[PATCH v3 2/2] Handle flushing of CPU backtraces during panic

[takakura]

From: Ryo Takakura <takakura@xxxxxxxxxxxxx>

After panic, non-panicked CPU's has been unable to flush ringbuffer
while they can still write into it. This can affect CPU backtrace
triggered in panic only able to write into ringbuffer incapable of
flushing them.

Fix the issue by letting the panicked CPU handle the flushing of
ringbuffer right after non-panicked CPUs finished writing their
backtraces.

Signed-off-by: Ryo Takakura <takakura@xxxxxxxxxxxxx>
---
 include/linux/console.h |  1 +
 kernel/panic.c          |  1 +
 kernel/printk/printk.c  | 14 ++++++++++++++
 3 files changed, 16 insertions(+)

diff --git a/include/linux/console.h b/include/linux/console.h
index 31a8f5b85..c7eb6f785 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -504,6 +504,7 @@ extern void console_unlock(void);
 extern void console_conditional_schedule(void);
 extern void console_unblank(void);
 extern void console_flush_on_panic(enum con_flush_mode mode);
+extern void console_try_flush(void);
 extern struct tty_driver *console_device(int *);
 extern void console_stop(struct console *);
 extern void console_start(struct console *);
diff --git a/kernel/panic.c b/kernel/panic.c
index 2a0449144..6519cc6bd 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -260,6 +260,7 @@ static void panic_other_cpus_shutdown(bool crash_kexec)
 		panic_triggering_all_cpu_backtrace = true;
 		trigger_all_cpu_backtrace();
 		panic_triggering_all_cpu_backtrace = false;
+		console_try_flush();
 	}
 
 	/*
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index c22b07049..517b8b02e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3284,6 +3284,20 @@ void console_flush_on_panic(enum con_flush_mode mode)
 	console_flush_all(false, &next_seq, &handover);
 }
 
+/**
+ * console_try_flush - try to flush consoles when safe
+ *
+ * Context: Any, except for NMI.
+ */
+void console_try_flush(void)
+{
+	if (is_printk_legacy_deferred())
+		return;
+
+	if (console_trylock())
+		console_unlock();
+}
+
 /*
  * Return the console tty driver structure and its associated index
  */
-- 
2.34.1