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=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,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 808D0C433DF for ; Wed, 20 May 2020 12:06:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5FA7220758 for ; Wed, 20 May 2020 12:06:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727011AbgETMGS (ORCPT ); Wed, 20 May 2020 08:06:18 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33828 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726435AbgETMGS (ORCPT ); Wed, 20 May 2020 08:06:18 -0400 Received: from Galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4A6CEC061A0E; Wed, 20 May 2020 05:06:18 -0700 (PDT) Received: from bigeasy by Galois.linutronix.de with local (Exim 4.80) (envelope-from ) id 1jbNU8-0007FV-Jb; Wed, 20 May 2020 14:06:08 +0200 Date: Wed, 20 May 2020 14:06:08 +0200 From: Sebastian Andrzej Siewior To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Steven Rostedt , Will Deacon , Thomas Gleixner , "Paul E . McKenney" , Linus Torvalds , Lai Jiangshan , Josh Triplett , Mathieu Desnoyers , rcu@vger.kernel.org Subject: Re: [PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu_data access Message-ID: <20200520120608.mwros5jurmidxxfv@linutronix.de> References: <20200519201912.1564477-1-bigeasy@linutronix.de> <20200519201912.1564477-4-bigeasy@linutronix.de> <20200520102407.GF317569@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200520102407.GF317569@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-05-20 12:24:07 [+0200], Peter Zijlstra wrote: > On Tue, May 19, 2020 at 10:19:07PM +0200, Sebastian Andrzej Siewior wrote: > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > > index 0c71505f0e19c..8d2b5f75145d7 100644 > > --- a/kernel/rcu/srcutree.c > > +++ b/kernel/rcu/srcutree.c > > @@ -25,6 +25,7 @@ > > #include > > #include > > #include > > +#include > > > > #include "rcu.h" > > #include "rcu_segcblist.h" > > @@ -735,6 +736,7 @@ static void srcu_flip(struct srcu_struct *ssp) > > smp_mb(); /* D */ /* Pairs with C. */ > > } > > > > +static DEFINE_LOCAL_LOCK(sda_lock); > > /* > > * If SRCU is likely idle, return true, otherwise return false. > > * > > @@ -765,13 +767,13 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp) > > unsigned long tlast; > > > > /* If the local srcu_data structure has callbacks, not idle. */ > > - local_irq_save(flags); > > + local_lock_irqsave(sda_lock, flags); > > sdp = this_cpu_ptr(ssp->sda); > > if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) { > > - local_irq_restore(flags); > > + local_unlock_irqrestore(sda_lock, flags); > > return false; /* Callbacks already present, so not idle. */ > > } > > - local_irq_restore(flags); > > + local_unlock_irqrestore(sda_lock, flags); > > Would it perhaps make sense to stick the local_lock in struct srcu_data ? In that case we would need something for pointer stability before the lock is acquired. I remember Paul looked at that patch a few years ago and he said that that disabling interrupts here is important and matches the other part instance where the interrupts are disabled. Looking at it now, it seems that there is just pointer stability but I can't tell if rcu_segcblist_pend_cbs() needs more than just this. Sebastian