summaryrefslogtreecommitdiffstats
path: root/debian/patches-rt/0046-printk-Avoid-false-positive-lockdep-report-for-legac.patch
blob: 7906760f715bc87df737026099af9054af59f901 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
From: John Ogness <john.ogness@linutronix.de>
Date: Mon, 11 Dec 2023 09:34:16 +0000
Subject: [PATCH 46/46] printk: Avoid false positive lockdep report for legacy
 printing
Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.9/older/patches-6.9-rt5.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
@@ -2980,6 +2980,33 @@ 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.
@@ -3034,7 +3061,7 @@ 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);
@@ -3056,7 +3083,9 @@ 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();