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 323FACCA47F for ; Tue, 7 Jun 2022 21:27:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1378318AbiFGV1T (ORCPT ); Tue, 7 Jun 2022 17:27:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38786 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1378209AbiFGVX4 (ORCPT ); Tue, 7 Jun 2022 17:23:56 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 27FE42271B1; Tue, 7 Jun 2022 12:00:56 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id D7F98B823A0; Tue, 7 Jun 2022 19:00:54 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 55743C385A2; Tue, 7 Jun 2022 19:00:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1654628453; bh=WOGbrWxzyZjlwEgeZMiKrO6Yp8c0OJ/aqaNZJ65xRjo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=Rw7vUjNfbsODbKOI5E6aW8i8a3mOUipAVrM5zZVjRMA0ZA/yWHceuj9CBpBxAyC4Y z0D0j8pXgYVYHSAOvJWl/lg1clrlNv9QlohLMQvg+gi1UvML2BjJfMSgGVOdJzF3Io UIVybriGGfP3dW/a8FaK2eOFPuzZtzo0r1Q9GSM8= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, John Ogness , Petr Mladek , Sasha Levin Subject: [PATCH 5.18 336/879] printk: add missing memory barrier to wake_up_klogd() Date: Tue, 7 Jun 2022 18:57:34 +0200 Message-Id: <20220607165012.610008844@linuxfoundation.org> X-Mailer: git-send-email 2.36.1 In-Reply-To: <20220607165002.659942637@linuxfoundation.org> References: <20220607165002.659942637@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: John Ogness [ Upstream commit 1f5d783094cf28b4905f51cad846eb5d1db6673e ] It is important that any new records are visible to preparing waiters before the waker checks if the wait queue is empty. Otherwise it is possible that: - there are new records available - the waker sees an empty wait queue and does not wake - the preparing waiter sees no new records and begins to wait This is exactly the problem that the function description of waitqueue_active() warns about. Use wq_has_sleeper() instead of waitqueue_active() because it includes the necessary full memory barrier. Signed-off-by: John Ogness Reviewed-by: Petr Mladek Signed-off-by: Petr Mladek Link: https://lore.kernel.org/r/20220421212250.565456-4-john.ogness@linutronix.de Signed-off-by: Sasha Levin --- kernel/printk/printk.c | 39 ++++++++++++++++++++++++++++++++++++--- 1 file changed, 36 insertions(+), 3 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index da03c15ecc89..ed6f20992915 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -746,8 +746,19 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf, goto out; } + /* + * Guarantee this task is visible on the waitqueue before + * checking the wake condition. + * + * The full memory barrier within set_current_state() of + * prepare_to_wait_event() pairs with the full memory barrier + * within wq_has_sleeper(). + * + * This pairs with wake_up_klogd:A. + */ ret = wait_event_interruptible(log_wait, - prb_read_valid(prb, atomic64_read(&user->seq), r)); + prb_read_valid(prb, + atomic64_read(&user->seq), r)); /* LMM(devkmsg_read:A) */ if (ret) goto out; } @@ -1513,7 +1524,18 @@ static int syslog_print(char __user *buf, int size) seq = syslog_seq; mutex_unlock(&syslog_lock); - len = wait_event_interruptible(log_wait, prb_read_valid(prb, seq, NULL)); + /* + * Guarantee this task is visible on the waitqueue before + * checking the wake condition. + * + * The full memory barrier within set_current_state() of + * prepare_to_wait_event() pairs with the full memory barrier + * within wq_has_sleeper(). + * + * This pairs with wake_up_klogd:A. + */ + len = wait_event_interruptible(log_wait, + prb_read_valid(prb, seq, NULL)); /* LMM(syslog_print:A) */ mutex_lock(&syslog_lock); if (len) @@ -3316,7 +3338,18 @@ void wake_up_klogd(void) return; preempt_disable(); - if (waitqueue_active(&log_wait)) { + /* + * Guarantee any new records can be seen by tasks preparing to wait + * before this context checks if the wait queue is empty. + * + * The full memory barrier within wq_has_sleeper() pairs with the full + * memory barrier within set_current_state() of + * prepare_to_wait_event(), which is called after ___wait_event() adds + * the waiter but before it has checked the wait condition. + * + * This pairs with devkmsg_read:A and syslog_print:A. + */ + if (wq_has_sleeper(&log_wait)) { /* LMM(wake_up_klogd:A) */ this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP); irq_work_queue(this_cpu_ptr(&wake_up_klogd_work)); } -- 2.35.1