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 A5559C433FE for ; Mon, 3 Oct 2022 13:36:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229941AbiJCNgO (ORCPT ); Mon, 3 Oct 2022 09:36:14 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52706 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229814AbiJCNgL (ORCPT ); Mon, 3 Oct 2022 09:36:11 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 392282E9EF; Mon, 3 Oct 2022 06:36:09 -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 5011C6109E; Mon, 3 Oct 2022 13:36:09 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 34A67C433C1; Mon, 3 Oct 2022 13:36:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1664804168; bh=APLpxixznGO5XHQlSSK1OhWUCftaVXjDWyjfb1FHWhY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=HvUDjr9LoeQP+mKWwzo8llsipcjHY1ym1/PWOko7gAFa6CutGcgrI63Klw//we31L KMfdGOnUB9GBvO8KDH1Ml8Ln1Hyu4WQK32zXUbisSuFRWoNzoHrb2Q+BPQ+ozs0o6J 6NQjb0ZiFfOb5Hv6JFnVwLtqBxj+UXsZJ83N0wDmbsq2w1pf6FSED9sXxw+gY2No3e wR6Lyn7zAGhF2MLTU8HUX11W4nVDL84JwX6R92QSBVukli37c41+/RaNpiTXxNesVt UT0OHLymPHFm4mQHAI42KE/gza2UQ9dkJfeVh6kdgux7NTWsD1YZkQa+6zUJYwk9ex b+HXJ+IGk3e3A== Date: Mon, 3 Oct 2022 15:36:05 +0200 From: Frederic Weisbecker To: "Paul E. McKenney" Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, rostedt@goodmis.org, Thomas Gleixner , John Ogness , Petr Mladek Subject: Re: [PATCH RFC v2 rcu 3/8] srcu: Check for consistent per-CPU per-srcu_struct NMI safety Message-ID: <20221003133605.GA306466@lothringen> References: <20220929180714.GA2874192@paulmck-ThinkPad-P17-Gen-1> <20220929180731.2875722-3-paulmck@kernel.org> <20221002220619.GA298433@lothringen> <20221002235103.GW4196@paulmck-ThinkPad-P17-Gen-1> <20221003101331.GA304186@lothringen> <20221003115718.GY4196@paulmck-ThinkPad-P17-Gen-1> <20221003123721.GA304426@lothringen> <20221003133210.GZ4196@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221003133210.GZ4196@paulmck-ThinkPad-P17-Gen-1> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Oct 03, 2022 at 06:32:10AM -0700, Paul E. McKenney wrote: > On Mon, Oct 03, 2022 at 02:37:21PM +0200, Frederic Weisbecker wrote: > > On Mon, Oct 03, 2022 at 04:57:18AM -0700, Paul E. McKenney wrote: > > > On Mon, Oct 03, 2022 at 12:13:31PM +0200, Frederic Weisbecker wrote: > > > > On Sun, Oct 02, 2022 at 04:51:03PM -0700, Paul E. McKenney wrote: > > > > > On Mon, Oct 03, 2022 at 12:06:19AM +0200, Frederic Weisbecker wrote: > > > > > > On Thu, Sep 29, 2022 at 11:07:26AM -0700, Paul E. McKenney wrote: > > > > > > > This commit adds runtime checks to verify that a given srcu_struct uses > > > > > > > consistent NMI-safe (or not) read-side primitives on a per-CPU basis. > > > > > > > > > > > > > > Link: https://lore.kernel.org/all/20220910221947.171557773@linutronix.de/ > > > > > > > > > > > > > > Signed-off-by: Paul E. McKenney > > > > > > > Cc: Thomas Gleixner > > > > > > > Cc: John Ogness > > > > > > > Cc: Petr Mladek > > > > > > > --- > > > > > > > include/linux/srcu.h | 4 ++-- > > > > > > > include/linux/srcutiny.h | 4 ++-- > > > > > > > include/linux/srcutree.h | 9 +++++++-- > > > > > > > kernel/rcu/srcutree.c | 38 ++++++++++++++++++++++++++++++++------ > > > > > > > 4 files changed, 43 insertions(+), 12 deletions(-) > > > > > > > > > > > > > > diff --git a/include/linux/srcu.h b/include/linux/srcu.h > > > > > > > index 2cc8321c0c86..565f60d57484 100644 > > > > > > > --- a/include/linux/srcu.h > > > > > > > +++ b/include/linux/srcu.h > > > > > > > @@ -180,7 +180,7 @@ static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) __acquires(ssp > > > > > > > int retval; > > > > > > > > > > > > > > if (IS_ENABLED(CONFIG_NEED_SRCU_NMI_SAFE)) > > > > > > > - retval = __srcu_read_lock_nmisafe(ssp); > > > > > > > + retval = __srcu_read_lock_nmisafe(ssp, true); > > > > > > > else > > > > > > > retval = __srcu_read_lock(ssp); > > > > > > > > > > > > Shouldn't it be checked also when CONFIG_NEED_SRCU_NMI_SAFE=n ? > > > > > > > > > > You are asking why there is no "true" argument to __srcu_read_lock()? > > > > > That is because it checks unconditionally. > > > > > > > > It checks unconditionally but it always assumes not to be called as nmisafe. > > > > > > > > For example on x86/arm64/loongarch, the same ssp used with both srcu_read_lock() and > > > > srcu_read_lock_nmisafe() won't report an issue. But on powerpc it will. > > > > > > > > My point is that strong archs should warn as well on behalf of others, to detect > > > > mistakes early. > > > > > > Good point, especially given that x86_64 and arm64 are a rather large > > > fraction of the uses. Not critically urgent, but definitely nice to have. > > > > No indeed. > > > > > > > > Did you by chance have a suggestion for a nice way to accomplish this? > > > > This could be like this: > > > > enum srcu_nmi_flags { > > SRCU_NMI_UNKNOWN = 0x0, > > SRCU_NMI_UNSAFE = 0x1, > > SRCU_NMI_SAFE = 0x2 > > }; > > > > #ifdef CONFIG_NEED_SRCU_NMI_SAFE > > static inline int __srcu_read_lock_nmisafe(struct srcu_struct *ssp, enum srcu_nmi_flags flags) > > { > > int idx; > > struct srcu_data *sdp = raw_cpu_ptr(ssp->sda); > > > > idx = READ_ONCE(ssp->srcu_idx) & 0x1; > > atomic_long_inc(&sdp->srcu_lock_count[idx]); > > smp_mb__after_atomic(); /* B */ /* Avoid leaking the critical section. */ > > > > srcu_check_nmi_safety(ssp, flags); > > > > return idx; > > } > > #else > > static inline int __srcu_read_lock_nmisafe(struct srcu_struct *ssp, enum srcu_nmi_flags flags) > > { > > srcu_check_nmi_safety(ssp, flags); > > return __srcu_read_lock(ssp); > > } > > #endif > > > > static inline int srcu_read_lock_nmisafe(struct srcu_struct *ssp) > > { > > return __srcu_read_lock_nmisafe(ssp, SRCU_NMI_SAFE); > > } > > > > // An __srcu_read_lock() caller in kernel/rcu/tasks.h must be > > // taken care of as well > > static inline int srcu_read_lock(struct srcu_struct *ssp) > > { > > srcu_check_nmi_safety(ssp, SRCU_NMI_UNSAFE); > > return __srcu_read_lock(ssp); > > } > > > > And then you can call __srcu_read_lock_nmisafe(ssp, SRCU_NMI_UNKNOWN) from > > initializers of gp. > > Not bad at all! > > Would you like to send a patch? > > I do not consider this to be something for the current merge window even > if the rest goes in because printk() is used heavily and because it is > easy to get access to powerpc and presumably also riscv systems. > > But as you say, it would be very good to have longer term for the case > where srcu_read_lock_nmisafe() is used for some more obscure purpose. Sure thing! Thanks.