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 06DE9C43217 for ; Sun, 27 Nov 2022 15:01:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229475AbiK0PB0 (ORCPT ); Sun, 27 Nov 2022 10:01:26 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47584 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229436AbiK0PBZ (ORCPT ); Sun, 27 Nov 2022 10:01:25 -0500 Received: from mail-pf1-x431.google.com (mail-pf1-x431.google.com [IPv6:2607:f8b0:4864:20::431]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8B6B0F5BD for ; Sun, 27 Nov 2022 07:01:24 -0800 (PST) Received: by mail-pf1-x431.google.com with SMTP id w129so8220722pfb.5 for ; Sun, 27 Nov 2022 07:01:24 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=LoVwI8EZP4bWHP6GzkMGMJewCJv5QLXzPPd1mGXyA8E=; b=ZWQG7HPQwCQmzxw0699/Ed6qUF/N1T0wPxyEFP7UXcVbcNvCiP/xIdimPILwYc5SHg 7QRIC3vNYc4y9G0pBbH3xr+lGboTg5GOJKsxavjgldeXhDKO3e61R01p2zHy/Hq/oRRw Y5q4zkPv+0dmi1Umi78XgV4X0qyH52CDI+cRHDJ0u0sSwRtH1omRdwN6WwSH238Lm2be LODAIdd9nTm8opgg8tof7OZcTI9V/D7Z1aJPLXUjA8a+Yoty9pMuwWpKtxaHR9HB2biy Ndou3ca6D2zKWgBP+mfREIxbbBrhPtYKfHItXaRuaaaX1CBNfTXYBrqLiVl18x85Kkak hXGg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=LoVwI8EZP4bWHP6GzkMGMJewCJv5QLXzPPd1mGXyA8E=; b=WNyhlYb36GPzyTvin5L/cP/VXPkKpsNJLE+MJEleys4vgDHIkbi4InyMIRohBNbQa7 DOheoj38NkJ7/vzeK9Zv2TOc1Atz3b3rbmx3SatuEXeOKSr31dsm5ZrUsujHpfVtf6H+ +kRiljKlnewjgDYTDAEpwgH1MUFB6QImgIWgU3TXD9O5Vxi7JK5iVrzkNbunvxA2kuBB 9r/WniOfsp+ZZCwcrM2ScEGA6mjHbCI+ggK4297stUun0SlVBm8ucd7ZZFt4ZXb7bGJO uDTyObWf0U1PtfZU3Ha36jEXleD622IrwXIMyV4/gkFVrKPV5Ipq6K3XRhttKC3NS+CH hF0Q== X-Gm-Message-State: ANoB5pnU/ryIsgkmUMcYv/j0GSF7gHs/15f3/ZxkDNlICixw8Kd4vCfj c+my6Uoo4rvrPZFEtyPMfBnQMh9DGg== X-Google-Smtp-Source: AA0mqf5oOf3L+svIo86mQFFi4noIYnDRpMm3u2jaLq7YiHXf+u9B4FZU3JhOZ7sEmlS3Oq+2kTEmyQ== X-Received: by 2002:a05:6a00:439a:b0:56b:c95f:34dd with SMTP id bt26-20020a056a00439a00b0056bc95f34ddmr27457895pfb.74.1669561283847; Sun, 27 Nov 2022 07:01:23 -0800 (PST) Received: from piliu.users.ipa.redhat.com ([209.132.188.80]) by smtp.gmail.com with ESMTPSA id l6-20020a170903120600b0018157b415dbsm6984934plh.63.2022.11.27.07.01.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 27 Nov 2022 07:01:23 -0800 (PST) Date: Sun, 27 Nov 2022 23:01:17 +0800 From: Pingfan Liu To: "Paul E. McKenney" Cc: rcu@vger.kernel.org, Lai Jiangshan , Frederic Weisbecker , Josh Triplett , Steven Rostedt , Mathieu Desnoyers Subject: Re: [PATCHv2 2/2] srcu: Eliminate the case that snp_seq bigger than snap in srcu_funnel_gp_start() Message-ID: References: <20221123135638.79021-1-kernelfans@gmail.com> <20221123135638.79021-3-kernelfans@gmail.com> <20221123192046.GH4001@paulmck-ThinkPad-P17-Gen-1> <20221124052417.GO4001@paulmck-ThinkPad-P17-Gen-1> <20221124161926.GA3495117@paulmck-ThinkPad-P17-Gen-1> <20221126180352.GD4001@paulmck-ThinkPad-P17-Gen-1> <20221126180704.GA3269129@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221126180704.GA3269129@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Sat, Nov 26, 2022 at 10:07:04AM -0800, Paul E. McKenney wrote: > On Sat, Nov 26, 2022 at 10:03:52AM -0800, Paul E. McKenney wrote: > > On Fri, Nov 25, 2022 at 08:37:03PM +0800, Pingfan Liu wrote: > > > On Thu, Nov 24, 2022 at 08:19:26AM -0800, Paul E. McKenney wrote: > > > > On Wed, Nov 23, 2022 at 09:24:17PM -0800, Paul E. McKenney wrote: > > > > > On Thu, Nov 24, 2022 at 12:06:39PM +0800, Pingfan Liu wrote: > > > > > > On Thu, Nov 24, 2022 at 11:33 AM Pingfan Liu wrote: > > > > > > > On Wed, Nov 23, 2022 at 11:20:46AM -0800, Paul E. McKenney wrote: > > > > > > > > On Wed, Nov 23, 2022 at 09:56:38PM +0800, Pingfan Liu wrote: > > > > > > > > > Since the srcu read lock is still held during srcu_funnel_gp_start(), > > > > > > > > > the seq snap should be the largest number for the slot > > > > > > > > > srcu_have_cbs[idx]. > > > > > > > > > > > > > > > > > > Signed-off-by: Pingfan Liu > > > > > > > > > Cc: Lai Jiangshan > > > > > > > > > Cc: "Paul E. McKenney" > > > > > > > > > Cc: Frederic Weisbecker > > > > > > > > > Cc: Josh Triplett > > > > > > > > > Cc: Steven Rostedt > > > > > > > > > Cc: Mathieu Desnoyers > > > > > > > > > To: rcu@vger.kernel.org > > > > > > > > > --- > > > > > > > > > include/linux/rcupdate.h | 1 + > > > > > > > > > kernel/rcu/srcutree.c | 11 ++++++----- > > > > > > > > > 2 files changed, 7 insertions(+), 5 deletions(-) > > > > > > > > > > > > > > > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > > > > > > > > index 08605ce7379d..a09007236660 100644 > > > > > > > > > --- a/include/linux/rcupdate.h > > > > > > > > > +++ b/include/linux/rcupdate.h > > > > > > > > > @@ -32,6 +32,7 @@ > > > > > > > > > #include > > > > > > > > > > > > > > > > > > #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b)) > > > > > > > > > +#define ULONG_CMP_GT(a, b) (ULONG_MAX / 2 > (a) - (b)) > > > > > > > > > > > > > > > > Please see below... > > > > > > > > > > > > > > > > > #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b)) > > > > > > > > > #define ulong2long(a) (*(long *)(&(a))) > > > > > > > > > #define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b))) > > > > > > > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > > > > > > > > > index 2fc0e775ade4..41902b823687 100644 > > > > > > > > > --- a/kernel/rcu/srcutree.c > > > > > > > > > +++ b/kernel/rcu/srcutree.c > > > > > > > > > @@ -905,14 +905,15 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, > > > > > > > > > for (snp = snp_leaf; snp != NULL; snp = snp->srcu_parent) { > > > > > > > > > spin_lock_irqsave_rcu_node(snp, flags); > > > > > > > > > snp_seq = snp->srcu_have_cbs[idx]; > > > > > > > > > - if (!srcu_invl_snp_seq(snp_seq) && ULONG_CMP_GE(snp_seq, s)) { > > > > > > > > > + /* > > > > > > > > > + * s should be the biggest in the current slot. Hence only LE is > > > > > > > > > + * valid > > > > > > > > > + */ > > > > > > > > > + BUG_ON(ULONG_CMP_GT(snp_seq, s)); > > > > > > > > > > > > > > > > Why not this? (Plus adjusting the comment above, of course.) > > > > > > > > > > > > > > > > WARN_ON_ONCE(ULONG_CMP_LT(s, snp_seq)); > > > > > > > > > > > > > > > > > > > > > > A neat solution! > > > > > > > > > > > > > > And what about the comment > > > > > > > /* s should be the biggest in the current slot. */ > > > > > > > > > > > > > > > That way we don't need ULONG_CMP_GT(). Plus if we are confused about > > > > > > > > s being the biggest in the current slot, we get a splat and can debug > > > > > > > > further. We both might be quite sure that we are not confused, but > > > > > > > > that is exactly when we are most prone to making mistakes. ;-) > > > > > > > > > > > > > > > > > > > > > > Same feeling :) > > > > > > > > > > > > > > > I also ask that you run with this check for some time. After all, if > > > > > > > > > > > > Forget to ask if the test > > > > > > "tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration > > > > > > 10h --configs 18*SRCU-P" > > > > > > satisfies your requirement? > > > > > > > > > > I am thinking more in terms of adding the WARN_ON_ONCE(), letting it be > > > > > in -next and possibly mainline for a couple of years, and then if there > > > > > are no splats, start feeling more confident in the asserted relationship. > > > > > > > > > > If there was a significant performance, scalability, energy-efficiency, or > > > > > simplification benefit, I would feel justified in being more aggressive. > > > > > > > > > > But I am not seeing a significant benefit. > > > > > > > > Ah, and before I forget -again-, have you thought through the counter-wrap > > > > scenarios? Please keep in mind that on a 32-bit system, those counters > > > > can wrap quite quickly compared to typical uptimes. > > > > > > I think 32-bit has not significant different since the statement > > > WARN_ON_ONCE(ULONG_CMP_LT(s, snp_seq)); > > > > > > where 's < snp_seq' is not associated with bits. Am I missing anything? > > > > Your point is that long before there was any danger of wrap, this > > WARN_ON_ONCE() would trigger? > > > > If so, the point is to avoid that WARN_ON_ONCE() from triggering. > > Oops! I failed to figure out the scene you pointed out, where snp_x did not have call_srcu event for a very long time, then comes the wrap, and when s is close enough to snp_x's snp_seq, ULONG_CMP_LT(s, snp_seq) will raise false alarm. Yes, this is a bug. > > Or am I missing your point? Sorry that I missed your point and confused you. Can I remedy it using the root snp? Because the root snp's snp_seq advances each GP. And it is the biggest one among snp-s. > > For example, are you instead saying that the WARN_ON_ONCE() proposed in > your patch will detect the problem either way? If so, what I am asking > is whether your earlier analysis considered wrap without the addition > of that WARN_ON_ONCE(). > Have I answer all of your concerns? Thanks, Pingfan > Thanx, Paul > > > > Thanks, > > > > > > Pingfan > > > > > > > Thanx, Paul > > > > > > > > > > Thanks > > > > > > > > > > > > Pingfan > > > > > > > Sure. I will try it immediately. > > > > > > > > > > > > > > > the assumption is incorrect, the resulting SRCU hangs off in various > > > > > > > > systems around the world will not be so much fun to debug. > > > > > > > > > > > > > > > > Given that this is slowpath code, it is much better to take an extra > > > > > > > > compare and branch than to introduce even an extremely small risk of > > > > > > > > hanging SRCU. > > > > > > > > > > > > > > > > > > > > > > Agree. > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > Pingfan > > > > > > > > > > > > > > > Thanx, Paul > > > > > > > > > > > > > > > > > + if (!srcu_invl_snp_seq(snp_seq) && (snp_seq == s)) { > > > > > > > > > if (snp == snp_leaf && snp_seq == s) > > > > > > > > > snp->srcu_data_have_cbs[idx] |= sdp->grpmask; > > > > > > > > > spin_unlock_irqrestore_rcu_node(snp, flags); > > > > > > > > > - if (snp == snp_leaf && snp_seq != s) { > > > > > > > > > - srcu_schedule_cbs_sdp(sdp, do_norm ? SRCU_INTERVAL : 0); > > > > > > > > > - return; > > > > > > > > > - } > > > > > > > > > if (!do_norm) > > > > > > > > > srcu_funnel_exp_start(ssp, snp, s); > > > > > > > > > return; > > > > > > > > > -- > > > > > > > > > 2.31.1 > > > > > > > > >