From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751398AbeELR0v (ORCPT ); Sat, 12 May 2018 13:26:51 -0400 Received: from mail.kernel.org ([198.145.29.99]:40662 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751010AbeELR0u (ORCPT ); Sat, 12 May 2018 13:26:50 -0400 Date: Sat, 12 May 2018 13:26:47 -0400 From: Steven Rostedt To: "Paul E. McKenney" Cc: Joel Fernandes , Byungchul Park , jiangshanlai@gmail.com, josh@joshtriplett.org, mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org, kernel-team@lge.com, peterz@infradead.org Subject: Re: [PATCH] rcu: Report a quiescent state when it's exactly in the state Message-ID: <20180512132647.2923f7cf@gandalf.local.home> In-Reply-To: <20180512144119.GJ26088@linux.vnet.ibm.com> References: <1526027434-21237-1-git-send-email-byungchul.park@lge.com> <3af4cec0-4019-e3ac-77f9-8631252fb6da@lge.com> <20180511161746.GX26088@linux.vnet.ibm.com> <20180511224138.GA89902@joelaf.mtv.corp.google.com> <20180512050824.GF26088@linux.vnet.ibm.com> <20180512063037.GC192642@joelaf.mtv.corp.google.com> <20180512144119.GJ26088@linux.vnet.ibm.com> X-Mailer: Claws Mail 3.16.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 12 May 2018 07:41:19 -0700 "Paul E. McKenney" wrote: > Don't get me wrong, this discussion was quite useful to me. We probably > need to at least change the comments, and perhaps the code as well. But > I agree that we need input from Peter and Steven to make much more forward > progress. It's the weekend so I skimmed more than read this thread, but I will just add this. The table Joel posted is interesting, and perhaps we should keep things consistent with that. But that said, with respect to task-RCU, as nothing on a trampoline should ever call cond_resched() (and perhaps I should add code in lockdep that verifies this), we just want a quiescent state that tells us that the task has left the trampoline. A cond_resched() should be one of those points that does. It really has nothing to do with scheduling or preemption. The issue is that if a task is on a trampoline and gets preempted, there's no knowing when it is off that trampoline where we can free it. We need to have places in the kernel that we know is a quiescent state to move task-RCU forward. cond_resched() seems to be one of them. schedule itself can not be, because it can be called from an interrupt preempting a task while it is on the trampoline. -- Steve