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 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 5F236C433B4 for ; Fri, 16 Apr 2021 19:30:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2435A60FF1 for ; Fri, 16 Apr 2021 19:30:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237566AbhDPTbU (ORCPT ); Fri, 16 Apr 2021 15:31:20 -0400 Received: from mail.efficios.com ([167.114.26.124]:35420 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231510AbhDPTbT (ORCPT ); Fri, 16 Apr 2021 15:31:19 -0400 Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id 4A4DB33A762; Fri, 16 Apr 2021 15:30:54 -0400 (EDT) Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id gcgjM8I8umO1; Fri, 16 Apr 2021 15:30:53 -0400 (EDT) Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id B7B4133A951; Fri, 16 Apr 2021 15:30:53 -0400 (EDT) DKIM-Filter: OpenDKIM Filter v2.10.3 mail.efficios.com B7B4133A951 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=default; t=1618601453; bh=p7XB97T5n4R7z4Fj+GUXbiw/P8xhfjfYKjRpfCWR4UA=; h=Date:From:To:Message-ID:MIME-Version; b=EJ4YF+QbW/Xw5UawLEnSZX+EW3DA9+PCNDauA383bIf6VDZ770rorpJBoGZ0/LnwZ RQ1IxraYTRm2xSuJ6v8lRUTp49vOHSPmH8TiHK6FKNCmPPaDHXv/1Imj+PdUN29rAX W5LbNA+eVC9X75jMBwnDciB14zG6i+BhSW7pYG1Krb09SonTFoaE12DYi4Vb4R46L7 E+QTW7l1nnX6r3xxgQYNOI97FYCZ3JCC46klNCnSccK7kqQNYIiUIdeTVxxfywmNS9 I9gRtyZiL8pZVsC37SVmvzlTexiBvOA2XhoAYWylUn8UjG7QReyoWQnv+OrWL+9Rah 20Xab0zeLIm+A== X-Virus-Scanned: amavisd-new at efficios.com Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id 79Qiehg1WzRj; Fri, 16 Apr 2021 15:30:53 -0400 (EDT) Received: from mail03.efficios.com (mail03.efficios.com [167.114.26.124]) by mail.efficios.com (Postfix) with ESMTP id AA81133A761; Fri, 16 Apr 2021 15:30:53 -0400 (EDT) Date: Fri, 16 Apr 2021 15:30:53 -0400 (EDT) From: Mathieu Desnoyers To: paulmck Cc: Peter Zijlstra , Will Deacon , linux-kernel , lttng-dev , Carlos O'Donell Message-ID: <2056094038.84390.1618601453585.JavaMail.zimbra@efficios.com> In-Reply-To: <20210416190244.GJ4212@paulmck-ThinkPad-P17-Gen-1> References: <1680415903.81652.1618584736742.JavaMail.zimbra@efficios.com> <20210416160139.GF4212@paulmck-ThinkPad-P17-Gen-1> <2089952450.84139.1618598408015.JavaMail.zimbra@efficios.com> <20210416190244.GJ4212@paulmck-ThinkPad-P17-Gen-1> Subject: Re: liburcu: LTO breaking rcu_dereference on arm64 and possibly other architectures ? MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.26.124] X-Mailer: Zimbra 8.8.15_GA_3996 (ZimbraWebClient - FF87 (Linux)/8.8.15_GA_4007) Thread-Topic: liburcu: LTO breaking rcu_dereference on arm64 and possibly other architectures ? Thread-Index: U+jByog9nw+xnur8wetlu9vzLfobNQ== Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ----- On Apr 16, 2021, at 3:02 PM, paulmck paulmck@kernel.org wrote: [...] > > If it can be done reasonably, I suggest also having some way for the > person building userspace RCU to say "I know what I am doing, so do > it with volatile rather than memory_order_consume." Like so ? #define CMM_ACCESS_ONCE(x) (*(__volatile__ __typeof__(x) *)&(x)) #define CMM_LOAD_SHARED(p) CMM_ACCESS_ONCE(p) /* * By defining URCU_DEREFERENCE_USE_VOLATILE, the user requires use of * volatile access to implement rcu_dereference rather than * memory_order_consume load from the C11/C++11 standards. * * This may improve performance on weakly-ordered architectures where * the compiler implements memory_order_consume as a * memory_order_acquire, which is stricter than required by the * standard. * * Note that using volatile accesses for rcu_dereference may cause * LTO to generate incorrectly ordered code starting from C11/C++11. */ #ifdef URCU_DEREFERENCE_USE_VOLATILE # define rcu_dereference(x) CMM_LOAD_SHARED(x) #else # if defined (__cplusplus) # if __cplusplus >= 201103L # include # define rcu_dereference(x) ((std::atomic<__typeof__(x)>)(x)).load(std::memory_order_consume) # else # define rcu_dereference(x) CMM_LOAD_SHARED(x) # endif # else # if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L) # include # define rcu_dereference(x) atomic_load_explicit(&(x), memory_order_consume) # else # define rcu_dereference(x) CMM_LOAD_SHARED(x) # endif # endif #endif Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com