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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 55DA5CA9ECF for ; Mon, 4 Nov 2019 14:55:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2D68821D7F for ; Mon, 4 Nov 2019 14:55:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1572879345; bh=qhFhtJ7kIs1LPobeI8yqIagmffHkLC//6dS3sSJVq14=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=Hul1LVw/UlorBSwhbw7pHFvjmbnJAgfOGtesDdS/w9OZ8hlZkDmXxfGD6DOngHxCp Wlvo3BBeHrLdVFwd/NoiUOaLX0u9bAAYRHsjQSMT3y17Kzc57o1gvtXJQjCT/hSwQB VzIPs0m6SPtvoQEfswHUpzuPVqB6vQoigFylCp4Y= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728783AbfKDOzo (ORCPT ); Mon, 4 Nov 2019 09:55:44 -0500 Received: from mail.kernel.org ([198.145.29.99]:53528 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727796AbfKDOzn (ORCPT ); Mon, 4 Nov 2019 09:55:43 -0500 Received: from paulmck-ThinkPad-P72.home (unknown [109.144.216.141]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id D1739218BA; Mon, 4 Nov 2019 14:55:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1572879342; bh=qhFhtJ7kIs1LPobeI8yqIagmffHkLC//6dS3sSJVq14=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=wCsPQPmjOZZqBPnGPQF00rU5fT0vPfJpx/6Y/X33mBGjMGU10tiWg/yA17r0XfhS9 Er53WkTQwjvLLtw1dkILFjJiGJ20ixK9csl70pc5eg4k2kRl89DLWbULLJDeljTfjK VSRRPFjfGdaB1paJFApRq16ecyTIuOTlvmjtFLrQ= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id B64DE3520B56; Mon, 4 Nov 2019 06:55:39 -0800 (PST) Date: Mon, 4 Nov 2019 06:55:39 -0800 From: "Paul E. McKenney" To: Lai Jiangshan Cc: Boqun Feng , linux-kernel@vger.kernel.org, Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , rcu@vger.kernel.org Subject: Re: [PATCH V2 2/7] rcu: cleanup rcu_preempt_deferred_qs() Message-ID: <20191104145539.GY20975@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20191102124559.1135-1-laijs@linux.alibaba.com> <20191102124559.1135-3-laijs@linux.alibaba.com> <20191103020150.GA23770@tardis> <7489f817-adaf-275b-b19d-18ad248b071f@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <7489f817-adaf-275b-b19d-18ad248b071f@linux.alibaba.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Nov 03, 2019 at 01:01:21PM +0800, Lai Jiangshan wrote: > > > On 2019/11/3 10:01 上午, Boqun Feng wrote: > > Hi Jiangshan, > > > > > > I haven't checked the correctness of this patch carefully, but.. > > > > > > On Sat, Nov 02, 2019 at 12:45:54PM +0000, Lai Jiangshan wrote: > > > Don't need to set ->rcu_read_lock_nesting negative, irq-protected > > > rcu_preempt_deferred_qs_irqrestore() doesn't expect > > > ->rcu_read_lock_nesting to be negative to work, it even > > > doesn't access to ->rcu_read_lock_nesting any more. > > > > rcu_preempt_deferred_qs_irqrestore() will report RCU qs, and may > > eventually call swake_up() or its friends to wake up, say, the gp > > kthread, and the wake up functions could go into the scheduler code > > path which might have RCU read-side critical section in it, IOW, > > accessing ->rcu_read_lock_nesting. > > Sure, thank you for pointing it out. > > I should rewrite the changelog in next round. Like this: > > rcu: cleanup rcu_preempt_deferred_qs() > > IRQ-protected rcu_preempt_deferred_qs_irqrestore() itself doesn't > expect ->rcu_read_lock_nesting to be negative to work. > > There might be RCU read-side critical section in it (from wakeup() > or so), 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once) > will ensure that ->rcu_read_unlock_special is zero and these RCU > read-side critical sections will not call rcu_read_unlock_special(). > > Thanks > Lai > > === > PS: Were 1711d15bf5ef(rcu: Clear ->rcu_read_unlock_special only once) > not applied earlier, it will be protected by previous patch (patch1) > in this series > "rcu: use preempt_count to test whether scheduler locks is held" > when rcu_read_unlock_special() is called. This one in -rcu, you mean? 5c5d9065e4eb ("rcu: Clear ->rcu_read_unlock_special only once") Some adjustment was needed due to my not applying the earlier patches that assumed nested interrupts. Please let me know if further adjustments are needed. Thanx, Paul > > Again, haven't checked closely, but this argument in the commit log > > seems untrue. > > > > Regards, > > Boqun > > > > > > > > It is true that NMI over rcu_preempt_deferred_qs_irqrestore() > > > may access to ->rcu_read_lock_nesting, but it is still safe > > > since rcu_read_unlock_special() can protect itself from NMI. > > > > > > Signed-off-by: Lai Jiangshan > > > --- > > > kernel/rcu/tree_plugin.h | 5 ----- > > > 1 file changed, 5 deletions(-) > > > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > > index aba5896d67e3..2fab8be2061f 100644 > > > --- a/kernel/rcu/tree_plugin.h > > > +++ b/kernel/rcu/tree_plugin.h > > > @@ -552,16 +552,11 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t) > > > static void rcu_preempt_deferred_qs(struct task_struct *t) > > > { > > > unsigned long flags; > > > - bool couldrecurse = t->rcu_read_lock_nesting >= 0; > > > if (!rcu_preempt_need_deferred_qs(t)) > > > return; > > > - if (couldrecurse) > > > - t->rcu_read_lock_nesting -= RCU_NEST_BIAS; > > > local_irq_save(flags); > > > rcu_preempt_deferred_qs_irqrestore(t, flags); > > > - if (couldrecurse) > > > - t->rcu_read_lock_nesting += RCU_NEST_BIAS; > > > } > > > /* > > > -- > > > 2.20.1 > > >