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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 8F488C19759 for ; Sun, 4 Aug 2019 14:53:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5F04F2070D for ; Sun, 4 Aug 2019 14:53:02 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="BQqOEfMD" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726190AbfHDOxB (ORCPT ); Sun, 4 Aug 2019 10:53:01 -0400 Received: from merlin.infradead.org ([205.233.59.134]:43260 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726181AbfHDOxB (ORCPT ); Sun, 4 Aug 2019 10:53:01 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=J0f8NcZWBTaPEpxB3Q2uAE6Fl3HGqWPvoXKHoJaYSuE=; b=BQqOEfMDnRDOqdI3zjyJSioLH ijupLADuURkUgqwPRHgc9JSsF10z9trwSk1z/n9GIyDi1Rk+ftCOhhGcAMirJTKkw5zsAEMND+vn4 qCboKxtKd1bFZuinJ8P/5nMFoxFM/rwgwCcECSo95NLCoAawaT+j4jbI4TcTe5hoXiv967vgehmmi +/0t1WGUCPuaDR04TOjHfVYe72OATg3E1IqKYi5AZDFfZqDXr7uguqgeQOd8B8bhZrqwxshVHIQTa S3C04pY0ZQVuT2pNemVwzDFh020uo7DLTZR69j57/a/J5n2qxmKPZR4teb2i41y5nmjs5pr+KSz5d XwEU7RpPg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1huHsN-0007Ru-Sb; Sun, 04 Aug 2019 14:52:48 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 9F1A320274D7B; Sun, 4 Aug 2019 16:52:46 +0200 (CEST) Date: Sun, 4 Aug 2019 16:52:46 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH RFC tip/core/rcu 01/14] rcu/nocb: Atomic ->len field in rcu_segcblist structure Message-ID: <20190804145246.GC2386@hirez.programming.kicks-ass.net> References: <20190802151435.GA1081@linux.ibm.com> <20190802151501.13069-1-paulmck@linux.ibm.com> <20190804145051.GG2349@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190804145051.GG2349@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Sun, Aug 04, 2019 at 04:50:51PM +0200, Peter Zijlstra wrote: > On Fri, Aug 02, 2019 at 08:14:48AM -0700, Paul E. McKenney wrote: > > +/* > > + * Exchange the numeric length of the specified rcu_segcblist structure > > + * with the specified value. This can cause the ->len field to disagree > > + * with the actual number of callbacks on the structure. This exchange is > > + * fully ordered with respect to the callers accesses both before and after. > > + */ > > +long rcu_segcblist_xchg_len(struct rcu_segcblist *rsclp, long v) > > +{ > > +#ifdef CONFIG_RCU_NOCB_CPU > > + return atomic_long_xchg(&rsclp->len, v); > > +#else > > + long ret = rsclp->len; > > + > > + smp_mb(); /* Up to the caller! */ > > + WRITE_ONCE(rsclp->len, v); > > + smp_mb(); /* Up to the caller! */ > > + return ret; > > +#endif > > +} > > That one's weird; for matching semantics the load needs to be between > the memory barriers. Also, since you WRITE_ONCE() the thing, the load needs to be a READ_ONCE().