linux-kernel - [PATCH v2] printk_legacy_map: use LD_WAIT_CONFIG instead of LD_WAIT

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Message-ID: <20251023103234.GA27415@redhat.com>
Date: Thu, 23 Oct 2025 12:32:34 +0200
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 v2] printk_legacy_map: use LD_WAIT_CONFIG instead of
 LD_WAIT_SLEEP

printk_legacy_map is used on !PREEMPT_RT to avoid false positives from
CONFIG_PROVE_RAW_LOCK_NESTING about raw_spinlock/spinlock nesting.

However, LD_WAIT_SLEEP is not exactly right; it fools lockdep as if it
is fine to acquire a sleeping lock.

Change DEFINE_WAIT_OVERRIDE_MAP(printk_legacy_map) to use LD_WAIT_CONFIG.

Also, update the comment to make it more clear as suggested by Petr.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Reviewed-by: Petr Mladek <pmladek@...e.com>
Reviewed-by: John Ogness <john.ogness@...utronix.de>
---
 kernel/printk/printk.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 5aee9ffb16b9..80f0bedf5cb7 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3003,11 +3003,12 @@ 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.
+ * raw_spinlock/spinlock nesting. However, on PREEMPT_RT the printing
+ * path from atomic context is always avoided and the console driver
+ * is always invoked from a dedicated thread. Thus the lockdep splat
+ * on !PREEMPT_RT is a false positive.
  *
- * This map is used to temporarily establish LD_WAIT_SLEEP context for the
+ * This map is used to temporarily establish LD_WAIT_CONFIG 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.
@@ -3016,7 +3017,7 @@ bool printk_get_next_message(struct printk_message *pmsg, u64 seq,
 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