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 0F9ECCA9ECF for ; Thu, 31 Oct 2019 14:31:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D7AFC208C0 for ; Thu, 31 Oct 2019 14:31:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1572532282; bh=kFk+rWG7J2BMqNZJiollmdbDl2nEfZWnzteJj+4Ol8M=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=YqZdFGt/P1edNtao6wRHoXCB4V3HWCKxXkopaaTr9dL09r7w2v7NtkSLG09Lg2OjL 31hMRZ+FXLj3nRcthuxNcMdohvmp5HUoJ326LAxXKUILpB2tG6zMqED3KjB2CutOTw wiO6+Jd7B9wGFkZ1jDhhr8vECUWwd8ZRPjM8JGJs= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728295AbfJaObV (ORCPT ); Thu, 31 Oct 2019 10:31:21 -0400 Received: from mail.kernel.org ([198.145.29.99]:59258 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727905AbfJaObV (ORCPT ); Thu, 31 Oct 2019 10:31:21 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (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 D68D02087E; Thu, 31 Oct 2019 14:31:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1572532279; bh=kFk+rWG7J2BMqNZJiollmdbDl2nEfZWnzteJj+4Ol8M=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=PEFFdBRHlenux1jomkA3Ry8xxPtd3d+2c+vVHiCR3143rhz/XZmWrNpkfzmz6Yaqv 7YdMvWm3+TnD6TXRATiOjEQuShvrP6Erp+kL00fNLTOl3doW1XAuou6SjjEvZsqWVe Emm40IDsVwkW5kHIyFzQgLBAC5O1WiRYp57Yjb6U= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id AA77D3520744; Thu, 31 Oct 2019 07:31:19 -0700 (PDT) Date: Thu, 31 Oct 2019 07:31:19 -0700 From: "Paul E. McKenney" To: Lai Jiangshan Cc: linux-kernel@vger.kernel.org, Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , rcu@vger.kernel.org Subject: Re: [PATCH 02/11] rcu: fix bug when rcu_exp_handler() in nested interrupt Message-ID: <20191031143119.GA15954@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20191031100806.1326-1-laijs@linux.alibaba.com> <20191031100806.1326-3-laijs@linux.alibaba.com> <20191031134731.GP20975@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191031134731.GP20975@paulmck-ThinkPad-P72> 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 Thu, Oct 31, 2019 at 06:47:31AM -0700, Paul E. McKenney wrote: > On Thu, Oct 31, 2019 at 10:07:57AM +0000, Lai Jiangshan wrote: > > These is a possible bug (although which I can't triger yet) > > since 2015 8203d6d0ee78 > > (rcu: Use single-stage IPI algorithm for RCU expedited grace period) > > > > rcu_read_unlock() > > ->rcu_read_lock_nesting = -RCU_NEST_BIAS; > > interrupt(); // before or after rcu_read_unlock_special() > > rcu_read_lock() > > fetch some rcu protected pointers > > // exp GP starts in other cpu. > > some works > > NESTED interrupt for rcu_exp_handler(); Also, which platforms support nested interrupts? Last I knew, this was prohibited. > > report exp qs! BUG! > > Why would a quiescent state for the expedited grace period be reported > here? This CPU is still in an RCU read-side critical section, isn't it? And I now see what you were getting at here. Yes, the current code assumes that interrupt-disabled regions, like hardware interrupt handlers, cannot be interrupted. But if interrupt-disabled regions such as hardware interrupt handlers can be interrupted (as opposed to being NMIed), wouldn't that break a whole lot of stuff all over the place in the kernel? So that sounds like an arch bug to me. Thanx, Paul > > // exp GP completes and pointers are freed in other cpu > > some works with the pointers. BUG > > rcu_read_unlock(); > > ->rcu_read_lock_nesting = 0; > > > > Although rcu_sched_clock_irq() can be in nested interrupt, > > there is no such similar bug since special.b.need_qs > > can only be set when ->rcu_read_lock_nesting > 0 > > > > Signed-off-by: Lai Jiangshan > > --- > > kernel/rcu/tree_exp.h | 5 +++-- > > kernel/rcu/tree_plugin.h | 9 ++++++--- > > 2 files changed, 9 insertions(+), 5 deletions(-) > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > index 6dec21909b30..c0d06bce35ea 100644 > > --- a/kernel/rcu/tree_exp.h > > +++ b/kernel/rcu/tree_exp.h > > @@ -664,8 +664,9 @@ static void rcu_exp_handler(void *unused) > > * Otherwise, force a context switch after the CPU enables everything. > > */ > > rdp->exp_deferred_qs = true; > > - if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) || > > - WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs())) { > > + if (rcu_preempt_need_deferred_qs(t) && > > + (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) || > > + WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs()))) { > > rcu_preempt_deferred_qs(t); > > } else { > > set_tsk_need_resched(t); > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index d4c482490589..59ef10da1e39 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -549,9 +549,12 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags) > > */ > > static bool rcu_preempt_need_deferred_qs(struct task_struct *t) > > { > > - return (__this_cpu_read(rcu_data.exp_deferred_qs) || > > - READ_ONCE(t->rcu_read_unlock_special.s)) && > > - t->rcu_read_lock_nesting <= 0; > > + return (__this_cpu_read(rcu_data.exp_deferred_qs) && > > + (!t->rcu_read_lock_nesting || > > + t->rcu_read_lock_nesting == -RCU_NEST_BIAS)) > > + || > > + (READ_ONCE(t->rcu_read_unlock_special.s) && > > + t->rcu_read_lock_nesting <= 0); > > } > > > > /* > > -- > > 2.20.1 > >