT

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Message-ID: <20251026150726.GA23223@redhat.com>
Date: Sun, 26 Oct 2025 16:07:26 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: John Ogness <john.ogness@...utronix.de>, Petr Mladek <pmladek@...e.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH v3] printk_legacy_map: use LD_WAIT_CONFIG instead of
 LD_WAIT_SLEEP

printk_legacy_map is used to hide lock nesting violations caused by
legacy drivers and is using the wrong override type. LD_WAIT_SLEEP is
for always sleeping lock types such as mutex_t. LD_WAIT_CONFIG is for
lock type which are sleeping while spinning on PREEMPT_RT such as
spinlock_t.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 kernel/printk/printk.c | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 5aee9ffb16b9..e9e65228f1ac 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3002,21 +3002,18 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
 }
 
 /*
- * Legacy console printing from printk() caller context does not respect
- * raw_spinlock/spinlock nesting. For !PREEMPT_RT the lockdep warning is a
- * false positive. For PREEMPT_RT the false positive condition does not
- * occur.
- *
- * This map is used to temporarily establish LD_WAIT_SLEEP context for the
- * console write() callback when legacy printing to avoid false positive
- * lockdep complaints, thus allowing lockdep to continue to function for
- * real issues.
- */
+ * The legacy console always acquires a spinlock_t from its printing
+ * callback. This violates lock nesting if the caller acquired an always
+ * spinning lock (raw_spinlock_t) while invoking printk(). This is not a
+ * problem on PREEMPT_RT because legacy consoles print always from a
+ * dedicated thread and never from within printk(). Therefore we tell
+ * lockdep that a sleeping spin lock (spinlock_t) is valid here.
+*/
 #ifdef CONFIG_PREEMPT_RT
 static inline void printk_legacy_allow_spinlock_enter(void) { }
 static inline void printk_legacy_allow_spinlock_exit(void) { }
 #else
-static DEFINE_WAIT_OVERRIDE_MAP(printk_legacy_map, LD_WAIT_SLEEP);
+static DEFINE_WAIT_OVERRIDE_MAP(printk_legacy_map, LD_WAIT_CONFIG);
 
 static inline void printk_legacy_allow_spinlock_enter(void)
 {
-- 
2.25.1.362.g51ebf55