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=-3.8 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 024A8C433E0 for ; Sat, 25 Jul 2020 20:21:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C7EE1206D8 for ; Sat, 25 Jul 2020 20:21:40 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="M82R7Oo1" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727912AbgGYUVj (ORCPT ); Sat, 25 Jul 2020 16:21:39 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42266 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726727AbgGYUVj (ORCPT ); Sat, 25 Jul 2020 16:21:39 -0400 Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D64A9C08C5C0 for ; Sat, 25 Jul 2020 13:21:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=nEVDy9qer+ckQqB0/s8J9oIrB0Y28OWoQKAT4jlVuho=; b=M82R7Oo1Bq7KxP1RWnK9b0Een7 ZePghvSlHu5G14h3UgVI23HBwSLhrQ2fRNvpsD8sN8qBZGELtpaPEcA1NtLSVBGaC9N70bngDsxYX zJM8fFRI5f+8xEweV1YZ4B+tlAz78KiwB4DLRp66mO3hts4NZgvXeUCx6V601ck4DUFmA8HHuZykL N9bxt82OE3YEaGKvNXy2pW/j+SfZIwOexkFLDZUA7lQpRTxUMfd03H653mhJRo0EMgzW6TTCohKqV aQjWrk4uHAunDY+OxGlwh9jgEyRr38S3w102tt8Mk+RBsST6WiSOE+7Ys6Nqk5w/eunNs1awcB56X nC7oCUlg==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1jzQfl-0000FO-VL; Sat, 25 Jul 2020 20:21:34 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 8A970301179; Sat, 25 Jul 2020 22:21:31 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 6E7582141FBB0; Sat, 25 Jul 2020 22:21:31 +0200 (CEST) Date: Sat, 25 Jul 2020 22:21:31 +0200 From: peterz@infradead.org To: "Paul E. McKenney" Cc: Marco Elver , Andrey Konovalov , Alexander Potapenko , Dmitry Vyukov , kasan-dev , LKML Subject: Re: [PATCH] kcsan: Add option to allow watcher interruptions Message-ID: <20200725202131.GM43129@hirez.programming.kicks-ass.net> References: <20200220141551.166537-1-elver@google.com> <20200220185855.GY2935@paulmck-ThinkPad-P72> <20200220213317.GA35033@google.com> <20200725145623.GZ9247@paulmck-ThinkPad-P72> <20200725174430.GH10769@hirez.programming.kicks-ass.net> <20200725193909.GB9247@paulmck-ThinkPad-P72> <20200725201013.GZ119549@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200725201013.GZ119549@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jul 25, 2020 at 10:10:13PM +0200, peterz@infradead.org wrote: > On Sat, Jul 25, 2020 at 12:39:09PM -0700, Paul E. McKenney wrote: > > This gets me the following for __rcu_read_lock(): > > > > 00000000000000e0 <__rcu_read_lock>: > > e0: 48 8b 14 25 00 00 00 mov 0x0,%rdx > > e7: 00 > > e8: 8b 82 e0 02 00 00 mov 0x2e0(%rdx),%eax > > ee: 83 c0 01 add $0x1,%eax > > f1: 89 82 e0 02 00 00 mov %eax,0x2e0(%rdx) > > f7: c3 retq > > f8: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1) > > ff: 00 > > > > One might hope for a dec instruction, but this isn't bad. We do lose > > a few instructions compared to the C-language case due to differences > > in address calculation: > > > > 00000000000000e0 <__rcu_read_lock>: > > e0: 48 8b 04 25 00 00 00 mov 0x0,%rax > > e7: 00 > > e8: 83 80 e0 02 00 00 01 addl $0x1,0x2e0(%rax) > > ef: c3 retq > > Shees, that's daft... I think this is one of the cases where GCC is > perhaps overly cautious when presented with 'volatile'. > > It has a history of generating excessively crap code around volatile, > and while it has improved somewhat, this seems to show there's still > room for improvement... > > I suppose this is the point where we go bug a friendly compiler person. Having had a play with godbolt.org, it seems clang isn't affected by this particular flavour of crazy, but GCC does indeed refuse to fuse the address calculation and the addition.