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 353FFC433EF for ; Thu, 24 Mar 2022 01:04:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347051AbiCXBFi (ORCPT ); Wed, 23 Mar 2022 21:05:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36422 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239850AbiCXBFf (ORCPT ); Wed, 23 Mar 2022 21:05:35 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3BB2133894 for ; Wed, 23 Mar 2022 18:04:05 -0700 (PDT) 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 dfw.source.kernel.org (Postfix) with ESMTPS id BFE3B61922 for ; Thu, 24 Mar 2022 01:04:04 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 10AABC340E8; Thu, 24 Mar 2022 01:04:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1648083844; bh=rHv7NJgpa63bJ+0QQF4yG6Kjje+PVdEiSiSy51DiNuY=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=fzGs3EbME7AKoL2gxxtUuBE9AamNUdryf5bW/lJ2MmO78Nx2bPLcbF1bnYlbuOcYN rYHpVIiaX2ZSaaVXiEW0CAvxfmGQMcWPXRC8Cwr5TRvff8iIBy5IlftwBxc0J2Vi75 8yxOfPLbQOWLMuCcx2m3EO0/PIdD4B9yHJOUcHBxYRoDq80TASqWiBo59csUjNiWE1 uwf5M86rn9Nae2VYG4AdZwv2SWpMe/UR+hTu3QMtwVYHR/vq9+mG1wEM0wWeU0m5j0 h7yPL40w2gmuv2DVfbi2Mngl33QZqsEUmxSJHfJtpSJUWbwQzn7tpyzE+sByNSfQ9S jdIEwnSd4dw4w== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id BCAB65C0192; Wed, 23 Mar 2022 18:04:02 -0700 (PDT) Date: Wed, 23 Mar 2022 18:04:02 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: LKML , Uladzislau Rezki , Boqun Feng , Neeraj Upadhyay , Joel Fernandes Subject: Re: [PATCH 4/4] rcu: Name internal polling flag Message-ID: <20220324010402.GU4285@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220316144255.336021-1-frederic@kernel.org> <20220316144255.336021-5-frederic@kernel.org> <20220322021107.GP4285@paulmck-ThinkPad-P17-Gen-1> <20220322103224.GA701946@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220322103224.GA701946@lothringen> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 22, 2022 at 11:32:24AM +0100, Frederic Weisbecker wrote: > On Mon, Mar 21, 2022 at 07:11:07PM -0700, Paul E. McKenney wrote: > > On Wed, Mar 16, 2022 at 03:42:55PM +0100, Frederic Weisbecker wrote: > > > Give a proper self-explanatory name to the expedited grace period > > > internal polling flag. > > > > > > Signed-off-by: Frederic Weisbecker > > > Cc: Neeraj Upadhyay > > > Cc: Boqun Feng > > > Cc: Uladzislau Rezki > > > Cc: Joel Fernandes > > > --- > > > kernel/rcu/rcu.h | 5 +++++ > > > kernel/rcu/tree.c | 2 +- > > > kernel/rcu/tree_exp.h | 9 +++++---- > > > 3 files changed, 11 insertions(+), 5 deletions(-) > > > > > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h > > > index eccbdbdaa02e..8a62bb416ba4 100644 > > > --- a/kernel/rcu/rcu.h > > > +++ b/kernel/rcu/rcu.h > > > @@ -30,6 +30,11 @@ > > > #define RCU_GET_STATE_USE_NORMAL 0x2 > > > #define RCU_GET_STATE_BAD_FOR_NORMAL (RCU_GET_STATE_FROM_EXPEDITED | RCU_GET_STATE_USE_NORMAL) > > > > > > +/* > > > + * Low-order bit definitions for polled grace-period internals. > > > + */ > > > +#define RCU_EXP_SEQ_POLL_DONE 0x1 > > > + > > > /* > > > * Return the counter portion of a sequence number previously returned > > > * by rcu_seq_snap() or rcu_seq_current(). > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > index 5da381a3cbe5..b3223b365f9f 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -4679,7 +4679,7 @@ static void __init rcu_init_one(void) > > > spin_lock_init(&rnp->exp_lock); > > > mutex_init(&rnp->boost_kthread_mutex); > > > raw_spin_lock_init(&rnp->exp_poll_lock); > > > - rnp->exp_seq_poll_rq = 0x1; > > > + rnp->exp_seq_poll_rq = RCU_EXP_SEQ_POLL_DONE; > > > INIT_WORK(&rnp->exp_poll_wq, sync_rcu_do_polled_gp); > > > } > > > } > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > > index c4a19c6a83cf..7ccb909d6355 100644 > > > --- a/kernel/rcu/tree_exp.h > > > +++ b/kernel/rcu/tree_exp.h > > > @@ -910,14 +910,14 @@ static void sync_rcu_do_polled_gp(struct work_struct *wp) > > > unsigned long s; > > > > > > s = READ_ONCE(rnp->exp_seq_poll_rq); > > > - if (s & 0x1) > > > + if (s & RCU_EXP_SEQ_POLL_DONE) > > > return; > > > while (!sync_exp_work_done(s)) > > > __synchronize_rcu_expedited(true); > > > > One additional question. If we re-read rnp->exp_seq_poll_rq on each pass > > through the loop, wouldn't we have less trouble with counter wrap? > > We can indeed do that, though it won't eliminate the possibility of wrapping. True. But in conjunction with an exact check for expired grace-period sequence number, it reduces the maximum addtional penalty for wrapping to two grace periods. Thanx, Paul > > > raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags); > > > s = rnp->exp_seq_poll_rq; > > > - if (!(s & 0x1) && sync_exp_work_done(s)) > > > - WRITE_ONCE(rnp->exp_seq_poll_rq, s | 0x1); > > > + if (!(s & RCU_EXP_SEQ_POLL_DONE) && sync_exp_work_done(s)) > > > + WRITE_ONCE(rnp->exp_seq_poll_rq, s | RCU_EXP_SEQ_POLL_DONE); > > > raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags); > > > } > > > > > > @@ -946,7 +946,8 @@ unsigned long start_poll_synchronize_rcu_expedited(void) > > > rnp = rdp->mynode; > > > if (rcu_init_invoked()) > > > raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags); > > > - if ((rnp->exp_seq_poll_rq & 0x1) || ULONG_CMP_LT(rnp->exp_seq_poll_rq, s)) { > > > + if ((rnp->exp_seq_poll_rq & RCU_EXP_SEQ_POLL_DONE) || > > > + ULONG_CMP_LT(rnp->exp_seq_poll_rq, s)) { > > > WRITE_ONCE(rnp->exp_seq_poll_rq, s); > > > if (rcu_init_invoked()) > > > queue_work(rcu_gp_wq, &rnp->exp_poll_wq); > > > -- > > > 2.25.1 > > >