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 X-Spam-Level: X-Spam-Status: No, score=-11.0 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 7EAACC07E9B for ; Fri, 9 Jul 2021 09:12:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 59BDB613D3 for ; Fri, 9 Jul 2021 09:12:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231854AbhGIJPk (ORCPT ); Fri, 9 Jul 2021 05:15:40 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]:35068 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229559AbhGIJPj (ORCPT ); Fri, 9 Jul 2021 05:15:39 -0400 Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out1.suse.de (Postfix) with ESMTP id 13BB22217F; Fri, 9 Jul 2021 09:12:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1625821975; 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=wuq8AXnGSyHPWHlBAgr2stU1z9+pJd/xXc0LihvQkIg=; b=pLKGEPz4RX9m9xhtNK/iehnkcQPfsO/AwJ5/p4NWMXdfaIK0vEyogXXGG7pqB7oaqjt3IQ lfPrkyT0SJWHh4hgIYA9i77fRlp5EnqR+cEJgpGzHXsYcpz5ofWwFL4d0Oobg24cvqcfaO 8foEucjWZLfQFVmWv/aamHQhOU9hWic= Received: from suse.cz (unknown [10.100.216.66]) (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 BBDDAA3BC7; Fri, 9 Jul 2021 09:12:54 +0000 (UTC) Date: Fri, 9 Jul 2021 11:12:54 +0200 From: Petr Mladek To: John Ogness Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org, kernel test robot , stable@vger.kernel.org Subject: Re: [PATCH v2] printk/console: Check consistent sequence number when handling race in console_unlock() Message-ID: References: <20210702150657.26760-1-pmladek@suse.com> <87y2an7w1x.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 2021-07-07 14:26:03, Petr Mladek wrote: > On Sat 2021-07-03 08:32:02, John Ogness wrote: > > On 2021-07-02, Petr Mladek wrote: > > > The standard printk() tries to flush the message to the console > > > immediately. It tries to take the console lock. If the lock is > > > already taken then the current owner is responsible for flushing > > > even the new message. > > > > > > There is a small race window between checking whether a new message is > > > available and releasing the console lock. It is solved by re-checking > > > the state after releasing the console lock. If the check is positive > > > then console_unlock() tries to take the lock again and process the new > > > message as well. > > > > > > The commit 996e966640ddea7b535c ("printk: remove logbuf_lock") causes that > > > console_seq is not longer read atomically. As a result, the re-check might > > > be done with an inconsistent 64-bit index. > > > > > > Solve it by using the last sequence number that has been checked under > > > the console lock. In the worst case, it will take the lock again only > > > to realized that the new message has already been proceed. But it > > > was possible even before. > > > > > > The variable next_seq is marked as __maybe_unused to call down compiler > > > warning when CONFIG_PRINTK is not defined. > > > > As Sergey already pointed out, this patch is not fixing a real > > problem. An inconsistent value (or an increased consistent value) would > > mean that another printer is actively printing, and thus a retry is not > > necessary anyway. > > Ah, I misunderstood that part. You are right. CPU_X might see wrong > console_seq only when CPU_Y incremented console_seq. If CPU_X does not do > retry because of racy console_seq. Then CPU_Y would do retry when > yet another CPU added yet another new message in the meantime. > > > But this patch will avoid a KASAN message about an unmarked > > (although safe) data race. > > Yup. > > OK, I am going to queue the patch for-5.15. There is no need to > rush it for-4.14. The patch has been committed into printk/linux.git, branch rework/fixup-for-5.15. Note that I am going to use topic branches rework/* for the printk rework from now on. It will allow to be more flexible with pushing big changes and fixes into linux-next and mainline. The "rework/" prefix will still allow to differ printk rework-related changes from "unrelated" printk features and fixes. As a result, "printk-rework" branch will not longer be merged into "for-next" or "for-linus" branches. But I am still going to merge "rework/*" branches there so that "printk-rework" branch shows the printk rework history. I think about renaming this branch to "rework/history" or "rework/HEAD". Best Regards, Petr