[PATCH printk] Flush console log from kernel_power_off()

["Paul E. McKenney" <paulmck@xxxxxxxxxx>]

Kernels built with CONFIG_PREEMPT_RT=y can lose significant console output
and shutdown time, which hides shutdown-time RCU issues from rcutorture.
Therefore, make pr_flush() public and invoke it after then last print
in kernel_power_off().

[ paulmck: Apply John Ogness feedback. ]
[ paulmck: Appy Sebastian Andrzej Siewior feedback. ]
[ paulmck: Apply kernel test robot feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Reviewed-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
---
 include/linux/printk.h |    6 ++++++
 kernel/printk/printk.c |    4 +---
 kernel/reboot.c        |    1 +
 3 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 4217a9f412b26..5b462029d03c1 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -207,6 +207,7 @@ void printk_legacy_allow_panic_sync(void);
 extern bool nbcon_device_try_acquire(struct console *con);
 extern void nbcon_device_release(struct console *con);
 void nbcon_atomic_flush_unsafe(void);
+bool pr_flush(int timeout_ms, bool reset_on_progress);
 #else
 static inline __printf(1, 0)
 int vprintk(const char *s, va_list args)
@@ -315,6 +316,11 @@ static inline void nbcon_atomic_flush_unsafe(void)
 {
 }
 
+static inline bool pr_flush(int timeout_ms, bool reset_on_progress)
+{
+	return true;
+}
+
 #endif
 
 bool this_cpu_in_panic(void);
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 07668433644b8..057db78876cd9 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2461,7 +2461,6 @@ asmlinkage __visible int _printk(const char *fmt, ...)
 }
 EXPORT_SYMBOL(_printk);
 
-static bool pr_flush(int timeout_ms, bool reset_on_progress);
 static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress);
 
 #else /* CONFIG_PRINTK */
@@ -2474,7 +2473,6 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 
 static u64 syslog_seq;
 
-static bool pr_flush(int timeout_ms, bool reset_on_progress) { return true; }
 static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress) { return true; }
 
 #endif /* CONFIG_PRINTK */
@@ -4466,7 +4464,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
  * Context: Process context. May sleep while acquiring console lock.
  * Return: true if all usable printers are caught up.
  */
-static bool pr_flush(int timeout_ms, bool reset_on_progress)
+bool pr_flush(int timeout_ms, bool reset_on_progress)
 {
 	return __pr_flush(NULL, timeout_ms, reset_on_progress);
 }
diff --git a/kernel/reboot.c b/kernel/reboot.c
index b5a8569e5d81f..41ab9e1ba3575 100644
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -704,6 +704,7 @@ void kernel_power_off(void)
 	migrate_to_reboot_cpu();
 	syscore_shutdown();
 	pr_emerg("Power down\n");
+	pr_flush(1000, true);
 	kmsg_dump(KMSG_DUMP_SHUTDOWN);
 	machine_power_off();
 }