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 96E4AC4167B for ; Wed, 21 Dec 2022 18:19:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234706AbiLUSTD (ORCPT ); Wed, 21 Dec 2022 13:19:03 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35716 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229728AbiLUSS7 (ORCPT ); Wed, 21 Dec 2022 13:18:59 -0500 Received: from mail-lf1-x12d.google.com (mail-lf1-x12d.google.com [IPv6:2a00:1450:4864:20::12d]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3B127BC4 for ; Wed, 21 Dec 2022 10:18:58 -0800 (PST) Received: by mail-lf1-x12d.google.com with SMTP id g13so8031033lfv.7 for ; Wed, 21 Dec 2022 10:18:58 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=1762m4MjNG9Yx9WLbbrufGklAfqCEfWd39rbtpBGmv4=; b=rMobi02QK5YPRDhSK+c+sHPLO3V96W8UQ1koW2/7B/0ozqibl8gAssMTYqgErtzz7S 794X5RoM+dkRcYU3w50UI3tWAN4+A9CJhPc/fdZoDTWgluyXY/4R/Kequ4xqk41uAl4c n1WDC5nqwcb8pii+fh6vjpK8rGZ1xeoaI4CEA= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=1762m4MjNG9Yx9WLbbrufGklAfqCEfWd39rbtpBGmv4=; b=vu6T2zUHhdcnL4GYv7totjLVdtCvpUy52UeRbPF7geN2yC8G0uJIJkj1V/qNGlDBKP ko4HjWmwYInnL4LwICNiSCyLcdX8vyGRouX40IbNXsL2CAVVs7s5Qy0Hj/MBBIuLwUNx JCgsK0G9dDQNeJwm2KaxNXsHYbqsdk9cuirqIVrqPPA3TbWPWyPSohSAWEojkUI41wX7 KlE8Jb2sIQ8I1oHIyrlnXtSL+V798I71hvWr+1oCjAqVk5LLjpZDHEoINEGYek/wX67b YvaOpwllcHxgiF1eLRnjsOE31yt+4iZHr4De7cBJAW1dGeYUj7QEspQmvzzRntJEXLlT JE8w== X-Gm-Message-State: AFqh2kqoH+tYwCrXPsIUBPAMjuBgHLCKmFsLxWsBUmDCGzLDvufJ5UfF JX98z9PqIfc100KFbEMmvtTODbnsH/lusYpphJnZLDCtKC8tGJjW X-Google-Smtp-Source: AMrXdXuINTDcRYR4N1IAAUnsWaKq2etXScrIz+bu7G+XdJJ4BRVQ/jCI1hX4Q3SjBgsAH6k4W5raAS7idbn+VYiOvjU= X-Received: by 2002:a05:6512:15a3:b0:4bc:bdf5:f163 with SMTP id bp35-20020a05651215a300b004bcbdf5f163mr153179lfb.583.1671646736579; Wed, 21 Dec 2022 10:18:56 -0800 (PST) MIME-Version: 1.0 References: <20221220140714.GB22763@lothringen> <20221220224459.GA25175@lothringen> <20221221004957.GA29021@lothringen> <20221221005858.GA29316@lothringen> <20221221121118.GA35081@lothringen> In-Reply-To: From: Joel Fernandes Date: Wed, 21 Dec 2022 18:18:44 +0000 Message-ID: Subject: Re: [RFC 0/2] srcu: Remove pre-flip memory barrier To: Mathieu Desnoyers Cc: Frederic Weisbecker , linux-kernel@vger.kernel.org, Josh Triplett , Lai Jiangshan , "Paul E. McKenney" , rcu@vger.kernel.org, Steven Rostedt Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 21, 2022 at 5:20 PM Mathieu Desnoyers wrote: > > On 2022-12-21 07:11, Frederic Weisbecker wrote: > > 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. > > Considering a coherent cache, the store-buffer will ensure that the > index flip eventually reaches all readers. This bounds the time during > which readers can flood the current index, and therefore guarantees > forward progress. AFAIK the Linux kernel does not support architectures > with incoherent caches. > > So I still don't see how having the barrier before or after the index > flip is useful for forward progress. Even though eventually the writes on either side will make it, I think you have little lost opportunity without the "D" memory barrier without satisfying the store-buffer pattern. i.e. even if the idx update was seen by the read-side, its unlock count contributions to the now-inactive idx may not be seen. I think that makes it slightly better for making the grace period end sooner. I think that's what you "scan both" idea also tries to achieve, so for that reason you should like "D" as well. But yes it depends on how forward-progress is defined. If defined as starvation - no the post/pref flip MBs are not helpful for that (eventually the writes will make it). If defined as not losing an opportunity to end the GP sooner, yes the flip-related memory barriers does help that. As for whether or not we need "E", that is up for debate. I am intrigued by Frederick's litmus tests showing that it helps, because my litmus tests don't show it. Thanks.