From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from simark.ca by simark.ca with LMTP id Gr0gLTPteWBZHgAAWB0awg (envelope-from ) for ; Fri, 16 Apr 2021 16:01:55 -0400 Received: by simark.ca (Postfix, from userid 112) id AB4981F104; Fri, 16 Apr 2021 16:01:55 -0400 (EDT) X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on simark.ca X-Spam-Level: X-Spam-Status: No, score=-1.1 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,MAILING_LIST_MULTI,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.2 Received: from lists.lttng.org (lists.lttng.org [167.114.26.123]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by simark.ca (Postfix) with ESMTPS id C9EB11E01F for ; Fri, 16 Apr 2021 16:01:54 -0400 (EDT) Received: from lists-lttng01.efficios.com (localhost [IPv6:::1]) by lists.lttng.org (Postfix) with ESMTP id 4FMRtp048hz1C1f; Fri, 16 Apr 2021 16:01:54 -0400 (EDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=lists.lttng.org; s=default; t=1618603314; bh=EoIRDe6Dmv0VhOTn6/kRWVUyhBH0qJZ/j901RrnbyTE=; h=Date:To:Cc:References:In-Reply-To:Subject:List-Id: List-Unsubscribe:List-Archive:List-Post:List-Help:List-Subscribe: From:Reply-To:From; b=OJh9ZYLCw4kFUkyDEnHnK8GfZM8mkNmH3qEURCCh9QvBzz8kfechg5eL0bo3rq1Nu x8pxbcJRWAsFGTuPknoYTpgYW4rZbFOkWSYvtoygUEvK7Rxw4RuvMX9sVXvUkwhNe0 F5t1qLNXkXBln8A1a3KGGhqIX7QBS9u23k7Gpt5ha5NAL27wL40csn484+ltx7Taep O2EylcWoBkfCKmKIFE4ASmJdVoifODo03iMJttPGc87sei1dUUl/r/xYjlsf5W+Gpx tySTmZrhkqO9eZYYLxJ5mJRSfHDlqMXxwA2B+t0dYg5eUMGCnzyJ38R0oxos8BcEfm EM5eYJAEtCZvQ== Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by lists.lttng.org (Postfix) with ESMTPS id 4FMRtm63nrz1C1d for ; Fri, 16 Apr 2021 16:01:52 -0400 (EDT) Received: by mail.kernel.org (Postfix) with ESMTPSA id 56B8F613C0; Fri, 16 Apr 2021 20:01:46 +0000 (UTC) Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 0E3645C0253; Fri, 16 Apr 2021 13:01:46 -0700 (PDT) Date: Fri, 16 Apr 2021 13:01:46 -0700 To: Mathieu Desnoyers Cc: Peter Zijlstra , Will Deacon , linux-kernel , lttng-dev , Carlos O'Donell Message-ID: <20210416200146.GK4212@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> <2056094038.84390.1618601453585.JavaMail.zimbra@efficios.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <2056094038.84390.1618601453585.JavaMail.zimbra@efficios.com> Subject: Re: [lttng-dev] liburcu: LTO breaking rcu_dereference on arm64 and possibly other architectures ? X-BeenThere: lttng-dev@lists.lttng.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: LTTng development list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: "Paul E. McKenney via lttng-dev" Reply-To: paulmck@kernel.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: lttng-dev-bounces@lists.lttng.org Sender: "lttng-dev" On Fri, Apr 16, 2021 at 03:30:53PM -0400, Mathieu Desnoyers wrote: > ----- 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 Looks good to me! Thanx, Paul _______________________________________________ lttng-dev mailing list lttng-dev@lists.lttng.org https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev