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=-8.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,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 46E9CC3A5A2 for ; Thu, 22 Aug 2019 13:40:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0EC5123401 for ; Thu, 22 Aug 2019 13:40:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="gUzki5iQ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388937AbfHVNj7 (ORCPT ); Thu, 22 Aug 2019 09:39:59 -0400 Received: from mail-pf1-f193.google.com ([209.85.210.193]:43023 "EHLO mail-pf1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387569AbfHVNj6 (ORCPT ); Thu, 22 Aug 2019 09:39:58 -0400 Received: by mail-pf1-f193.google.com with SMTP id v12so3990325pfn.10 for ; Thu, 22 Aug 2019 06:39:58 -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=laipNW3j2EKDk6tHY1/Qg9ASE0QU4QNK68diornVwyk=; b=gUzki5iQcgN9eKgKbYbwEB8ujk1xhGm5CJx3myOWdpRsvNssZS86hS00mL8Obyavbp TyHWu6O9657sPIiVnBEi9oMpybv3KDtXr/w+a9NDBmTE4VSzFaiY5Alt8iMP2PBlpKez 6Hfr02E55lUb3udv2MO4mXWh3olSeuYL4hHPU= 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=laipNW3j2EKDk6tHY1/Qg9ASE0QU4QNK68diornVwyk=; b=bnk8/azToM1KDIclNbHRsxRH7rSZ9KnB2/B4jhFqxYfqEGQ4GSLHje71wEfrzn5CVR ii7YhCxsM0chBysP5NiB5o1GcBhQPYv6WudcdezYcxIanjjPm6iUOjvjo7FuZKImI3sM jhjm+Z5Muz3fL5b6ZTrxNScOkq2v3QCOtcBy1Am1qSyu8VUGgNV8lKqRyiIrn3l5VATg 7CTVHZrSkj6AqvGkCfr4wuY7L5l4NneJs8IE2WER0Mex7b9vxRnv7icBsmSwSEzFGXOH X8zU7dzhvUm/SlMN8Ny5Ll6fLkDjKb9/pr4N/zuqqcwbyOkk3wKHdNpC4Jkyy6omxdcR og0Q== X-Gm-Message-State: APjAAAV2yHZdi9N/733zIvgOBfG66jFeOPoEO8PtS3Qbw4zJS/WaLFel QWdsI0pEqVbsJagbvJGJrWlCuQ== X-Google-Smtp-Source: APXvYqz0A3D27GjS3wXlB2R997d/+71Javbq9sAagNY3kGdGFnFrwr7sCE/uOmcn9Ws5ftJYwGWJiA== X-Received: by 2002:a17:90a:342d:: with SMTP id o42mr5663380pjb.27.1566481197680; Thu, 22 Aug 2019 06:39:57 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id n185sm24733438pga.16.2019.08.22.06.39.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 22 Aug 2019 06:39:56 -0700 (PDT) Date: Thu, 22 Aug 2019 09:39:55 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: Scott Wood , Sebastian Andrzej Siewior , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, Thomas Gleixner , Peter Zijlstra , Juri Lelli , Clark Williams Subject: Re: [PATCH RT v2 1/3] rcu: Acquire RCU lock when disabling BHs Message-ID: <20190822133955.GA29841@google.com> References: <20190821231906.4224-1-swood@redhat.com> <20190821231906.4224-2-swood@redhat.com> <20190821233358.GU28441@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190821233358.GU28441@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-rt-users-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rt-users@vger.kernel.org On Wed, Aug 21, 2019 at 04:33:58PM -0700, Paul E. McKenney wrote: > On Wed, Aug 21, 2019 at 06:19:04PM -0500, Scott Wood wrote: > > A plain local_bh_disable() is documented as creating an RCU critical > > section, and (at least) rcutorture expects this to be the case. However, > > in_softirq() doesn't block a grace period on PREEMPT_RT, since RCU checks > > preempt_count() directly. Even if RCU were changed to check > > in_softirq(), that wouldn't allow blocked BH disablers to be boosted. > > > > Fix this by calling rcu_read_lock() from local_bh_disable(), and update > > rcu_read_lock_bh_held() accordingly. > > Cool! Some questions and comments below. > > Thanx, Paul > > > Signed-off-by: Scott Wood > > --- > > Another question is whether non-raw spinlocks are intended to create an > > RCU read-side critical section due to implicit preempt disable. > > Hmmm... Did non-raw spinlocks act like rcu_read_lock_sched() > and rcu_read_unlock_sched() pairs in -rt prior to the RCU flavor > consolidation? If not, I don't see why they should do so after that > consolidation in -rt. May be I am missing something, but I didn't see the connection between consolidation and this patch. AFAICS, this patch is so that rcu_read_lock_bh_held() works at all on -rt. Did I badly miss something? > > If they > > are, then we'd need to add rcu_read_lock() there as well since RT doesn't > > disable preemption (and rcutorture should explicitly test with a > > spinlock). If not, the documentation should make that clear. > > True enough! > > > include/linux/rcupdate.h | 4 ++++ > > kernel/rcu/update.c | 4 ++++ > > kernel/softirq.c | 12 +++++++++--- > > 3 files changed, 17 insertions(+), 3 deletions(-) > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > index 388ace315f32..d6e357378732 100644 > > --- a/include/linux/rcupdate.h > > +++ b/include/linux/rcupdate.h > > @@ -615,10 +615,12 @@ static inline void rcu_read_unlock(void) > > static inline void rcu_read_lock_bh(void) > > { > > local_bh_disable(); > > +#ifndef CONFIG_PREEMPT_RT_FULL > > __acquire(RCU_BH); > > rcu_lock_acquire(&rcu_bh_lock_map); > > RCU_LOCKDEP_WARN(!rcu_is_watching(), > > "rcu_read_lock_bh() used illegally while idle"); > > +#endif > > Any chance of this using "if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL))"? > We should be OK providing a do-nothing __maybe_unused rcu_bh_lock_map > for lockdep-enabled -rt kernels, right? Since this function is small, I prefer if -rt defines their own rcu_read_lock_bh() which just does the local_bh_disable(). That would be way cleaner IMO. IIRC, -rt does similar things for spinlocks, but it has been sometime since I look at the -rt patchset. > > } > > > > /* > > @@ -628,10 +630,12 @@ static inline void rcu_read_lock_bh(void) > > */ > > static inline void rcu_read_unlock_bh(void) > > { > > +#ifndef CONFIG_PREEMPT_RT_FULL > > RCU_LOCKDEP_WARN(!rcu_is_watching(), > > "rcu_read_unlock_bh() used illegally while idle"); > > rcu_lock_release(&rcu_bh_lock_map); > > __release(RCU_BH); > > +#endif > > Ditto. > > > local_bh_enable(); > > } > > > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > > index 016c66a98292..a9cdf3d562bc 100644 > > --- a/kernel/rcu/update.c > > +++ b/kernel/rcu/update.c > > @@ -296,7 +296,11 @@ int rcu_read_lock_bh_held(void) > > return 0; > > if (!rcu_lockdep_current_cpu_online()) > > return 0; > > +#ifdef CONFIG_PREEMPT_RT_FULL > > + return lock_is_held(&rcu_lock_map) || irqs_disabled(); > > +#else > > return in_softirq() || irqs_disabled(); > > +#endif > > And globally. And could be untangled a bit as well: if (irqs_disabled()) return 1; if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) return lock_is_held(&rcu_lock_map); return in_softirq(); > > } > > EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held); > > > > diff --git a/kernel/softirq.c b/kernel/softirq.c > > index d16d080a74f7..6080c9328df1 100644 > > --- a/kernel/softirq.c > > +++ b/kernel/softirq.c > > @@ -115,8 +115,10 @@ void __local_bh_disable_ip(unsigned long ip, unsigned int cnt) > > long soft_cnt; > > > > WARN_ON_ONCE(in_irq()); > > - if (!in_atomic()) > > + if (!in_atomic()) { > > local_lock(bh_lock); > > + rcu_read_lock(); > > + } > > soft_cnt = this_cpu_inc_return(softirq_counter); > > WARN_ON_ONCE(soft_cnt == 0); > > current->softirq_count += SOFTIRQ_DISABLE_OFFSET; > > @@ -151,8 +153,10 @@ void _local_bh_enable(void) > > #endif > > > > current->softirq_count -= SOFTIRQ_DISABLE_OFFSET; > > - if (!in_atomic()) > > + if (!in_atomic()) { > > + rcu_read_unlock(); > > local_unlock(bh_lock); > > + } > > } > > > > void _local_bh_enable_rt(void) > > @@ -185,8 +189,10 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt) > > WARN_ON_ONCE(count < 0); > > local_irq_enable(); > > > > - if (!in_atomic()) > > + if (!in_atomic()) { > > + rcu_read_unlock(); > > local_unlock(bh_lock); > > + } > > The return from in_atomic() is guaranteed to be the same at > local_bh_enable() time as was at the call to the corresponding > local_bh_disable()? > > I could have sworn that I ran afoul of this last year. Might these > added rcu_read_lock() and rcu_read_unlock() calls need to check for > CONFIG_PREEMPT_RT_FULL? Great point! I think they should be guarded but will let Scott answer that one. thanks, - Joel