From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5F4BF881F for ; Fri, 10 Mar 2023 14:47:24 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id D870DC4339B; Fri, 10 Mar 2023 14:47:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1678459644; bh=eYnGH6XaFfJO7JNnGkMlQddojF3ytL1PC2+eqy1Hu1k=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=tPSHVagPGDi3c3hzeVzevx0W7XQvcJqIH5GTXfKd2WXZj+vmrfgWk3rhHNYpvdvre FkhUa1rSbBmJGXwU8tvaXOCHDY3tQNBgoZaZEBI6LgiXdAGDgZL6pf697C08SMqGZq Bvpjj0gzmbG4yLSHkAo45j76y1mNEY9hicB8LWRY= 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 5.10 072/529] rcu-tasks: Improve comments explaining tasks_rcu_exit_srcu purpose Date: Fri, 10 Mar 2023 14:33:35 +0100 Message-Id: <20230310133808.297341522@linuxfoundation.org> X-Mailer: git-send-email 2.39.2 In-Reply-To: <20230310133804.978589368@linuxfoundation.org> References: <20230310133804.978589368@linuxfoundation.org> User-Agent: quilt/0.67 Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 8b51e6a5b3869..372644126b0de 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -416,11 +416,21 @@ 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); } @@ -487,7 +497,10 @@ 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(); } @@ -576,7 +589,11 @@ static void show_rcu_tasks_classic_gp_kthread(void) } #endif /* #ifndef 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(); @@ -584,7 +601,11 @@ 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