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 C29AEC678DB for ; Tue, 7 Mar 2023 18:12:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232466AbjCGSMZ (ORCPT ); Tue, 7 Mar 2023 13:12:25 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50140 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232460AbjCGSMH (ORCPT ); Tue, 7 Mar 2023 13:12:07 -0500 Received: from sin.source.kernel.org (sin.source.kernel.org [145.40.73.55]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6CF76B1EDB for ; Tue, 7 Mar 2023 10:07:17 -0800 (PST) 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 sin.source.kernel.org (Postfix) with ESMTPS id BF616CE1C79 for ; Tue, 7 Mar 2023 18:07:15 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C1B7EC433D2; Tue, 7 Mar 2023 18:07:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1678212434; bh=XTErf9Aln/p/7tr6d/NCkem36TtCVxIr9dQxahm+ZAs=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=Gsr7agxYc71/ZLVf0jJfOEtDx1IDZ2Qu8N05PnyYnqUwszgA7KXZr6P7F6DOIlAl/ rzUJ07MAuyRCCWtJ2HFDyUx0tuEbP7Fua2NlWacCLqiMMk/cUOeo/FOqBJ4+f3HPcr PHg1n/RO+rIRHVvrSYL3E8jx+nhKes6lPelTO0y4= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Boqun Feng , Neeraj Upadhyay , "Paul E. McKenney" , Oleg Nesterov , Lai Jiangshan , "Eric W. Biederman" , Frederic Weisbecker , Sasha Levin Subject: [PATCH 6.1 143/885] rcu-tasks: Improve comments explaining tasks_rcu_exit_srcu purpose Date: Tue, 7 Mar 2023 17:51:17 +0100 Message-Id: <20230307170008.076373636@linuxfoundation.org> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230307170001.594919529@linuxfoundation.org> References: <20230307170001.594919529@linuxfoundation.org> User-Agent: quilt/0.67 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: Frederic Weisbecker [ Upstream commit e4e1e8089c5fd948da12cb9f4adc93821036945f ] Make sure we don't need to look again into the depths of git blame in order not to miss a subtle part about how rcu-tasks is dealing with exiting tasks. Suggested-by: Boqun Feng Suggested-by: Neeraj Upadhyay Suggested-by: Paul E. McKenney Cc: Oleg Nesterov Cc: Lai Jiangshan Cc: Eric W. Biederman Signed-off-by: Frederic Weisbecker Signed-off-by: Paul E. McKenney Stable-dep-of: 28319d6dc5e2 ("rcu-tasks: Fix synchronize_rcu_tasks() VS zap_pid_ns_processes()") Signed-off-by: Sasha Levin --- kernel/rcu/tasks.h | 37 +++++++++++++++++++++++++++++-------- 1 file changed, 29 insertions(+), 8 deletions(-) diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index f5bf6fb430dab..688c461036f57 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -827,11 +827,21 @@ static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop) static void rcu_tasks_postscan(struct list_head *hop) { /* - * Wait for tasks that are in the process of exiting. This - * does only part of the job, ensuring that all tasks that were - * previously exiting reach the point where they have disabled - * preemption, allowing the later synchronize_rcu() to finish - * the job. + * Exiting tasks may escape the tasklist scan. Those are vulnerable + * until their final schedule() with TASK_DEAD state. To cope with + * this, divide the fragile exit path part in two intersecting + * read side critical sections: + * + * 1) An _SRCU_ read side starting before calling exit_notify(), + * which may remove the task from the tasklist, and ending after + * the final preempt_disable() call in do_exit(). + * + * 2) An _RCU_ read side starting with the final preempt_disable() + * call in do_exit() and ending with the final call to schedule() + * with TASK_DEAD state. + * + * This handles the part 1). And postgp will handle part 2) with a + * call to synchronize_rcu(). */ synchronize_srcu(&tasks_rcu_exit_srcu); } @@ -898,7 +908,10 @@ static void rcu_tasks_postgp(struct rcu_tasks *rtp) * * In addition, this synchronize_rcu() waits for exiting tasks * to complete their final preempt_disable() region of execution, - * cleaning up after the synchronize_srcu() above. + * cleaning up after synchronize_srcu(&tasks_rcu_exit_srcu), + * enforcing the whole region before tasklist removal until + * the final schedule() with TASK_DEAD state to be an RCU TASKS + * read side critical section. */ synchronize_rcu(); } @@ -988,7 +1001,11 @@ void show_rcu_tasks_classic_gp_kthread(void) EXPORT_SYMBOL_GPL(show_rcu_tasks_classic_gp_kthread); #endif // !defined(CONFIG_TINY_RCU) -/* Do the srcu_read_lock() for the above synchronize_srcu(). */ +/* + * Contribute to protect against tasklist scan blind spot while the + * task is exiting and may be removed from the tasklist. See + * corresponding synchronize_srcu() for further details. + */ void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu) { preempt_disable(); @@ -996,7 +1013,11 @@ void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu) preempt_enable(); } -/* Do the srcu_read_unlock() for the above synchronize_srcu(). */ +/* + * Contribute to protect against tasklist scan blind spot while the + * task is exiting and may be removed from the tasklist. See + * corresponding synchronize_srcu() for further details. + */ void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu) { struct task_struct *t = current; -- 2.39.2