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=-8.6 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT 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 DFCC6C4360F for ; Mon, 25 Mar 2019 16:44:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9BDFD20879 for ; Mon, 25 Mar 2019 16:44:45 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="MOvLGG5t" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729862AbfCYQoo (ORCPT ); Mon, 25 Mar 2019 12:44:44 -0400 Received: from mail-pg1-f196.google.com ([209.85.215.196]:43980 "EHLO mail-pg1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726024AbfCYQon (ORCPT ); Mon, 25 Mar 2019 12:44:43 -0400 Received: by mail-pg1-f196.google.com with SMTP id z9so563416pgu.10 for ; Mon, 25 Mar 2019 09:44:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=t9zy/7o/wXxn8o9xiennO56tqny6qYj28o2PkgbCnMU=; b=MOvLGG5tLn0biNvCmTTbgLuQZ4wsa0zeH2hjvDbu6XPQNhHyKLPuvOWEmvKaJL2Wtm P6+tZ/8XD+nwYi+BNde9/qrPsgN/VrpKZLhQ7VSMDb6xO6xH2yRxmUd+LXSQYQc8odOz EiVLbFLi4VQvv2j9LDA6TOIbjhaOaml7u5p/4= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=t9zy/7o/wXxn8o9xiennO56tqny6qYj28o2PkgbCnMU=; b=OepltJCPL/6xDoFvaF+2h0rspyAl3F1UNgXYANXbplkJhNcuL1j7iECxYUGCY72esz ygzRySCdeV/qk0KLGmSwm1EpMfpG9zRsFpYxyRpsBt1S/a3eyqB0ggavv0w2iqy0tajv 0++5XTOUJoT4u3kmY1QdgH2/I8yEcUF/RfG13uZypJV8F7mBA1IU83RsiiY9SjQJL4Hh g8fasNL5kCbrJszpmEe5bP7suOWGoVhctOL/nAss8O51vIgV0e3k6dLcLUHO+tWt3Lmm PLJ9m3+qVbQEU9jXZHh5yo+XFYHZiyDKfgqvWo++HHidzCAMJhWndjW8FxVxY1QjlaYe 82oA== X-Gm-Message-State: APjAAAWNMC/4kt0fKYwp36tHSWtyNPG+Zox1kxtM3rLd++QFy/6QYaOR PLrc4/qaCIEFyofButxXM6iESw== X-Google-Smtp-Source: APXvYqz4GmgKSuLoWMlEHsTlpg6TaOd4kBaJQALreNIV1Nd1+OY7jww4ruWKlGKq60XFMFS8Iqs5ow== X-Received: by 2002:a62:7049:: with SMTP id l70mr25082091pfc.78.1553532282631; Mon, 25 Mar 2019 09:44:42 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id e21sm10803828pfd.177.2019.03.25.09.44.41 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 25 Mar 2019 09:44:41 -0700 (PDT) Date: Mon, 25 Mar 2019 12:44:40 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, byungchul.park@lge.com, kernel-team@android.com, rcu@vger.kernel.org, Ingo Molnar , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , Peter Zijlstra , Steven Rostedt , Will Deacon Subject: Re: [RFC 2/2] rcutree: Add checks for dynticks counters in rcu_is_cpu_rrupt_from_idle Message-ID: <20190325164440.GA25975@google.com> References: <20190323012939.15185-1-joel@joelfernandes.org> <20190323012939.15185-2-joel@joelfernandes.org> <20190323030251.GB136835@google.com> <20190324234351.GX4102@linux.ibm.com> <20190325133646.GA182885@google.com> <20190325155341.GC4102@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190325155341.GC4102@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 25, 2019 at 08:53:41AM -0700, Paul E. McKenney wrote: > On Mon, Mar 25, 2019 at 09:36:46AM -0400, Joel Fernandes wrote: > > On Sun, Mar 24, 2019 at 04:43:51PM -0700, Paul E. McKenney wrote: > > > On Fri, Mar 22, 2019 at 11:02:51PM -0400, Joel Fernandes wrote: > > > > On Fri, Mar 22, 2019 at 09:29:39PM -0400, Joel Fernandes (Google) wrote: > > > > > In the future we would like to combine the dynticks and dynticks_nesting > > > > > counters thus leading to simplifying the code. At the moment we cannot > > > > > do that due to concerns about usermode upcalls appearing to RCU as half > > > > > of an interrupt. Byungchul tried to do it in [1] but the > > > > > "half-interrupt" concern was raised. It is half because, what RCU > > > > > expects is rcu_irq_enter() and rcu_irq_exit() pairs when the usermode > > > > > exception happens. However, only rcu_irq_enter() is observed. This > > > > > concern may not be valid anymore, but at least it used to be the case. > > > > > > > > > > Out of abundance of caution, Paul added warnings [2] in the RCU code > > > > > which if not fired by 2021 may allow us to assume that such > > > > > half-interrupt scenario cannot happen any more, which can lead to > > > > > simplification of this code. > > > > > > > > > > Summary of the changes are the following: > > > > > > > > > > (1) In preparation for this combination of counters in the future, we > > > > > first need to first be sure that rcu_rrupt_from_idle cannot be called > > > > > from anywhere but a hard-interrupt because previously, the comments > > > > > suggested otherwise so let us be sure. We discussed this here [3]. We > > > > > use the services of lockdep to accomplish this. > > > > > > > > > > (2) Further rcu_rrupt_from_idle() is not explicit about how it is using > > > > > the counters which can lead to weird future bugs. This patch therefore > > > > > makes it more explicit about the specific counter values being tested > > > > > > > > > > (3) Lastly, we check for counter underflows just to be sure these are > > > > > not happening, because the previous code in rcu_rrupt_from_idle() was > > > > > allowing the case where the counters can underflow, and the function > > > > > would still return true. Now we are checking for specific values so let > > > > > us be confident by additional checking, that such underflows don't > > > > > happen. Any case, if they do, we should fix them and the screaming > > > > > warning is appropriate. All these checks checks are NOOPs if PROVE_RCU > > > > > and PROVE_LOCKING are disabled. > > > > > > > > > > [1] https://lore.kernel.org/patchwork/patch/952349/ > > > > > [2] Commit e11ec65cc8d6 ("rcu: Add warning to detect half-interrupts") > > > > > [3] https://lore.kernel.org/lkml/20190312150514.GB249405@google.com/ > > > > > > > > > > Cc: byungchul.park@lge.com > > > > > Cc: kernel-team@android.com > > > > > Cc: rcu@vger.kernel.org > > > > > Signed-off-by: Joel Fernandes (Google) > > > > > --- > > > > > kernel/rcu/tree.c | 21 +++++++++++++++++---- > > > > > 1 file changed, 17 insertions(+), 4 deletions(-) > > > > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > > index 9180158756d2..d94c8ed29f6b 100644 > > > > > --- a/kernel/rcu/tree.c > > > > > +++ b/kernel/rcu/tree.c > > > > > @@ -381,16 +381,29 @@ static void __maybe_unused rcu_momentary_dyntick_idle(void) > > > > > } > > > > > > > > > > /** > > > > > - * rcu_is_cpu_rrupt_from_idle - see if idle or immediately interrupted from idle > > > > > + * rcu_is_cpu_rrupt_from_idle - see if interrupted from idle > > > > > * > > > > > - * If the current CPU is idle or running at a first-level (not nested) > > > > > + * If the current CPU is idle and running at a first-level (not nested) > > > > > * interrupt from idle, return true. The caller must have at least > > > > > * disabled preemption. > > > > > */ > > > > > static int rcu_is_cpu_rrupt_from_idle(void) > > > > > { > > > > > - return __this_cpu_read(rcu_data.dynticks_nesting) <= 0 && > > > > > - __this_cpu_read(rcu_data.dynticks_nmi_nesting) <= 1; > > > > > + /* Called only from within the scheduling-clock interrupt */ > > > > > + lockdep_assert_in_irq(); > > > > > + > > > > > + /* Check for counter underflows */ > > > > > + RCU_LOCKDEP_WARN( > > > > > + (__this_cpu_read(rcu_data.dynticks_nesting) < 0) && > > > > > + (__this_cpu_read(rcu_data.dynticks_nmi_nesting) < 0), > > > > > > > > > > > > This condition for the warning is supposed to be || instead of &&. Sorry. > > > > > > > > Or, I will just use 2 RCU_LOCKDEP_WARN(s) here, that's better. > > > > > > Also, the dynticks_nmi_nesting being zero is a bug given that we know > > > we are in an interrupt handler, right? Or am I off by one again? > > > > You are right, we can do additional checking for making sure its never zero. > > I refreshed the patch as below, does this look Ok? > > > > ---8<----------------------- > > > > From: "Joel Fernandes (Google)" > > Subject: [RFC v2] rcutree: Add checks for dynticks counters in > > > > In the future we would like to combine the dynticks and dynticks_nesting > > counters thus leading to simplifying the code. At the moment we cannot > > do that due to concerns about usermode upcalls appearing to RCU as half > > of an interrupt. Byungchul tried to do it in [1] but the > > "half-interrupt" concern was raised. It is half because, what RCU > > expects is rcu_irq_enter() and rcu_irq_exit() pairs when the usermode > > exception happens. However, only rcu_irq_enter() is observed. This > > concern may not be valid anymore, but at least it used to be the case. > > > > Out of abundance of caution, Paul added warnings [2] in the RCU code > > which if not fired by 2021 may allow us to assume that such > > half-interrupt scenario cannot happen any more, which can lead to > > simplification of this code. > > > > Summary of the changes are the following: > > > > (1) In preparation for this combination of counters in the future, we > > first need to first be sure that rcu_rrupt_from_idle cannot be called > > from anywhere but a hard-interrupt because previously, the comments > > suggested otherwise so let us be sure. We discussed this here [3]. We > > use the services of lockdep to accomplish this. > > > > (2) Further rcu_rrupt_from_idle() is not explicit about how it is using > > the counters which can lead to weird future bugs. This patch therefore > > makes it more explicit about the specific counter values being tested > > > > (3) Lastly, we check for counter underflows just to be sure these are > > not happening, because the previous code in rcu_rrupt_from_idle() was > > allowing the case where the counters can underflow, and the function > > would still return true. Now we are checking for specific values so let > > us be confident by additional checking, that such underflows don't > > happen. Any case, if they do, we should fix them and the screaming > > warning is appropriate. All these checks checks are NOOPs if PROVE_RCU > > and PROVE_LOCKING are disabled. > > > > [1] https://lore.kernel.org/patchwork/patch/952349/ > > [2] Commit e11ec65cc8d6 ("rcu: Add warning to detect half-interrupts") > > [3] https://lore.kernel.org/lkml/20190312150514.GB249405@google.com/ > > > > Cc: byungchul.park@lge.com > > Cc: kernel-team@android.com > > Cc: rcu@vger.kernel.org > > Signed-off-by: Joel Fernandes (Google) > > Looks better! I have applied this and its predecessor, if in backwards > order. (Will fix, rebase coming up anyway.) > > I do like your Cc-ing kernel-team@android.com -- one less thing for me > to remember! ;-) Thanks a lot Paul! :) - Joel