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 46992C54EE9 for ; Mon, 19 Sep 2022 12:50:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230202AbiISMt7 (ORCPT ); Mon, 19 Sep 2022 08:49:59 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33124 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229952AbiISMtz (ORCPT ); Mon, 19 Sep 2022 08:49:55 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CC770266B for ; Mon, 19 Sep 2022 05:49:52 -0700 (PDT) From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1663591791; h=from:from:reply-to:subject:subject: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=H2fk5UgooGqmYDFl5idROCKpAEIlHVlp9hl/WZSTLHI=; b=teD7/h7ErOV6ct5FPG9wslM1TEXZBhMAqGhoJXpu7Quzay3klIbRxR2IhjXceR1Y/fE4Wy O2U3jnSgiMwIR9kLdGuUQ9FQguASbLxO5a61hydTMaHqcPzmnKp3u7ULhMtHLYNjY0ehUM fuSrbF30Z+CMLYYOnHswrQSWR99N+K/HzBDqtSfp1elT2dU0Lwo2XYpZFU/pfWviEGGHkN N10c0jVTMTGVSlYUFove962aex78olT5jiGpu+abEMP2feOBtCU+TrzDJZ5brmYZNgQ5HH AXaT5BFEG+ePshAxdaVXLfinU9kzGAUKtvmwPJgP+1DhWxka1O3ByD96J7G31g== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1663591791; h=from:from:reply-to:subject:subject: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=H2fk5UgooGqmYDFl5idROCKpAEIlHVlp9hl/WZSTLHI=; b=bAbspF65yuZdzkxSYZ8aKEx1jQgQUspXn+mi6iAwAA61OCqLyLBT7SbcbTx24eXGBYokr6 0/U7pwQK3itFY2Cw== To: Sergey Senozhatsky , Thomas Gleixner Cc: LKML , Petr Mladek , 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 In-Reply-To: References: <20220910221947.171557773@linutronix.de> <20220910222300.712668210@linutronix.de> Date: Mon, 19 Sep 2022 14:55:50 +0206 Message-ID: <87pmfrlea9.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022-09-14, Sergey Senozhatsky wrote: > On (22/09/11 00:27), Thomas Gleixner wrote: > [..] >> + * 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); > [..] >> /* >> * Helper macros to handle lockdep when locking/unlocking console_sem. We use >> * macros instead of functions so that _RET_IP_ contains useful information. >> @@ -2978,17 +3008,21 @@ struct tty_driver *console_device(int *i >> void console_stop(struct console *console) >> { >> __pr_flush(console, 1000, true); >> + console_list_lock(); >> console_lock(); >> console->flags &= ~CON_ENABLED; >> console_unlock(); >> + console_list_unlock(); >> } >> EXPORT_SYMBOL(console_stop); >> >> void console_start(struct console *console) >> { >> + console_list_lock(); >> console_lock(); >> console->flags |= CON_ENABLED; >> console_unlock(); >> + console_list_unlock(); >> __pr_flush(console, 1000, true); >> } >> EXPORT_SYMBOL(console_start); > > So the comment says that list lock (console_mutex) is to serialize > register/unregister, but then we take it in stop/start as well. What > does list lock protect us against in start/stop? console->flags reader > (console_is_usable()) does not take list lock, it's called under console > lock and console->flags writers (console_unregister() and console_stop()) > modify console->flags under console_lock. Currently all writers to console->flags are holding the console_lock. However, there are console->flags readers that do _not_ hold the console_lock (register_console, unregister_console, printk_late_init). Aside from adding list synchronization, the list lock also provides the missing console->flags synchronization. Now all console->flags writers hold the list lock _and_ console_lock. A console->flags reader can hold either the list lock or the console_lock. Since console_start and console_stop are console->flags writers, they also need to take the list lock. I agree that this should be mentioned in the commit message and code comments. The follow-up patch in the series only deals with list/flags readers. Therefore I think the change to console_stop/console_start belongs in this patch, which focusses on fixing synchronization. John