linux-kernel - Re: [RFC][PATCHv5 07/13] printk: register syscore notifier

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Date:   Tue, 15 Aug 2017 13:56:56 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Andreas Mohr <andi@...as.de>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv5 07/13] printk: register syscore notifier

On Tuesday, August 15, 2017 4:56:19 AM CEST Sergey Senozhatsky wrote:
> We need to switch to emergency printk mode in kernel_kexec(). One
> kernel_kexec() branch calls kernel_restart_prepare(), which updates
> `system_state', however, the other one, when user requested to
> ->preserve_context, does not and we are lacking the information
> in printk about kexec being executed. Register a syscore notifier
> so printk will be notified by syscore_suspend().
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> ---
>  kernel/printk/printk.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index d3f149fad85c..b14e35f94596 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -50,6 +50,7 @@
>  #include <linux/sched/task_stack.h>
>  #include <linux/kthread.h>
>  #include <linux/suspend.h>
> +#include <linux/syscore_ops.h>
>  
>  #include <linux/uaccess.h>
>  #include <asm/sections.h>
> @@ -2941,6 +2942,22 @@ static struct notifier_block printk_pm_nb = {
>  	.notifier_call = printk_pm_notify,
>  };
>  
> +static int printk_syscore_suspend(void)
> +{
> +	printk_emergency_begin();
> +	return 0;
> +}

Somebody looking at suspend/resume code paths only will be wondering why
you need this, because we should be in the emergency mode already when
it is called.

It would be good to add a comment about the kexec context here.

> +
> +static void printk_syscore_resume(void)
> +{
> +	printk_emergency_end();
> +}

This will turn the emergency mode off during resume before the notifier
added by the previous patch AFAICS.  Is this intentional?

Thanks,
Rafael