From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1757855AbcH3JjM (ORCPT <rfc822;w@1wt.eu>);
        Tue, 30 Aug 2016 05:39:12 -0400
Received: from mail-pa0-f65.google.com ([209.85.220.65]:33968 "EHLO
        mail-pa0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1757384AbcH3JjL (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 30 Aug 2016 05:39:11 -0400
Date: Tue, 30 Aug 2016 18:39:18 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] printk/nmi: avoid direct printk()-s from
 __printk_nmi_flush()
Message-ID: <20160830093918.GA23693@swordfish>
References: <20160829123220.1295-1-sergey.senozhatsky@gmail.com>
 <20160829151600.GM4866@pathway.suse.cz>
 <20160830075834.GA494@swordfish>
 <20160830090436.GO4866@pathway.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20160830090436.GO4866@pathway.suse.cz>
User-Agent: Mutt/1.7.0 (2016-08-17)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On (08/30/16 11:04), Petr Mladek wrote:
> On Tue 2016-08-30 16:58:34, Sergey Senozhatsky wrote:
> > Petr,
> > one more question. Not related to the patch, but still related to NMI.
> > 
> > can NMI nest?
> 
> AFAIK, they cannot. NMIs should be disabled until iret is called.
> Therefore we should be on the safe side if iret is not called
> inside the NMI handler. But this should not happen because
> it would cause other problems, like using wrong return address.
> 
> Well, x86 nmi code has some hacks to handle exceptions inside
> NMI handlers that use iret. But printk_nmi_enter()/printk_nmi_exit()
> are never nested there. It is prevented by the nmi_state per-CPU
> variable. See do_nmi() in arch/x86/kernel/nmi.c.

yes, x86 has a per-cpu nmi_state to handle the case when NMI is
loosing its NMI context. But other arch-s, as far as I can see,
don't do that. Does it mean that we are safe only on x86?

this printk_func_saved thing is still will be needed, I think,
for alt_printk.

Example:

process abc
	printk()
		alt_printk_enter()
			this_cpu_write(printk_func, vprintk_alt);
->	NMI
	:	printk_nmi_enter()
	:		this_cpu_write(printk_func, vprintk_nmi);
	:	printk_nmi_exit()
	:		this_cpu_write(printk_func, vprintk_default);
	return NMI

		printk()  <<<<  nested printk -> vprintk_default(), set by nmi_exit()
		alt_printk_exit()
	...

	-ss