linux-kernel - Re: [RFC][PATCH v6 1/2] printk: Make printk() completely async

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Date:	Tue, 22 Mar 2016 17:36:12 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.com>, Tejun Heo <tj@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org,
	Byungchul Park <byungchul.park@....com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Jan Kara <jack@...e.cz>
Subject: Re: [RFC][PATCH v6 1/2] printk: Make printk() completely async

On Tue 2016-03-22 02:25:29, Sergey Senozhatsky wrote:
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index e38579d..99c105d6 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1619,12 +1721,21 @@ asmlinkage int vprintk_emit(int facility, int level,
>  	int printed_len = 0;
>  	int nmi_message_lost;
>  	bool in_sched = false;
> -	/* cpu currently holding logbuf_lock in this function */
> -	static unsigned int logbuf_cpu = UINT_MAX;
> +	bool in_panic = console_loglevel == CONSOLE_LOGLEVEL_MOTORMOUTH;

I am just looking at the printk in NMI patchset and I will need to
deal with the panic state as well. I am not sure if this detection
is secure.

This console level is set also by kdb_show_stack()
and kdb_dumpregs(). I am not sure how this kdb stuff works
and if it affects normal kernel but...

Anyway, it seems that many locations detects the panic situation
via the variable oops_in_progress. It has another advantage
that it can be easily checked and we would not need any extra
variable here.

> +	bool sync_print = printk_sync;
> +
> +	/* disable async printk */
> +	if (in_panic)
> +		printk_sync = true;

In each case, we should not switch the global state if we
only print debug traces.

Best Regards,
Petr