linux-kernel - Re: [PATCH 1/2] printk: Add an option to flush all messages on panic

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Date:   Tue, 2 Apr 2019 11:14:19 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 1/2] printk: Add an option to flush all messages on panic

On (04/01/19 18:48), Feng Tang wrote:
> diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
> index 1fd45a8..58d9580 100644
> --- a/arch/powerpc/kernel/traps.c
> +++ b/arch/powerpc/kernel/traps.c
> @@ -179,7 +179,7 @@ extern void panic_flush_kmsg_end(void)
>  	kmsg_dump(KMSG_DUMP_PANIC);
>  	bust_spinlocks(0);
>  	debug_locks_off();
> -	console_flush_on_panic();
> +	console_flush_on_panic(false);
>  }
[..]
> @@ -277,7 +277,7 @@ void panic(const char *fmt, ...)
>  	 * panic() is not being callled from OOPS.
>  	 */
>  	debug_locks_off();
> -	console_flush_on_panic();
> +	console_flush_on_panic(false);
[..]
> -void console_flush_on_panic(void)
> +void console_flush_on_panic(bool flush_all)
>  {
>  	/*
>  	 * If someone else is holding the console lock, trylock will fail
> @@ -2539,6 +2540,12 @@ void console_flush_on_panic(void)
>  	 */
>  	console_trylock();
>  	console_may_schedule = 0;
> +
> +	/* User may want to see all the printk messages on panic */
> +	if (flush_all) {
> +		console_seq = log_first_seq;
> +		console_idx = log_first_idx;
> +	}
>  	console_unlock();

So my first thought was - let's not add a `bool flag', but instead add
an `enum' with clear flag names, e.g. DUMP_ALL/DUMP_PENDING, etc. Something
similar to what ftrace_dump(DUMP_ALL) does. And we already have panic_print
bit-mask and panic_print_sys_info(), which controls what we print when in
panic. So we can move console_flush_on_panic() to panic_print_sys_info()
and handle different types of console_flush_on_panic() there:

---

diff --git a/kernel/panic.c b/kernel/panic.c
index 0ae0d7332f12..e142faaebbcd 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -134,6 +134,11 @@ EXPORT_SYMBOL(nmi_panic);
 
 static void panic_print_sys_info(void)
 {
+	if (panic_print & PANIC_PRINT_REPLAY_LOGBUF)
+		console_flush_on_panic(DUMP_ALL);
+	else
+		console_flush_on_panic(DUMP_PENDING);
+
 	if (panic_print & PANIC_PRINT_TASK_INFO)
 		show_state();
 
@@ -277,7 +282,6 @@ void panic(const char *fmt, ...)
 	 * panic() is not being callled from OOPS.
 	 */
 	debug_locks_off();
-	console_flush_on_panic();
 
 	panic_print_sys_info();
 
---

After looking at this more - DUMP_ALL/DUMP_PENDING clearly depend
on panic_print. So... May be we can move panic_print_sys_info() to
printk.c, and for PANIC_PRINT_REPLAY_LOGBUF case just reset console
seq/idx (console seq/idx are internal to printk) before we
flush_on_panic. This way we would not need to modify
console_flush_on_panic() prototype at all.

Let's hear from people.

	-ss