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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 86E0CC4167B for ; Wed, 21 Dec 2022 12:11:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229713AbiLUML3 (ORCPT ); Wed, 21 Dec 2022 07:11:29 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56022 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229472AbiLUML0 (ORCPT ); Wed, 21 Dec 2022 07:11:26 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 146332628; Wed, 21 Dec 2022 04:11:24 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id BA1DCB81B80; Wed, 21 Dec 2022 12:11:22 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C44B4C433D2; Wed, 21 Dec 2022 12:11:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1671624681; bh=Yi1sF85S9MiMleKMdbGeKCC7UjazMd8nprfYlJnsE3w=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=S4coD86hzo9BMcqjlDxy02GuxStiRxBs7xL/tkcssh1jWhItk0rGK2OU6Y+XqdM+m 5bCE/Dvs25Y8XxNOf/CZBK5cTmRJcCeRcqKyiDe4Bnu4OCTQ7MEIaSzAHGC0QFVSba 8LasEW8ba5HXLLdduosoIp5JXLoVPLvCt6pULlbHDcqp1mP/qTaarv3nm3vuNQXrkW Oiv/KPZDSZixi0sPUpL/Bv4jaknANlXpZb67jJCbfdqA8Y+HkEJ++L8zaa8UKLefwN dpIVZr/qXVfJ/NCEpKj+cVjmA3rtzmHnLTNuhUFDBPBdgDeHOE89mBRejlQlaQqeN4 miJmNHppBoxmg== Date: Wed, 21 Dec 2022 13:11:18 +0100 From: Frederic Weisbecker To: Mathieu Desnoyers Cc: Joel Fernandes , linux-kernel@vger.kernel.org, Josh Triplett , Lai Jiangshan , "Paul E. McKenney" , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [RFC 0/2] srcu: Remove pre-flip memory barrier Message-ID: <20221221121118.GA35081@lothringen> References: <20221220140714.GB22763@lothringen> <20221220224459.GA25175@lothringen> <20221221004957.GA29021@lothringen> <20221221005858.GA29316@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 20, 2022 at 10:43:25PM -0500, Mathieu Desnoyers wrote: > On 2022-12-20 19:58, Frederic Weisbecker wrote: > > On Wed, Dec 21, 2022 at 01:49:57AM +0100, Frederic Weisbecker wrote: > > > On Tue, Dec 20, 2022 at 07:15:00PM -0500, Joel Fernandes wrote: > > > > On Tue, Dec 20, 2022 at 5:45 PM Frederic Weisbecker wrote: > > > > Agreed about (1). > > > > > > > > > _ In (2), E pairs with the address-dependency between idx and lock_count. > > > > > > > > But that is not the only reason. If that was the only reason for (2), > > > > then there is an smp_mb() just before the next-scan post-flip before > > > > the lock counts are read. > > > > > > The post-flip barrier makes sure the new idx is visible on the next READER's > > > turn, but it doesn't protect against the fact that "READ idx then WRITE lock[idx]" > > > may appear unordered from the update side POV if there is no barrier between the > > > scan and the flip. > > > > > > If you remove the smp_mb() from the litmus test I sent, things explode. > > > > Or rather, look at it the other way, if there is no barrier between the lock > > scan and the index flip (E), then the index flip can appear to be written before the > > lock is read. Which means you may start activating the index before you finish > > reading it (at least it appears that way from the readers pont of view). > > Considering that you can have pre-existing readers from arbitrary index > appearing anywhere in the grace period (because a reader can fetch the > index and be preempted for an arbitrary amount of time before incrementing > the lock count), the grace period algorithm needs to deal with the fact that > a newcoming reader can appear in a given index either before or after the > flip. True but the number of preempted tasks is bound and there is a forward progress guarantee. > I don't see how flipping the index before or after loading the unlock/lock > values would break anything (except for unlikely counter overflow situations > as previously discussed). Forward progress guarantee. Thanks. > > Thanks, > > Mathieu > > -- > Mathieu Desnoyers > EfficiOS Inc. > https://www.efficios.com >