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From: John Ogness <john.ogness@linutronix.de>
Date: Mon, 11 Dec 2023 09:34:16 +0000
Subject: [PATCH 48/48] printk: Avoid false positive lockdep report for legacy
printing
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.8/older/patches-6.8.2-rt11.tar.xz
Legacy console printing from printk() caller context may invoke
the console driver from atomic context. This leads to a lockdep
splat because the console driver will acquire a sleeping lock
and the caller may already hold a spinning lock. This is noticed
by lockdep on !PREEMPT_RT configurations because it will lead to
a problem on PREEMPT_RT.
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.
For !PREEMPT_RT override the lock-context before invoking the
console driver to avoid the false positive.
Do not override the lock-context for PREEMPT_RT in order to
allow lockdep to catch any real locking context issues related
to the write callback usage.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/printk/printk.c | 31 ++++++++++++++++++++++++++++++-
1 file changed, 30 insertions(+), 1 deletion(-)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2985,6 +2985,33 @@ bool printk_get_next_message(struct prin
}
/*
+ * 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 establish LD_WAIT_SLEEP context for the console write
+ * callbacks when legacy printing to avoid false positive lockdep complaints,
+ * thus allowing lockdep to continue to function for real issues.
+ */
+#ifdef CONFIG_PREEMPT_RT
+static inline void printk_legacy_lock_map_acquire_try(void) { }
+static inline void printk_legacy_lock_map_release(void) { }
+#else
+static DEFINE_WAIT_OVERRIDE_MAP(printk_legacy_map, LD_WAIT_SLEEP);
+
+static inline void printk_legacy_lock_map_acquire_try(void)
+{
+ lock_map_acquire_try(&printk_legacy_map);
+}
+
+static inline void printk_legacy_lock_map_release(void)
+{
+ lock_map_release(&printk_legacy_map);
+}
+#endif /* CONFIG_PREEMPT_RT */
+
+/*
* Used as the printk buffers for non-panic, serialized console printing.
* This is for legacy (!CON_NBCON) as well as all boot (CON_BOOT) consoles.
* Its usage requires the console_lock held.
@@ -3039,7 +3066,7 @@ static bool console_emit_next_record(str
/*
* With forced threading this function is either in a thread
* or panic context. So there is no need for concern about
- * printk reentrance or handovers.
+ * printk reentrance, handovers, or lockdep complaints.
*/
con->write(con, outbuf, pmsg.outbuf_len);
@@ -3061,7 +3088,9 @@ static bool console_emit_next_record(str
/* Do not trace print latency. */
stop_critical_timings();
+ printk_legacy_lock_map_acquire_try();
con->write(con, outbuf, pmsg.outbuf_len);
+ printk_legacy_lock_map_release();
start_critical_timings();
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