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 7DC23C4332F for ; Tue, 15 Nov 2022 13:22:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229557AbiKONWT (ORCPT ); Tue, 15 Nov 2022 08:22:19 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51936 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230123AbiKONWS (ORCPT ); Tue, 15 Nov 2022 08:22:18 -0500 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 00792DEC6; Tue, 15 Nov 2022 05:22:17 -0800 (PST) Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id AFDB21F8CD; Tue, 15 Nov 2022 13:22:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1668518536; 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=QmX7RJNNVBj1FU2joU/lqNhMfCU9lMkhGvr+3DdJhfg=; b=iRMOUOKOWNnBtw6+eGOtN4tmTznrw3/yio0CZFdwk76Npxu5xZrQMnLuPWOJZaZRJhr8bU OuFgBhGGQMQLaOoIJ2fF/z9dxbUcOLYu0CaEHVNetJKx8C7V4C0b5tqrI+hjNkNq+mQ5x8 2G+HZgSGsI5B+D3aEntVGEYNZsUZGT4= Received: from suse.cz (unknown [10.100.208.146]) (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 6B80D2C142; Tue, 15 Nov 2022 13:22:16 +0000 (UTC) Date: Tue, 15 Nov 2022 14:22:15 +0100 From: Petr Mladek To: John Ogness Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org, Helge Deller , Greg Kroah-Hartman , Javier Martinez Canillas , Thomas Zimmermann , Juergen Gross , Boris Ostrovsky , Tom Rix , linux-fbdev@vger.kernel.org, dri-devel@lists.freedesktop.org Subject: Re: [PATCH printk v4 31/39] printk, xen: fbfront: create/use safe function for forcing preferred Message-ID: References: <20221114162932.141883-1-john.ogness@linutronix.de> <20221114162932.141883-32-john.ogness@linutronix.de> <87mt8tfh6x.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87mt8tfh6x.fsf@jogness.linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-fbdev@vger.kernel.org On Mon 2022-11-14 20:57:18, John Ogness wrote: > Hi, > > After more detailed runtime testing I discovered that I didn't re-insert > the console to the correct place in the list. More below... > > > --- a/kernel/printk/printk.c > > +++ b/kernel/printk/printk.c > > @@ -3461,6 +3462,48 @@ int unregister_console(struct console *console) > > } > > EXPORT_SYMBOL(unregister_console); > > > > +/** > > + * console_force_preferred_locked - force a registered console preferred > > + * @con: The registered console to force preferred. > > + * > > + * Must be called under console_list_lock(). > > + */ > > +void console_force_preferred_locked(struct console *con) > > +{ > > + struct console *cur_pref_con; > > + > > + if (!console_is_registered_locked(con)) > > + return; > > + > > + cur_pref_con = console_first(); > > + > > + /* Already preferred? */ > > + if (cur_pref_con == con) > > + return; > > + > > + /* > > + * Delete, but do not re-initialize the entry. This allows the console > > + * to continue to appear registered (via any hlist_unhashed_lockless() > > + * checks), even though it was briefly removed from the console list. > > + */ > > + hlist_del_rcu(&con->node); > > + > > + /* > > + * Ensure that all SRCU list walks have completed so that the console > > + * can be added to the beginning of the console list and its forward > > + * list pointer can be re-initialized. > > + */ > > + synchronize_srcu(&console_srcu); > > + > > + con->flags |= CON_CONSDEV; > > + WARN_ON(!con->device); > > + > > + /* Only the new head can have CON_CONSDEV set. */ > > + console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV); > > + hlist_add_behind_rcu(&con->node, console_list.first); > > This is adding the console as the 2nd item. It should be the new > head. The patch below fixes it. > > I have done careful runtime testing with this fixup. After the > force_preferred, the console is the new head and sending data to > /dev/console redirects to that console. Great catch! > It would be nice if we could fold this in. Sorry. I have missed it as well :-/ > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 8d635467882f..4b77586cf4cb 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -3494,7 +3494,7 @@ void console_force_preferred_locked(struct console *con) > > /* Only the new head can have CON_CONSDEV set. */ > console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV); > - hlist_add_behind_rcu(&con->node, console_list.first); > + hlist_add_head_rcu(&con->node, &console_list); > } > EXPORT_SYMBOL(console_force_preferred_locked); With this change: Reviewed-by: Petr Mladek Best Regards, Petr