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 EFBAFC43217 for ; Mon, 7 Nov 2022 14:18:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231924AbiKGOSG (ORCPT ); Mon, 7 Nov 2022 09:18:06 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55684 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232056AbiKGOQq (ORCPT ); Mon, 7 Nov 2022 09:16:46 -0500 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1A9D41CFFE for ; Mon, 7 Nov 2022 06:16:45 -0800 (PST) From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1667830603; 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=5TCVppZa0E2AuHd0nGaCpwvvOyi1zDUsbUnYVLs1Wys=; b=q0O+msnTIqJDVliJS2mLPmiTeN1bT4M4lS4vv1ta4Iles3swgem1QVh35gabpuNYnUkW9o kumUOwPGqpUFtE9uIVqJVyyJusic9EbpwXtVXM/DphHFhu7j/llHcsOvNhdoXOvKW8MwAQ J78ruFn8rKOWjEbg/78XMS7uTzmJ0ui6PHsH4xVdm6WM1flZAoz4X6TCt3/2V92m0+iDBx sC+yKZCWx+xbCVsF7NxRyQDNHLlA6cBfu2CHxYQN0D53Mjjz9COGM9xALCiHtPORtU1PtI 8M97XR5at4c2Z232jKNiAH5yZuym/gYqEsz5toH1Kq4UKFF4sVPpB5fzAj40Yw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1667830603; 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=5TCVppZa0E2AuHd0nGaCpwvvOyi1zDUsbUnYVLs1Wys=; b=yzRUX7qMIVq9AxG/h9VUumLt/k5928UDKof4oMa1D5IebwjjtNBshv84ecqWvcCu5r9Fi1 i2W2BEAaV5h3zaDg== To: Petr Mladek Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org, Greg Kroah-Hartman Subject: [PATCH printk v3 07/40] console: introduce console_is_enabled() wrapper Date: Mon, 7 Nov 2022 15:22:05 +0106 Message-Id: <20221107141638.3790965-8-john.ogness@linutronix.de> In-Reply-To: <20221107141638.3790965-1-john.ogness@linutronix.de> References: <20221107141638.3790965-1-john.ogness@linutronix.de> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. The primary reason for readers to access console->flags is to check if the console is enabled. Introduce console_is_enabled() to mark such access as a data race. Signed-off-by: John Ogness --- include/linux/console.h | 27 +++++++++++++++++++++++++++ kernel/printk/printk.c | 10 +++++----- 2 files changed, 32 insertions(+), 5 deletions(-) diff --git a/include/linux/console.h b/include/linux/console.h index f4f0c9523835..d9c636011364 100644 --- a/include/linux/console.h +++ b/include/linux/console.h @@ -172,6 +172,33 @@ extern void console_srcu_read_unlock(int cookie); extern struct hlist_head console_list; +/** + * console_is_enabled - Locklessly check if the console is enabled + * @con: struct console pointer of console to check + * + * Unless the caller is explicitly synchronizing against the console + * register/unregister/stop/start functions, this function should be + * used instead of manually readings console->flags and testing for + * the CON_ENABLED bit. + * + * 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. + * + * Context: Any context. + */ +static inline bool console_is_enabled(const struct console *con) +{ + /* + * 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)) & CON_ENABLED); +} + /** * 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 8974523f3107..79811984da34 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -3021,7 +3021,7 @@ void console_stop(struct console *console) { __pr_flush(console, 1000, true); console_lock(); - console->flags &= ~CON_ENABLED; + WRITE_ONCE(console->flags, console->flags & ~CON_ENABLED); console_unlock(); /* @@ -3037,7 +3037,7 @@ EXPORT_SYMBOL(console_stop); void console_start(struct console *console) { console_lock(); - console->flags |= CON_ENABLED; + WRITE_ONCE(console->flags, console->flags | CON_ENABLED); console_unlock(); __pr_flush(console, 1000, true); } @@ -3256,7 +3256,7 @@ void register_console(struct console *newcon) } else if (newcon->flags & CON_CONSDEV) { /* Only the new head can have CON_CONSDEV set. */ - console_first()->flags &= ~CON_CONSDEV; + WRITE_ONCE(console_first()->flags, console_first()->flags & ~CON_CONSDEV); hlist_add_head_rcu(&newcon->node, &console_list); } else { @@ -3308,7 +3308,7 @@ int unregister_console(struct console *console) console_lock(); /* Disable it unconditionally */ - console->flags &= ~CON_ENABLED; + WRITE_ONCE(console->flags, console->flags & ~CON_ENABLED); if (hlist_unhashed(&console->node)) { console_unlock(); @@ -3327,7 +3327,7 @@ int unregister_console(struct console *console) * console has any device attached. Oh well.... */ if (!hlist_empty(&console_list) && console->flags & CON_CONSDEV) - console_first()->flags |= CON_CONSDEV; + WRITE_ONCE(console_first()->flags, console_first()->flags | CON_CONSDEV); console_unlock(); -- 2.30.2