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-rw-r--r--debian/patches-rt/0046-printk-Avoid-false-positive-lockdep-report-for-legac.patch86
1 files changed, 86 insertions, 0 deletions
diff --git a/debian/patches-rt/0046-printk-Avoid-false-positive-lockdep-report-for-legac.patch b/debian/patches-rt/0046-printk-Avoid-false-positive-lockdep-report-for-legac.patch
new file mode 100644
index 0000000000..7906760f71
--- /dev/null
+++ b/debian/patches-rt/0046-printk-Avoid-false-positive-lockdep-report-for-legac.patch
@@ -0,0 +1,86 @@
+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();
+