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 87AC2C43460 for ; Mon, 19 Apr 2021 08:35:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6845A61029 for ; Mon, 19 Apr 2021 08:35:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238261AbhDSIgK (ORCPT ); Mon, 19 Apr 2021 04:36:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52212 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238262AbhDSIf4 (ORCPT ); Mon, 19 Apr 2021 04:35:56 -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 DE328C06174A; Mon, 19 Apr 2021 01:35:26 -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=MzstDp0klphgOp7a2sZHFPQlmjiV2TLi0ke2ptY9o0k=; b=aC9S2N9D2XUKcx2oukFZkmx0Ws lK/9B4aUWUI64d7CzgHYvB0yRX7gHRrEoLHHOn6e65yVn4zoVytrteYeC+VGzGI/0yGxGRrXxfoTg X2qCNDlHXGMseBgBLSVPWvet3Dh37edBEQitwC/oqx5HY3gicvhS+rseDsNNIGfaPk0VEWhtGvqge HTKvkpDbsF6bbzOW5aryOFMCkvK5qtBSzcXN34IZmn6/XYrjf2rcjZMHnwvsLMusMdnswnY2mxbIg n+M3NF+hg08LeUnFpJQuMSK8bpIb5qPM232YJ5v4boP+vqK5aSWzEyLO0OJs89m/zAgIznzizvIW9 rrreqJaw==; 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 1lYPNJ-009RUB-Vt; Mon, 19 Apr 2021 08:35:22 +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) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 3AC7630020C; Mon, 19 Apr 2021 10:35:21 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 28F972C14C5BA; Mon, 19 Apr 2021 10:35:21 +0200 (CEST) Date: Mon, 19 Apr 2021 10:35:21 +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: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Apr 19, 2021 at 10:26:57AM +0200, Peter Zijlstra wrote: > https://godbolt.org/z/85xoPxeE5 That wants _Atomic on the seq definition for clang. > 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. Put differently, what you seem to want is store-acquire, but there ain't no such thing.