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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 C0C13C43460 for ; Mon, 19 Apr 2021 08:27:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 936D361159 for ; Mon, 19 Apr 2021 08:27:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232568AbhDSI1c (ORCPT ); Mon, 19 Apr 2021 04:27:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50358 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229790AbhDSI1c (ORCPT ); Mon, 19 Apr 2021 04:27:32 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D63F4C06174A; Mon, 19 Apr 2021 01:27:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; 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=DBMoV/cX0cKYwtE9SxJVrR1aR8cR7qkDyUhKQJS3Otk=; b=A0baORrD+G2oufCB44y+zQaf6Z tzVFCPRFc3jisZMvJFjuS0b3X+MdxYOHuDFAhOzwEfmw7on3iIp7vVA9ISvN85wEi7Z9xNukphDER qc83ko727XedAalO3uJ1SRUG6e4yZYxpU+y2vntG8Up8piSMhL1CkvGp1UtJNuGIWlVBVGuD0wx/1 e/hZzi8CVnL4sM/dsC9Ec1ZeYMvJWwYaPLzXWN4zsdb1hHegQRzE9yTImLKifb6iq+kFTCje/Rwjw a2TZNV4ThUO8Xu0ilaIBiL9PvmGQikjaa2qBD6j8wsBzu1VsgbJhG6IH4BQS0slHduK/ZmG+w1TnO ed5muYIA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.94 #2 (Red Hat Linux)) id 1lYPFC-009QQ4-EQ; Mon, 19 Apr 2021 08:26:58 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 050AC300212; Mon, 19 Apr 2021 10:26:57 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id D96F12C14C5BA; Mon, 19 Apr 2021 10:26:57 +0200 (CEST) Date: Mon, 19 Apr 2021 10:26:57 +0200 From: Peter Zijlstra To: Paolo Bonzini Cc: Wedson Almeida Filho , ojeda@kernel.org, Linus Torvalds , Greg Kroah-Hartman , rust-for-linux@vger.kernel.org, linux-kbuild@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Will Deacon Subject: Re: [PATCH 00/13] [RFC] Rust support Message-ID: References: <20210414184604.23473-1-ojeda@kernel.org> <7287eac3-f492-bab1-9ea8-b89ceceed560@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7287eac3-f492-bab1-9ea8-b89ceceed560@redhat.com> Precedence: bulk List-ID: X-Mailing-List: rust-for-linux@vger.kernel.org On Mon, Apr 19, 2021 at 09:53:06AM +0200, Paolo Bonzini wrote: > On 19/04/21 09:32, Peter Zijlstra wrote: > > On Sat, Apr 17, 2021 at 04:51:58PM +0200, Paolo Bonzini wrote: > > > On 16/04/21 09:09, Peter Zijlstra wrote: > > > > Well, the obvious example would be seqlocks. C11 can't do them > > > > > > Sure it can. C11 requires annotating with (the equivalent of) READ_ONCE all > > > reads of seqlock-protected fields, but the memory model supports seqlocks > > > just fine. > > > > How does that help? > > > > IIRC there's two problems, one on each side the lock. On the write side > > we have: > > > > seq++; > > smp_wmb(); > > X = r; > > Y = r; > > smp_wmb(); > > seq++; > > > > Which C11 simply cannot do right because it does't have wmb. > > It has atomic_thread_fence(memory_order_release), and > atomic_thread_fence(memory_order_acquire) on the read side. https://godbolt.org/z/85xoPxeE5 void writer(void) { atomic_store_explicit(&seq, seq+1, memory_order_relaxed); atomic_thread_fence(memory_order_acquire); X = 1; Y = 2; atomic_store_explicit(&seq, seq+1, memory_order_release); } gives: writer: adrp x1, .LANCHOR0 add x0, x1, :lo12:.LANCHOR0 ldr w2, [x1, #:lo12:.LANCHOR0] add w2, w2, 1 str w2, [x0] dmb ishld ldr w1, [x1, #:lo12:.LANCHOR0] mov w3, 1 mov w2, 2 stp w3, w2, [x0, 4] add w1, w1, w3 stlr w1, [x0] ret Which, afaict, is completely buggered. What it seems to be doing is turning the seq load into a load-acquire, but what we really need is to make sure the seq store (increment) is ordered before the other stores.