panic: add note that panic_print sysctl interface is deprecated

Add a dedicated core parameter 'panic_console_replay' for controlling
console replay, and add note that 'panic_print' sysctl interface will be
obsoleted by 'panic_sys_info' and 'panic_console_replay'.  When it
happens, the SYS_INFO_PANIC_CONSOLE_REPLAY can be removed as well.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Feng Tang <[email protected]>
Suggested-by: Petr Mladek <[email protected]>
Cc: John Ogness <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Lance Yang <[email protected]>
Cc: "Paul E . McKenney" <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Feng Tang

Documentation/admin-guide/kernel-parameters.txt

@@ -4578,6 +4578,10 @@
 
                         This is a human readable alternative to the 'panic_print' option.
 
+	panic_console_replay
+			When panic happens, replay all kernel messages on
+			consoles at the end of panic.
+
 	parkbd.port=	[HW] Parallel port number the keyboard adapter is
 			connected to, default is 0.
 			Format: <parport#>

kernel/panic.c

@@ -64,6 +64,7 @@ int panic_on_warn __read_mostly;
 unsigned long panic_on_taint;
 bool panic_on_taint_nousertaint = false;
 static unsigned int warn_limit __read_mostly;
+static bool panic_console_replay;
 
 bool panic_triggering_all_cpu_backtrace;
 
@@ -77,6 +78,13 @@ ATOMIC_NOTIFIER_HEAD(panic_notifier_list);
 EXPORT_SYMBOL(panic_notifier_list);
 
 #ifdef CONFIG_SYSCTL
+static int sysctl_panic_print_handler(const struct ctl_table *table, int write,
+			   void *buffer, size_t *lenp, loff_t *ppos)
+{
+	pr_info_once("Kernel: 'panic_print' sysctl interface will be obsoleted by both 'panic_sys_info' and 'panic_console_replay'\n");
+	return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
+}
+
 static const struct ctl_table kern_panic_table[] = {
 #ifdef CONFIG_SMP
 	{
@@ -108,7 +116,7 @@ static const struct ctl_table kern_panic_table[] = {
 		.data		= &panic_print,
 		.maxlen		= sizeof(unsigned long),
 		.mode		= 0644,
-		.proc_handler	= proc_doulongvec_minmax,
+		.proc_handler	= sysctl_panic_print_handler,
 	},
 	{
 		.procname	= "panic_on_warn",
@@ -247,12 +255,6 @@ void nmi_panic(struct pt_regs *regs, const char *msg)
 }
 EXPORT_SYMBOL(nmi_panic);
 
-static void panic_console_replay(void)
-{
-	if (panic_print & SYS_INFO_PANIC_CONSOLE_REPLAY)
-		console_flush_on_panic(CONSOLE_REPLAY_ALL);
-}
-
 void check_panic_on_warn(const char *origin)
 {
 	unsigned int limit;
@@ -427,7 +429,9 @@ void panic(const char *fmt, ...)
 	debug_locks_off();
 	console_flush_on_panic(CONSOLE_FLUSH_PENDING);
 
-	panic_console_replay();
+	if ((panic_print & SYS_INFO_PANIC_CONSOLE_REPLAY) ||
+		panic_console_replay)
+		console_flush_on_panic(CONSOLE_REPLAY_ALL);
 
 	if (!panic_blink)
 		panic_blink = no_blink;
@@ -869,6 +873,7 @@ core_param(panic_print, panic_print, ulong, 0644);
 core_param(pause_on_oops, pause_on_oops, int, 0644);
 core_param(panic_on_warn, panic_on_warn, int, 0644);
 core_param(crash_kexec_post_notifiers, crash_kexec_post_notifiers, bool, 0644);
+core_param(panic_console_replay, panic_console_replay, bool, 0644);
 
 static int __init oops_setup(char *s)
 {