From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2047BC54EE9 for ; Tue, 27 Sep 2022 09:56:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231615AbiI0J4f (ORCPT ); Tue, 27 Sep 2022 05:56:35 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41860 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230376AbiI0J4d (ORCPT ); Tue, 27 Sep 2022 05:56:33 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [IPv6:2001:67c:2178:6::1c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 34E6260529 for ; Tue, 27 Sep 2022 02:56:32 -0700 (PDT) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id E0E8C21CFD; Tue, 27 Sep 2022 09:56:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1664272590; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=SsFlZVhnAlWJT1H2/PkcPlbNRl2sk4GBcclzoTxgr48=; b=dsDtG87TjPPnF+pS0LkQS82nBxvxETj/La9Ls95Y0X1+dE3FWjwAaLByxz+jFvYWV2EZnT zhYWD1iFxs1uts95te6m6RDiFtZhKel2XnrEX0JITr5uwAJXyPy2woaG5Kzm8SlPd9xzKA q9pdJTYTv3iu7EiwmluLr6mZh6Z//2U= Received: from suse.cz (unknown [10.100.201.202]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id DD4CD2C162; Tue, 27 Sep 2022 09:56:28 +0000 (UTC) Date: Tue, 27 Sep 2022 11:56:27 +0200 From: Petr Mladek To: Thomas Gleixner Cc: LKML , John Ogness , Sergey Senozhatsky , Steven Rostedt , Linus Torvalds , Peter Zijlstra , "Paul E. McKenney" , Daniel Vetter , Greg Kroah-Hartman , Helge Deller , Jason Wessel , Daniel Thompson Subject: Re: [patch RFC 06/29] printk: Protect [un]register_console() with a mutex Message-ID: References: <20220910221947.171557773@linutronix.de> <20220910222300.712668210@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220910222300.712668210@linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun 2022-09-11 00:27:41, Thomas Gleixner wrote: > Unprotected list walks are a brilliant idea. Especially in the context of > hotpluggable consoles. Yeah, it is crazy. And it is there probably since the beginning. > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -79,10 +79,14 @@ int oops_in_progress; > EXPORT_SYMBOL(oops_in_progress); > > /* > - * console_sem protects the console_drivers list, and also > - * provides serialisation for access to the entire console > - * driver system. > + * console_sem protects the console_drivers list, and also provides > + * serialization for access to the entire console driver system. > + * > + * console_mutex serializes register/unregister. console_sem has to be > + * taken for any list manipulation inside the console_mutex locked > + * section to keep the console BKL machinery happy. > */ > +static DEFINE_MUTEX(console_mutex); > static DEFINE_SEMAPHORE(console_sem); > struct console *console_drivers; > EXPORT_SYMBOL_GPL(console_drivers); > @@ -220,6 +230,26 @@ int devkmsg_sysctl_set_loglvl(struct ctl > } > #endif /* CONFIG_PRINTK && CONFIG_SYSCTL */ > > +/** > + * console_list_lock - Lock the console list > + * > + * For non-console related list walks, e.g. procfs, sysfs... > + */ > +void console_list_lock(void) > +{ > + mutex_lock(&console_mutex); > +} > + > +/** > + * console_list_unlock - Unlock the console list > + * > + * Counterpart to console_list_lock() > + */ > +void console_list_unlock(void) > +{ > + mutex_unlock(&console_mutex); > +} > + > /* > * Helper macros to handle lockdep when locking/unlocking console_sem. We use > * macros instead of functions so that _RET_IP_ contains useful information. > @@ -3107,13 +3143,14 @@ void register_console(struct console *ne > bool realcon_enabled = false; > int err; > > - for_each_console(con) { > + console_list_lock(); Hmm, the new mutex is really nasty. It has very strange semantic. It makes the locking even more complicated. The ideal solution would be take console_lock() here. We (me and Sergey) never did it because con->match() and con->setup() callbacks were called in try_enable_*console(). We were afraid that some might want to take console_lock() and it could create a deadlock. There were too many drivers and we did not found time to check them all. And it had low priority because nobody reported problems. A good enough solution might be call this under the later added srcu_read_lock(&console_srcu) and use for_each_console_srcu(). The srcu walk would prevent seeing broken list. Obviously, the code might see outdated list and do bad decisions: + try to enable the same console twice + enable more consoles by default in try_enable_default_console() + associate more consoles with /dev/console, see CON_CONSDEV in try_enable_preferred_console() and try_enable_default_console() If we race then we could end up with more consoles enabled by default and with more consoles with CON_CONSDEV flag. IMHO, the rcu walk is an acceptable and conservative solution. Registering the same driver twice is hard to imagine at all. And I have never seen reports about too many default consoles or CON_CONSDEV flags. Anyway, I would like to avoid adding console_mutex. From my POV, it is a hack that complicates the code. Taking console_lock() should be enough. Using rcu walk would be good enough. Do I miss something, please? > + for_each_registered_console(con) { > if (WARN(con == newcon, "console '%s%d' already registered\n", > con->name, con->index)) > - return; > + goto unlock; > } > > - for_each_console(con) { > + for_each_registered_console(con) { > if (con->flags & CON_BOOT) > bootcon_enabled = true; > else Best Regards, Petr