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=-3.6 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 19851C433E1 for ; Sun, 26 Jul 2020 11:52:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E91F72065E for ; Sun, 26 Jul 2020 11:52:53 +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="2TtrIcN2" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727794AbgGZLww (ORCPT ); Sun, 26 Jul 2020 07:52:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46222 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725972AbgGZLww (ORCPT ); Sun, 26 Jul 2020 07:52:52 -0400 Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D782BC0619D2 for ; Sun, 26 Jul 2020 04:52:51 -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=KjnRKLEHDr8Ifsf9arCUZI0KOq8oLtgssys0oHriYEo=; b=2TtrIcN2jOiRX3Q/kYQ11GmmlL WYx+GA09hXwgczIdN8KelbmcJK2ULiU7Ni1+sCobXjmuRtUBzmcs8CqTx3LnsjLQ6yGZklkMcuLOX mYH6xES74KhqiWQVVpZBBpSYPzjN3/lFBFabJWWG8gJ+dOVw5vW8SkRoPq82hnJ4yn69OtOD1ar3E OawOJoranHOkFWeHr2TLrLdDFXdqOpCOQ0WztALrMCHP7bG6yCllPPjjmf7Yy70ndXeZFX47u206H rZIfuuxzZcShaxu6KIHptWPMZvzKYB8HgDj8q2TyY8mloimv+Im7wno4Y524zY/0mWcuQdaYBJwBb bIKMnybQ==; 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 1jzfCv-00086M-D6; Sun, 26 Jul 2020 11:52:45 +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 D0C1A301AC6; Sun, 26 Jul 2020 13:52:42 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 279DC2141FBAD; Sun, 26 Jul 2020 13:52:42 +0200 (CEST) Date: Sun, 26 Jul 2020 13:52:42 +0200 From: peterz@infradead.org To: "Paul E. McKenney" Cc: Marco Elver , Andrey Konovalov , Alexander Potapenko , Dmitry Vyukov , kasan-dev , LKML , jakub@redhat.com, hjl.tools@gmail.com Subject: Re: [PATCH] kcsan: Add option to allow watcher interruptions Message-ID: <20200726115242.GA119549@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> <20200725202131.GM43129@hirez.programming.kicks-ass.net> <20200725220750.GC9247@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200725220750.GC9247@paulmck-ThinkPad-P72> 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 03:07:50PM -0700, Paul E. McKenney wrote: > On Sat, Jul 25, 2020 at 10:21:31PM +0200, peterz@infradead.org wrote: > > 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. > > Sounds very good! Do you have someone specific in mind? Jakub perhaps?, Cc'ed > > 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. > > So there is hope, then! > > And even GCC's current state is an improvement. Last I messed with this, > the ACCESS_ONCE()++ approach generated a load, a register increment, > and a store. > > Do you still have the godbolt.org URLs? I would be happy to file > a bugzilla. https://godbolt.org/z/rP8rYM