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 106F9EC8748 for ; Sun, 10 Sep 2023 10:23:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1346574AbjIJKXD (ORCPT ); Sun, 10 Sep 2023 06:23:03 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52060 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231314AbjIJKXD (ORCPT ); Sun, 10 Sep 2023 06:23:03 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D76C2CCC; Sun, 10 Sep 2023 03:22:58 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 75E09C433C7; Sun, 10 Sep 2023 10:22:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1694341378; bh=21hq7RbQ9RzoMcSnEyECe577YZG2+AwiaJOi/Lu2oiQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=baAaNQycJWBnn7U+kheCUCJS8LCDjlPoS2APe6DnLw0nduJoi4slyhoEf9u72mvs4 6uEeoJfNfIJ7TT8JvI+MI1xL8XmWjZlFWxbVkLo1s/3K68kP+kO8wUEtyhLQM7S9+e XL/HnTFUafp1yHW56AtNeECFtz65uEaK2j3VdFr/15Iyodzttzoz/cr6f5jR70MUvx sVEejB/QdoMfIC8EqZg6Bd3tJFRsFgAo87njFWtGJ3VtQVXRMHh6FcAalkZD0Sxjfw eEojl0xCIOXkfP3EP7+4jGsvF/Sivc5Uymf6CEHWRP1+zuAn9/aqihXXJsLfKL6BRA QMv7Yrloehhng== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 1BD84CE080A; Sun, 10 Sep 2023 03:22:58 -0700 (PDT) Date: Sun, 10 Sep 2023 03:22:58 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: Boqun Feng , Frederic Weisbecker , LKML , rcu , Uladzislau Rezki , Neeraj Upadhyay Subject: Re: [PATCH 04/10] rcu/nocb: Remove needless full barrier after callback advancing Message-ID: <5bb46b87-963a-4e6d-a689-dee59db7025f@paulmck-laptop> Reply-To: paulmck@kernel.org References: <20230908203603.5865-1-frederic@kernel.org> <20230908203603.5865-5-frederic@kernel.org> <20230909043125.GA3920383@google.com> <20230910040923.GA762577@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230910040923.GA762577@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Sep 10, 2023 at 12:09:23AM -0400, Joel Fernandes wrote: > On Sat, Sep 09, 2023 at 11:22:48AM -0700, Boqun Feng wrote: > > On Sat, Sep 09, 2023 at 04:31:25AM +0000, Joel Fernandes wrote: > > > On Fri, Sep 08, 2023 at 10:35:57PM +0200, Frederic Weisbecker wrote: > > > > A full barrier is issued from nocb_gp_wait() upon callbacks advancing > > > > to order grace period completion with callbacks execution. > > > > > > > > However these two events are already ordered by the > > > > smp_mb__after_unlock_lock() barrier within the call to > > > > raw_spin_lock_rcu_node() that is necessary for callbacks advancing to > > > > happen. > > > > > > > > The following litmus test shows the kind of guarantee that this barrier > > > > provides: > > > > > > > > C smp_mb__after_unlock_lock > > > > > > > > {} > > > > > > > > // rcu_gp_cleanup() > > > > P0(spinlock_t *rnp_lock, int *gpnum) > > > > { > > > > // Grace period cleanup increase gp sequence number > > > > spin_lock(rnp_lock); > > > > WRITE_ONCE(*gpnum, 1); > > > > spin_unlock(rnp_lock); > > > > } > > > > > > > > // nocb_gp_wait() > > > > P1(spinlock_t *rnp_lock, spinlock_t *nocb_lock, int *gpnum, int *cb_ready) > > > > { > > > > int r1; > > > > > > > > // Call rcu_advance_cbs() from nocb_gp_wait() > > > > spin_lock(nocb_lock); > > > > spin_lock(rnp_lock); > > > > smp_mb__after_unlock_lock(); > > > > r1 = READ_ONCE(*gpnum); > > > > WRITE_ONCE(*cb_ready, 1); > > > > spin_unlock(rnp_lock); > > > > spin_unlock(nocb_lock); > > > > } > > > > > > > > // nocb_cb_wait() > > > > P2(spinlock_t *nocb_lock, int *cb_ready, int *cb_executed) > > > > { > > > > int r2; > > > > > > > > // rcu_do_batch() -> rcu_segcblist_extract_done_cbs() > > > > spin_lock(nocb_lock); > > > > r2 = READ_ONCE(*cb_ready); > > > > spin_unlock(nocb_lock); > > > > > > > > // Actual callback execution > > > > WRITE_ONCE(*cb_executed, 1); > > > > > > So related to this something in the docs caught my attention under "Callback > > > Invocation" [1] > > > > > > > > > However, if the callback function communicates to other CPUs, for example, > > > doing a wakeup, then it is that function's responsibility to maintain > > > ordering. For example, if the callback function wakes up a task that runs on > > > some other CPU, proper ordering must in place in both the callback function > > > and the task being awakened. To see why this is important, consider the top > > > half of the grace-period cleanup diagram. The callback might be running on a > > > CPU corresponding to the leftmost leaf rcu_node structure, and awaken a task > > > that is to run on a CPU corresponding to the rightmost leaf rcu_node > > > structure, and the grace-period kernel thread might not yet have reached the > > > rightmost leaf. In this case, the grace period's memory ordering might not > > > yet have reached that CPU, so again the callback function and the awakened > > > task must supply proper ordering. > > > > > > > > > I believe this text is for non-nocb but if we apply that to the nocb case, > > > lets see what happens. > > > > > > In the litmus, he rcu_advance_cbs() happened on P1, however the callback is > > > executing on P2. That sounds very similar to the non-nocb world described in > > > the text where a callback tries to wake something up on a different CPU and > > > needs to take care of all the ordering. > > > > > > So unless I'm missing something (quite possible), P2 must see the update to > > > gpnum as well. However, per your limus test, the only thing P2 does is > > > acquire the nocb_lock. I don't see how it is guaranteed to see gpnum == 1. > > > > Because P1 writes cb_ready under nocb_lock, and P2 reads cb_ready under > > nocb_lock as well and if P2 read P1's write, then we know the serialized > > order of locking is P1 first (i.e. the spin_lock(nocb_lock) on P2 reads > > from the spin_unlock(nocb_lock) on P1), in other words: > > > > (fact #1) > > > > unlock(nocb_lock) // on P1 > > ->rfe > > lock(nocb_lock) // on P2 > > > > so if P1 reads P0's write on gpnum > > > > (assumption #1) > > > > W(gpnum)=1 // on P0 > > ->rfe > > R(gpnum)=1 // on P1 > > > > and we have > > > > (fact #2) > > > > R(gpnum)=1 // on P1 > > ->(po; [UL]) > > unlock(nocb_lock) // on P1 > > > > combine them you get > > > > W(gpnum)=1 // on P0 > > ->rfe // fact #1 > > ->(po; [UL]) // fact #2 > > ->rfe // assumption #1 > > lock(nocb_lock) // on P2 > > ->([LKR]; po) > > M // any access on P2 after spin_lock(nocb_lock); > > > > so > > W(gpnum)=1 // on P0 > > ->rfe ->po-unlock-lock-po > > M // on P2 > > > > and po-unlock-lock-po is A-culum, hence "->rfe ->po-unlock-lock-po" or > > "rfe; po-unlock-lock-po" is culum-fence, hence it's a ->prop, which > > means the write of gpnum on P0 propagates to P2 before any memory > > accesses after spin_lock(nocb_lock)? > > You and Frederic are right. I confirmed this by running herd7 as well. > > Also he adds a ->co between P2 and P3, so that's why the > smp_mb__after_lock_unlock() helps to keep the propogation intact. Its pretty > much the R-pattern extended across 4 CPUs. > > We should probably document these in the RCU memory ordering docs. I never have heard of R showing up in real code before, so that is worth a few words in the documentation as well. ;-) Thanx, Paul