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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 C8F78C4CED1 for ; Thu, 3 Oct 2019 13:58:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9FC10215EA for ; Thu, 3 Oct 2019 13:58:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1570111087; bh=l1Q8lHXuoN3TJqBllSFbScMN9RwGTZhbVho5omx4P/0=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=XuD8WR0fTybmWz4Y0GFTQRiYWAh5kCm7J/2CqDlZpz3zWKk9FYAn2Rz4NotDdEpJV K4VRJwRvl+XrdB/Cg9A4+V0/IVbC+99vJBKCcI8nJDPpfl8EuVqseFTPApWnpLzsbK GaPVREeBJCPQJm/j7n4oTVrzW6q9xDKydKEeAPPs= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729950AbfJCN6G (ORCPT ); Thu, 3 Oct 2019 09:58:06 -0400 Received: from mail.kernel.org ([198.145.29.99]:59618 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725907AbfJCN6G (ORCPT ); Thu, 3 Oct 2019 09:58:06 -0400 Received: from paulmck-ThinkPad-P72 (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 7A73020865; Thu, 3 Oct 2019 13:58:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1570111085; bh=l1Q8lHXuoN3TJqBllSFbScMN9RwGTZhbVho5omx4P/0=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=LA26TD21ueBZ3XwCFg+wA8fztaYSGZU2is/9B+0cZxU6cnA16Zl9z4pmd7gqO+izz DN/tmGHUX/6DzwTPV8RIYhIovfAkcfZi2odqadsQ5vFgrFF1J8o2LNyklJ3IRAm3zd gBjEfEE30Gd72478SVO2c+M6l9HuU6cW0TjnjOcE= Date: Thu, 3 Oct 2019 06:58:04 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Steven Rostedt , David Howells , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, Bart Van Assche , Christoph Hellwig , Hannes Reinecke , Johannes Thumshirn , Shane M Seymour , "Martin K . Petersen" Subject: Re: [PATCH tip/core/rcu 1/9] rcu: Upgrade rcu_swap_protected() to rcu_replace() Message-ID: <20191003135804.GU2689@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20191003014310.13262-1-paulmck@kernel.org> <20191003014153.GA13156@paulmck-ThinkPad-P72> <25408.1570091957@warthog.procyon.org.uk> <20191003090850.1e2561b3@gandalf.local.home> <20191003133315.GN2689@paulmck-ThinkPad-P72> <20191003134131.GS4536@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191003134131.GS4536@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 03, 2019 at 03:41:31PM +0200, Peter Zijlstra wrote: > On Thu, Oct 03, 2019 at 06:33:15AM -0700, Paul E. McKenney wrote: > > On Thu, Oct 03, 2019 at 09:08:50AM -0400, Steven Rostedt wrote: > > > On Thu, 03 Oct 2019 09:39:17 +0100 > > > David Howells wrote: > > > > > > > paulmck@kernel.org wrote: > > > > > > > > > +#define rcu_replace(rcu_ptr, ptr, c) \ > > > > > +({ \ > > > > > + typeof(ptr) __tmp = rcu_dereference_protected((rcu_ptr), (c)); \ > > > > > + rcu_assign_pointer((rcu_ptr), (ptr)); \ > > > > > + __tmp; \ > > > > > +}) > > > > > > > > Does it make sense to actually use xchg() if that's supported by the arch? > > > > Historically, xchg() has been quite a bit slower than a pair of assignment > > statements, in part due to the strong memory ordering guaranteed by > > xchg(). Has that changed? If so, then, agreed, it might make sense to > > use xchg(). > > Nope, still the case. xchg() is an atomic op with full ordering. OK, let's stick with the pair of assignments, then. ;-) Thanx, Paul