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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 A471DC433DF for ; Thu, 4 Jun 2020 14:17:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7C0C7207D8 for ; Thu, 4 Jun 2020 14:17:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1591280248; bh=I/yRg+bsP+cNSTc4qUwZLr6xT+q1LIQeHuYrZkUYQDQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=LZPGwcEVrn9ro5I7II2mKkt7mzoTUIR8r/nFXN2MPEfH1mkMEIX49ZShLTFGGtmDj At6ltBGP6oTb35pZcBnBGf0MeXUxnoaf4RWUmhI3GNmkkzKQfJxwee1MD8WUlb69Ka AtNiQGVSJmZdBCiKTGayenYPe8EaxKuHeYn3TyqE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728933AbgFDOR1 (ORCPT ); Thu, 4 Jun 2020 10:17:27 -0400 Received: from mail.kernel.org ([198.145.29.99]:54000 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728682AbgFDOR1 (ORCPT ); Thu, 4 Jun 2020 10:17:27 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 9D86920663; Thu, 4 Jun 2020 14:17:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1591280246; bh=I/yRg+bsP+cNSTc4qUwZLr6xT+q1LIQeHuYrZkUYQDQ=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=EdPxM+570FQebvv1nWIopmvMhUQ4lvRtII1Q8pz8N/VDyiE8nwtwY15/MsZc1X0C0 1tSNOiD11PO573Bg0Eg7gMeTRurC7YP5yq4OsNb8oJMBPwnzkt2W8sYWdKf0F2GZad tfayAewJuV4uWfDfPDLfVgvKKoaUoPe2sDzurJXY= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 8558935228BC; Thu, 4 Jun 2020 07:17:26 -0700 (PDT) Date: Thu, 4 Jun 2020 07:17:26 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: tglx@linutronix.de, x86@kernel.org, elver@google.com, kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org, will@kernel.org, dvyukov@google.com, glider@google.com, andreyknvl@google.com Subject: Re: [PATCH 2/9] rcu: Fixup noinstr warnings Message-ID: <20200604141726.GZ29598@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200603114014.152292216@infradead.org> <20200603114051.896465666@infradead.org> <20200603164600.GQ29598@paulmck-ThinkPad-P72> <20200603171320.GE2570@hirez.programming.kicks-ass.net> <20200604033409.GX29598@paulmck-ThinkPad-P72> <20200604080512.GA2587@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200604080512.GA2587@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 04, 2020 at 10:05:12AM +0200, Peter Zijlstra wrote: > On Wed, Jun 03, 2020 at 08:34:09PM -0700, Paul E. McKenney wrote: > > On Wed, Jun 03, 2020 at 07:13:20PM +0200, Peter Zijlstra wrote: > > > On Wed, Jun 03, 2020 at 09:46:00AM -0700, Paul E. McKenney wrote: > > > > > > @@ -313,7 +313,7 @@ static __always_inline bool rcu_dynticks > > > > > { > > > > > struct rcu_data *rdp = this_cpu_ptr(&rcu_data); > > > > > > > > > > - return !(atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR); > > > > > + return !(arch_atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR); > > > > > > The above is actually instrumented by KCSAN, due to arch_atomic_read() > > > being a READ_ONCE() and it now understanding volatile. > > > > > > > Also instrument_atomic_write(&rdp->dynticks, sizeof(rdp->dynticks)) as > > > > Right, this should instead be instrument_read(...). > > > > Though if KCSAN is unconditionally instrumenting volatile, how does > > this help? Or does KCSAN's instrumentation of volatile somehow avoid > > causing trouble? > > As Marco already explained, when used inside noinstr no instrumentation > will be emitted, when used outside noinstr it will emit the right > instrumentation. > > > > > o In theory in rcu_irq_exit_preempt(), but as this generates code > > > > only in lockdep builds, it might not be worth worrying about. > > > > > > > > o Ditto for rcu_irq_exit_check_preempt(). > > > > > > > > o Ditto for __rcu_irq_enter_check_tick(). > > > > > > Not these, afaict they're all the above arch_atomic_read(), which is > > > instrumented due to volatile in these cases. > > I this case, the above call-sites are all not noinstr (double negative!) > and will thus cause instrumentation to be emitted. > > This is all a 'special' case for arch_atomic_read() (and _set()), > because they're basically READ_ONCE() (and WRITE_ONCE() resp.). The > normal atomics are asm() and it doesn't do anything for those (although > I suppose clang could, since it has this internal assembler to parse the > inline asm, but afaiu that's not something GCC ever wants to do). Got it, and I had missed the inlining. Again, commenting this will be interesting. And your earlier comment about the compiler refusing to inline now makes sense... Thanx, Paul