From nobody Tue Apr 7 22:02:25 2026 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 7F774C43217 for ; Wed, 16 Nov 2022 16:23:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238531AbiKPQXQ (ORCPT ); Wed, 16 Nov 2022 11:23:16 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58690 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234117AbiKPQWF (ORCPT ); Wed, 16 Nov 2022 11:22:05 -0500 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B01B143856 for ; Wed, 16 Nov 2022 08:22:04 -0800 (PST) From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1668615723; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=dwik/3763uUHWTxRRunPLlxyK1IUw8NqJotbljpAOY4=; b=JaroB9hPopOfMxbrqK090Z6M7wyOVm0NA0cKZ/SDYVDZGLIkZT9+z6j2ZZwuc03pmrhTbf eux7prTSPhbkaV7o+K8JCq3wHjSfQBJhtez+j65ip10w9XcNvBOruUuIJ7k6YWFkSIhK8l CYPGprrzT+Ybcf3PEIDSohCq/5yNXbnpKbq1BS51PUZU2bAktoTAlvpazS3mTtYn7oPWuE j+VWpzgR/+8mJ2ibx7C3hxVVVtN0OVa+iQLPGCbXznNFQU/M8E/MqmLhG6tQufxdiCmYmj oncn8PFDf8RnWTnyreefgQ9xHQZF1LLdVi4Rmcc3al8S7oRvm3fceP69GYfZNA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1668615723; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=dwik/3763uUHWTxRRunPLlxyK1IUw8NqJotbljpAOY4=; b=C8fSR7WZSQABfcv0J42d47XCNSOx+CBmi+brrOhA6dvtvdINUAEBbCH7+QH107YGjCCeAB H2Lo20plN1YMKYCg== To: Petr Mladek Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org, Greg Kroah-Hartman Subject: [PATCH printk v5 12/40] console: introduce wrappers to read/write console flags Date: Wed, 16 Nov 2022 17:27:24 +0106 Message-Id: <20221116162152.193147-13-john.ogness@linutronix.de> In-Reply-To: <20221116162152.193147-1-john.ogness@linutronix.de> References: <20221116162152.193147-1-john.ogness@linutronix.de> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" After switching to SRCU for console list iteration, some readers will begin readings console->flags as a data race. Locklessly reading console->flags provides a consistent value because there is at most one CPU modifying console->flags and that CPU is using only read-modify-write operations. Introduce a wrapper for SRCU iterators to read console flags. Introduce a matching wrapper to write to flags of registered consoles. Writing to flags of registered consoles is synchronized by the console_list_lock. Signed-off-by: John Ogness Reviewed-by: Petr Mladek --- include/linux/console.h | 45 +++++++++++++++++++++++++++++++++++++++++ kernel/printk/printk.c | 10 ++++----- 2 files changed, 50 insertions(+), 5 deletions(-) diff --git a/include/linux/console.h b/include/linux/console.h index 24d83e24840b..c1ca461d088a 100644 --- a/include/linux/console.h +++ b/include/linux/console.h @@ -183,6 +183,51 @@ extern void console_list_unlock(void) __releases(conso= le_mutex); =20 extern struct hlist_head console_list; =20 +/** + * console_srcu_read_flags - Locklessly read the console flags + * @con: struct console pointer of console to read flags from + * + * This function provides the necessary READ_ONCE() and data_race() + * notation for locklessly reading the console flags. The READ_ONCE() + * in this function matches the WRITE_ONCE() when @flags are modified + * for registered consoles with console_srcu_write_flags(). + * + * Only use this function to read console flags when locklessly + * iterating the console list via srcu. + * + * Context: Any context. + */ +static inline short console_srcu_read_flags(const struct console *con) +{ + WARN_ON_ONCE(!console_srcu_read_lock_is_held()); + + /* + * Locklessly reading console->flags provides a consistent + * read value because there is at most one CPU modifying + * console->flags and that CPU is using only read-modify-write + * operations to do so. + */ + return data_race(READ_ONCE(con->flags)); +} + +/** + * console_srcu_write_flags - Write flags for a registered console + * @con: struct console pointer of console to write flags to + * @flags: new flags value to write + * + * Only use this function to write flags for registered consoles. It + * requires holding the console_list_lock. + * + * Context: Any context. + */ +static inline void console_srcu_write_flags(struct console *con, short fla= gs) +{ + lockdep_assert_console_list_lock_held(); + + /* This matches the READ_ONCE() in console_srcu_read_flags(). */ + WRITE_ONCE(con->flags, flags); +} + /** * for_each_console_srcu() - Iterator over registered consoles * @con: struct console pointer used as loop cursor diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index f7479fd73114..35018f18f5aa 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -3068,7 +3068,7 @@ void console_stop(struct console *console) __pr_flush(console, 1000, true); console_list_lock(); console_lock(); - console->flags &=3D ~CON_ENABLED; + console_srcu_write_flags(console, console->flags & ~CON_ENABLED); console_unlock(); console_list_unlock(); =20 @@ -3086,7 +3086,7 @@ void console_start(struct console *console) { console_list_lock(); console_lock(); - console->flags |=3D CON_ENABLED; + console_srcu_write_flags(console, console->flags | CON_ENABLED); console_unlock(); console_list_unlock(); __pr_flush(console, 1000, true); @@ -3342,7 +3342,7 @@ void register_console(struct console *newcon) =20 } else if (newcon->flags & CON_CONSDEV) { /* Only the new head can have CON_CONSDEV set. */ - console_first()->flags &=3D ~CON_CONSDEV; + console_srcu_write_flags(console_first(), console_first()->flags & ~CON_= CONSDEV); hlist_add_head_rcu(&newcon->node, &console_list); =20 } else { @@ -3399,7 +3399,7 @@ static int unregister_console_locked(struct console *= console) console_lock(); =20 /* Disable it unconditionally */ - console->flags &=3D ~CON_ENABLED; + console_srcu_write_flags(console, console->flags & ~CON_ENABLED); =20 if (hlist_unhashed(&console->node)) { console_unlock(); @@ -3418,7 +3418,7 @@ static int unregister_console_locked(struct console *= console) * console has any device attached. Oh well.... */ if (!hlist_empty(&console_list) && console->flags & CON_CONSDEV) - console_first()->flags |=3D CON_CONSDEV; + console_srcu_write_flags(console_first(), console_first()->flags | CON_C= ONSDEV); =20 console_unlock(); =20 --=20 2.30.2