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 5F2E7C43381 for ; Sat, 23 Mar 2019 03:02:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2927E218D3 for ; Sat, 23 Mar 2019 03:02:57 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="bt0aYpi1" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728407AbfCWDCz (ORCPT ); Fri, 22 Mar 2019 23:02:55 -0400 Received: from mail-pf1-f196.google.com ([209.85.210.196]:44412 "EHLO mail-pf1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728208AbfCWDCz (ORCPT ); Fri, 22 Mar 2019 23:02:55 -0400 Received: by mail-pf1-f196.google.com with SMTP id a3so2723374pff.11 for ; Fri, 22 Mar 2019 20:02:54 -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=GO/8l8d1+eXZTrJqKmIH2kHR9q+gBa7alFugBZAjfcQ=; b=bt0aYpi1XmmRurMTyY9oBVhcitWJ0Tgzw/F6EhxCNTNMzP6i+8thAneNkt6kqK5M6y GIuMzunHV+AwGh2uPfecKIuOr4vv0aSgm6sjRe9qeqGs5C12dW/VZbqjQtAol4OlX9Hv Kyy1RSHDX3arUNjgrVif36CmL2DcTxxDI+fZ0= 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=GO/8l8d1+eXZTrJqKmIH2kHR9q+gBa7alFugBZAjfcQ=; b=HiH5Qd96/fQKsu6YXNXPhfl3TnBJSlC1MwB6HCyH/IwKoNHcaGf0T+7Po1nRfVRmXw qcdbrPr4p7Pi1T5obMJNd3J0bPyKayh2+qR+tJcRvee4hWEHcImGwmhyOlxA6P539lYJ dlLcw+aN6ZBQUhMU705xQmKI8eLyPqUiZj0VhVFxwc0rmFXt56FAWOtpJu74J1yAsMz4 lerBLhw81f6b722DAd1/9bUEzvUs6+6KP8FpFCCH+9wTOJQBpECc1nv3nen55OPhRhR2 0fbbGDmFm3NM0YUzouBxOT62nrEuU2o2o2KTMRWLmS3yA8BPOrNrFbnDCqe8QFVHbIcw 1c9A== X-Gm-Message-State: APjAAAURDrWVCGs6GdrKFAn8MQ8JpT1HRL/rM9Du5kx1uUAcXnO0tI+I JxBar4PDm32CFqv2Iy+zb5oxbzPfhJw= X-Google-Smtp-Source: APXvYqyVwVeQLm1FGDYEkaoGt6VX3Z9trGNs0EtjsYhV7NRSmx8CNRO2RyiAoMDUWGIRaWzu7NjFUQ== X-Received: by 2002:a17:902:2989:: with SMTP id h9mr13010151plb.26.1553310173786; Fri, 22 Mar 2019 20:02:53 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id m67sm13261674pfj.70.2019.03.22.20.02.52 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Fri, 22 Mar 2019 20:02:52 -0700 (PDT) Date: Fri, 22 Mar 2019 23:02:51 -0400 From: Joel Fernandes To: linux-kernel@vger.kernel.org Cc: byungchul.park@lge.com, kernel-team@android.com, rcu@vger.kernel.org, Ingo Molnar , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , "Paul E. McKenney" , 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: <20190323030251.GB136835@google.com> References: <20190323012939.15185-1-joel@joelfernandes.org> <20190323012939.15185-2-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190323012939.15185-2-joel@joelfernandes.org> 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 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. thanks, - Joel