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=-11.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 D3497C4741F for ; Thu, 5 Nov 2020 03:57:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 77C6E2078E for ; Thu, 5 Nov 2020 03:57:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1604548636; bh=qIk9u05EWbRNA4eKZJAdxo+1qirS9mO9h6wHQ8e1bcY=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=Cz3GIXZ0BQx4R35Cqq7uLsxXoljm22lpDIAEBo5VecokNrtIocQQWhIIWm2BO3T31 by5tKRxH3x+1ExH76ebFcyhUPSfLhfE2wVM3m0MWiTexEXhQgxq/PPwFVdQqzdYr+P GQz3LwQr3xsBxS5PuXA4ox5jYrt0bCF3VHSaCCiI= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729833AbgKED5P (ORCPT ); Wed, 4 Nov 2020 22:57:15 -0500 Received: from mail.kernel.org ([198.145.29.99]:43818 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725856AbgKED5O (ORCPT ); Wed, 4 Nov 2020 22:57:14 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-104-11.bvtn.or.frontiernet.net [50.39.104.11]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 5742F20786; Thu, 5 Nov 2020 03:57:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1604548634; bh=qIk9u05EWbRNA4eKZJAdxo+1qirS9mO9h6wHQ8e1bcY=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=hFEcJZXTgiWX3DxNomAa86O2q4gKtQmVlmgTeI/mNcbxC1rJhj833Zt/b4/Wq0bgI GFjZ4fJSwoSk7ww2OkQdvr2hjv2e3oiJu02hO9q4k6nmw04C1iz9FdK5kgAw0/oJiW glhV9B1R9OKJOydhWAS2UOlOc2P9df21wYP1SrxQ= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id F3E423522683; Wed, 4 Nov 2020 19:57:13 -0800 (PST) Date: Wed, 4 Nov 2020 19:57:13 -0800 From: "Paul E. McKenney" To: "Joel Fernandes (Google)" Cc: linux-kernel@vger.kernel.org, Frederic Weisbecker , Josh Triplett , Lai Jiangshan , Marco Elver , Mathieu Desnoyers , rcu@vger.kernel.org, Steven Rostedt , "Uladzislau Rezki (Sony)" , fweisbec@gmail.com, neeraj.iitr10@gmail.com Subject: Re: [PATCH v9 6/7] rcu/tree: segcblist: Remove redundant smp_mb()s Message-ID: <20201105035713.GL3249@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20201103142603.1302207-1-joel@joelfernandes.org> <20201103142603.1302207-7-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201103142603.1302207-7-joel@joelfernandes.org> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 03, 2020 at 09:26:02AM -0500, Joel Fernandes (Google) wrote: > This memory barrier is not needed as rcu_segcblist_add_len() already > includes a memory barrier *before* and *after* the length of the list is > updated. > > Same reasoning for rcu_segcblist_enqueue(). I suggest a commit log like the following: ------------------------------------------------------------------------ The full memory barriers in rcu_segcblist_enqueue() and in rcu_do_batch() are not needed because rcu_segcblist_add_len(), and thus also rcu_segcblist_inc_len(), already includes a memory barrier *before* and *after* the length of the list is updated. This commit therefore removes these redundant smp_mb() invocations. ------------------------------------------------------------------------ Other than that, looks good! I could hand-apply it, but that would just cause more churn with the addition of the call to rcu_segcblist_inc_seglen(). So could you please update the commit log when you repost, whether to the mailing list or from your git tree? Thanx, Paul > Reviewed-by: Frederic Weisbecker > Signed-off-by: Joel Fernandes (Google) > --- > kernel/rcu/rcu_segcblist.c | 1 - > kernel/rcu/tree.c | 1 - > 2 files changed, 2 deletions(-) > > diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c > index e9e72d72f7a6..d96272e8d604 100644 > --- a/kernel/rcu/rcu_segcblist.c > +++ b/kernel/rcu/rcu_segcblist.c > @@ -268,7 +268,6 @@ void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp, > struct rcu_head *rhp) > { > rcu_segcblist_inc_len(rsclp); > - smp_mb(); /* Ensure counts are updated before callback is enqueued. */ > rcu_segcblist_inc_seglen(rsclp, RCU_NEXT_TAIL); > rhp->next = NULL; > WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp); > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index f6c6653b3ec2..fb2a5ac4a59c 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2525,7 +2525,6 @@ static void rcu_do_batch(struct rcu_data *rdp) > > /* Update counts and requeue any remaining callbacks. */ > rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl); > - smp_mb(); /* List handling before counting for rcu_barrier(). */ > rcu_segcblist_add_len(&rdp->cblist, -count); > > /* Reinstate batch limit if we have worked down the excess. */ > -- > 2.29.1.341.ge80a0c044ae-goog >