From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=vL1f=RL=vger.kernel.org=linux-kernel-owner@kernel.org>
X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on
	aws-us-west-2-korg-lkml-1.web.codeaurora.org
X-Spam-Level: 
X-Spam-Status: No, score=-3.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,
	MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_NEOMUTT autolearn=ham
	autolearn_force=no version=3.4.0
Received: from mail.kernel.org (mail.kernel.org [198.145.29.99])
	by smtp.lore.kernel.org (Postfix) with ESMTP id C823CC43381
	for <linux-kernel@archiver.kernel.org>; Fri,  8 Mar 2019 10:31:30 +0000 (UTC)
Received: from vger.kernel.org (vger.kernel.org [209.132.180.67])
	by mail.kernel.org (Postfix) with ESMTP id A046320854
	for <linux-kernel@archiver.kernel.org>; Fri,  8 Mar 2019 10:31:30 +0000 (UTC)
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1726463AbfCHKb3 (ORCPT
        <rfc822;linux-kernel@archiver.kernel.org>);
        Fri, 8 Mar 2019 05:31:29 -0500
Received: from mx2.suse.de ([195.135.220.15]:59196 "EHLO mx1.suse.de"
        rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP
        id S1725975AbfCHKb3 (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 8 Mar 2019 05:31:29 -0500
X-Virus-Scanned: by amavisd-new at test-mx.suse.de
Received: from relay2.suse.de (unknown [195.135.220.254])
        by mx1.suse.de (Postfix) with ESMTP id C968DAF7B;
        Fri,  8 Mar 2019 10:31:27 +0000 (UTC)
Date:   Fri, 8 Mar 2019 11:31:27 +0100
From:   Petr Mladek <pmladek@suse.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc:     John Ogness <john.ogness@linutronix.de>,
        linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Daniel Wang <wonderfly@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@suse.com>,
        Peter Feiner <pfeiner@google.com>,
        linux-serial@vger.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [RFC PATCH v1 19/25] printk: introduce emergency messages
Message-ID: <20190308103127.txsgv3d6lqlf6pad@pathway.suse.cz>
References: <20190212143003.48446-1-john.ogness@linutronix.de>
 <20190212143003.48446-20-john.ogness@linutronix.de>
 <20190307073029.GA489@jagdpanzerIV>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190307073029.GA489@jagdpanzerIV>
User-Agent: NeoMutt/20170421 (1.8.2)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu 2019-03-07 16:30:29, Sergey Senozhatsky wrote:
> On (02/12/19 15:29), John Ogness wrote:
> [..]
> > +static bool console_can_emergency(int level)
> > +{
> > +	struct console *con;
> > +
> > +	for_each_console(con) {
> > +		if (!(con->flags & CON_ENABLED))
> > +			continue;
> > +		if (con->write_atomic && level < emergency_console_loglevel)
> > +			return true;
> > +		if (con->write && (con->flags & CON_BOOT))
> > +			return true;
> > +	}
> > +	return false;
> > +}
> > +
> > +static void call_emergency_console_drivers(int level, const char *text,
> > +					   size_t text_len)
> > +{
> > +	struct console *con;
> > +
> > +	for_each_console(con) {
> > +		if (!(con->flags & CON_ENABLED))
> > +			continue;
> > +		if (con->write_atomic && level < emergency_console_loglevel) {
> > +			con->write_atomic(con, text, text_len);
> > +			continue;
> > +		}
> > +		if (con->write && (con->flags & CON_BOOT)) {
> > +			con->write(con, text, text_len);
> > +			continue;
> > +		}
> > +	}
> > +}
> > +
> > +static void printk_emergency(char *buffer, int level, u64 ts_nsec, u16 cpu,
> > +			     char *text, u16 text_len)
> > +{
> > +	struct printk_log msg;
> > +	size_t prefix_len;
> > +
> > +	if (!console_can_emergency(level))
> > +		return;
> > +
> > +	msg.level = level;
> > +	msg.ts_nsec = ts_nsec;
> > +	msg.cpu = cpu;
> > +	msg.facility = 0;
> > +
> > +	/* "text" must have PREFIX_MAX preceding bytes available */
> > +
> > +	prefix_len = print_prefix(&msg,
> > +				  console_msg_format & MSG_FORMAT_SYSLOG,
> > +				  printk_time, buffer);
> > +	/* move the prefix forward to the beginning of the message text */
> > +	text -= prefix_len;
> > +	memmove(text, buffer, prefix_len);
> > +	text_len += prefix_len;
> > +
> > +	text[text_len++] = '\n';
> > +
> > +	call_emergency_console_drivers(level, text, text_len);
> 
> So this iterates the console list and calls consoles' callbacks, but what
> prevents console driver to be rmmod-ed under us?
> 
> 	CPU0						CPU1
> 
> 	printk_emergency()				rmmod netcon
> 	 call_emergency_console_drivers()		
> 	  con_foo->flags & CON_ENABLED == 1
> 							unregister_console(con_foo)
> 							con_foo->flags &= ~CON_ENABLED
> 							__exit // con_foo gone ?
> 	  con_foo->write()
> 
> We use console_lock()/console_trylock() in order to protect the list and
> console drivers; but this brings scheduler to the picture, with all its
> locks.

Great catch!

I think that it is doable to guard the list using RCU.

Best Regards,
Petr