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From: "Rafael J. Wysocki" <rjw@rjwysocki.net>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Petr Mladek <pmladek@suse.com>, Steven Rostedt <rostedt@goodmis.org>,
        Jan Kara <jack@suse.cz>, Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Eric Biederman <ebiederm@xmission.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>, Pavel Machek <pavel@ucw.cz>,
        Andreas Mohr <andi@lisas.de>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        linux-kernel@vger.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [RFC][PATCHv5 06/13] printk: register PM notifier
Date: Tue, 15 Aug 2017 13:51:19 +0200
Message-ID: <10043624.VvthE8rrlX@aspire.rjw.lan>
In-Reply-To: <20170815025625.1977-7-sergey.senozhatsky@gmail.com>
References: <20170815025625.1977-1-sergey.senozhatsky@gmail.com> <20170815025625.1977-7-sergey.senozhatsky@gmail.com>
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On Tuesday, August 15, 2017 4:56:18 AM CEST Sergey Senozhatsky wrote:
> It's not always possible/safe to wake_up() printk kernel
> thread. For example, late suspend/early resume may printk()
> while timekeeping is not initialized yet, so calling into the
> scheduler may result in recursive warnings.
> 
> Another thing to notice is the fact that PM at some point
> freezes user space and kernel threads: freeze_processes()
> and freeze_kernel_threads(), correspondingly. Thus we need
> printk() to operate in emergency mode there and attempt to
> immediately flush pending kernel message to the console.
> 
> This patch registers PM notifier, so PM can switch printk
> to emergency mode from PM_FOO_PREPARE notifiers and return

Isn't that too early?  That's before user space is frozen even.

> back to printk threaded mode from PM_POST_FOO notifiers.

And isn't that too late?

> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> Suggested-by: Andreas Mohr <andi@lisas.de>
> ---
>  kernel/printk/printk.c | 35 +++++++++++++++++++++++++++++++++++
>  1 file changed, 35 insertions(+)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 05165f008bc8..d3f149fad85c 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -49,6 +49,7 @@
>  #include <linux/sched/debug.h>
>  #include <linux/sched/task_stack.h>
>  #include <linux/kthread.h>
> +#include <linux/suspend.h>
>  
>  #include <linux/uaccess.h>
>  #include <asm/sections.h>
> @@ -2913,6 +2914,33 @@ static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = {
>  	.flags = IRQ_WORK_LAZY,
>  };
>  
> +static int printk_pm_notify(struct notifier_block *notify_block,
> +			    unsigned long mode, void *unused)
> +{
> +	switch (mode) {
> +	case PM_HIBERNATION_PREPARE:
> +	case PM_SUSPEND_PREPARE:
> +	case PM_RESTORE_PREPARE:
> +		printk_emergency_begin_sync();

I'm not sure what would be wrong with calling this directly
from dpm_suspend_noirq().

> +		break;
> +
> +	case PM_POST_SUSPEND:
> +	case PM_POST_HIBERNATION:
> +	case PM_POST_RESTORE:
> +		printk_emergency_end_sync();

And this could be called from dpm_resume_noirq().

In which case you wouldn't really need the stuff below.

Thanks,
Rafael