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 X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 16591C4332B for ; Fri, 20 Mar 2020 22:36:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EA5172072D for ; Fri, 20 Mar 2020 22:36:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727456AbgCTWgg (ORCPT ); Fri, 20 Mar 2020 18:36:36 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:37622 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726855AbgCTWgf (ORCPT ); Fri, 20 Mar 2020 18:36:35 -0400 Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1jFQFI-0005GA-2c; Fri, 20 Mar 2020 23:36:04 +0100 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id 7EC4C1039FC; Fri, 20 Mar 2020 23:36:03 +0100 (CET) From: Thomas Gleixner To: paulmck@kernel.org Cc: LKML , Peter Zijlstra , Linus Torvalds , Ingo Molnar , Will Deacon , Joel Fernandes , Steven Rostedt , Randy Dunlap , Sebastian Andrzej Siewior , Logan Gunthorpe , Kurt Schwemmer , Bjorn Helgaas , linux-pci@vger.kernel.org, Felipe Balbi , Greg Kroah-Hartman , linux-usb@vger.kernel.org, Kalle Valo , "David S. Miller" , linux-wireless@vger.kernel.org, netdev@vger.kernel.org, Oleg Nesterov , Davidlohr Bueso , Michael Ellerman , Arnd Bergmann , linuxppc-dev@lists.ozlabs.org Subject: Re: [patch V2 08/15] Documentation: Add lock ordering and nesting documentation In-Reply-To: <20200320210243.GT3199@paulmck-ThinkPad-P72> References: <20200320160145.GN3199@paulmck-ThinkPad-P72> <87mu8apzxr.fsf@nanos.tec.linutronix.de> <20200320210243.GT3199@paulmck-ThinkPad-P72> Date: Fri, 20 Mar 2020 23:36:03 +0100 Message-ID: <874kuipsbw.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org "Paul E. McKenney" writes: > On Fri, Mar 20, 2020 at 08:51:44PM +0100, Thomas Gleixner wrote: >> "Paul E. McKenney" writes: >> > >> > - The soft interrupt related suffix (_bh()) still disables softirq >> > handlers. However, unlike non-PREEMPT_RT kernels (which disable >> > preemption to get this effect), PREEMPT_RT kernels use a per-CPU >> > lock to exclude softirq handlers. >> >> I've made that: >> >> - The soft interrupt related suffix (_bh()) still disables softirq >> handlers. >> >> Non-PREEMPT_RT kernels disable preemption to get this effect. >> >> PREEMPT_RT kernels use a per-CPU lock for serialization. The lock >> disables softirq handlers and prevents reentrancy by a preempting >> task. > > That works! At the end, I would instead say "prevents reentrancy > due to task preemption", but what you have works. Yours is better. >> - Task state is preserved across spinlock acquisition, ensuring that the >> task-state rules apply to all kernel configurations. Non-PREEMPT_RT >> kernels leave task state untouched. However, PREEMPT_RT must change >> task state if the task blocks during acquisition. Therefore, it >> saves the current task state before blocking and the corresponding >> lock wakeup restores it. A regular not lock related wakeup sets the >> task state to RUNNING. If this happens while the task is blocked on >> a spinlock then the saved task state is changed so that correct >> state is restored on lock wakeup. >> >> Hmm? > > I of course cannot resist editing the last two sentences: > > ... Other types of wakeups unconditionally set task state to RUNNING. > If this happens while a task is blocked while acquiring a spinlock, > then the task state is restored to its pre-acquisition value at > lock-wakeup time. Errm no. That would mean state = UNINTERRUPTIBLE lock() block() real_state = state state = SLEEPONLOCK non lock wakeup state = RUNNING <--- FAIL #1 lock wakeup state = real_state <--- FAIL #2 How it works is: state = UNINTERRUPTIBLE lock() block() real_state = state state = SLEEPONLOCK non lock wakeup real_state = RUNNING lock wakeup state = real_state == RUNNING If there is no 'non lock wakeup' before the lock wakeup: state = UNINTERRUPTIBLE lock() block() real_state = state state = SLEEPONLOCK lock wakeup state = real_state == UNINTERRUPTIBLE I agree that what I tried to express is hard to parse, but it's at least halfways correct :) Thanks, tglx