From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751500AbdEPGWn (ORCPT ); Tue, 16 May 2017 02:22:43 -0400 Received: from mail-wm0-f54.google.com ([74.125.82.54]:35128 "EHLO mail-wm0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751027AbdEPGWm (ORCPT ); Tue, 16 May 2017 02:22:42 -0400 Date: Tue, 16 May 2017 08:22:33 +0200 From: Ingo Molnar To: "Paul E. McKenney" Cc: rostedt@goodmis.org, linux-kernel@vger.kernel.org Subject: Re: Use case for TASKS_RCU Message-ID: <20170516062233.tyz7ze7ilmbkxtjc@gmail.com> References: <20170515182354.GA25440@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170515182354.GA25440@linux.vnet.ibm.com> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Paul E. McKenney wrote: > Hello! > > The question of the use case for TASKS_RCU came up, and here is my > understanding. Steve will not be shy about correcting any misconceptions > I might have. ;-) > > The use case is to support freeing of trampolines used in tracing/probing > in CONFIG_PREEMPT=y kernels. It is necessary to wait until any task > executing in the trampoline in question has left it, taking into account > that the trampoline's code might be interrupted and preempted. However, > the code in the trampolines is guaranteed never to context switch. > > Note that in CONFIG_PREEMPT=n kernels, synchronize_sched() suffices. > It is therefore tempting to think in terms of disabling preemption across > the trampolines, but there is apparently not enough room to accommodate > the needed preempt_disable() and preempt_enable() in the code invoking > the trampoline, and putting the preempt_disable() and preempt_enable() > in the trampoline itself fails because of the possibility of preemption > just before the preempt_disable() and just after the preempt_enable(). > Similar reasoning rules out use of rcu_read_lock() and rcu_read_unlock(). So how was this solved before TASKS_RCU? Also, nothing uses call_rcu_tasks() at the moment, so it's hard for me to review its users. What am I missing? Thanks, Ingo