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=-6.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 0AE78C433E9 for ; Tue, 16 Mar 2021 15:18:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E8F67650DE for ; Tue, 16 Mar 2021 15:18:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234812AbhCPPSb (ORCPT ); Tue, 16 Mar 2021 11:18:31 -0400 Received: from mail.kernel.org ([198.145.29.99]:36462 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235966AbhCPPRx (ORCPT ); Tue, 16 Mar 2021 11:17:53 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 798F065095; Tue, 16 Mar 2021 15:17:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1615907873; bh=IqNjw5OS5jsCFW8fnWpfOQWhT1YkR4zpIX9U7YlL3zM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=haNLULuBUIKgpX0ls5CtJk7LNzFkCawRlb9zGxfW06zeRm5dhEB4jSGD+ahZhkj8o l+R06v6Nj0mWZfyQs+BMi4zuAGOaZjmXeRFeJOtEY1Xxd59kU/OGek1CD8NC1dTZXC nMJ5Rv71X1dBncmQ/GFulJ/yrC99hpphCExZLB4J97MVhMgsjzm0uHrIkj+hPRtSJ1 WVpktjCvks2O1VBaFLQX0XifIL4qDL1gkYBr/GgEnL9AVE9acj97ie71OLlHgCHM4e RdbgZewbErb8cyS6C5pn3DNTf1EVG2glq2r52PRG2OzRFYrXdgp+TDdG3ku2Iou56L 70DIuVobeEHPg== Date: Tue, 16 Mar 2021 16:17:50 +0100 From: Frederic Weisbecker To: paulmck@kernel.org Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH tip/core/rcu 1/3] rcu: Provide polling interfaces for Tree RCU grace periods Message-ID: <20210316151750.GF639918@lothringen> References: <20210304002605.GA23785@paulmck-ThinkPad-P72> <20210304002632.23870-1-paulmck@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210304002632.23870-1-paulmck@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 03, 2021 at 04:26:30PM -0800, paulmck@kernel.org wrote: > +/** > + * poll_state_synchronize_rcu - Conditionally wait for an RCU grace period > + * > + * @oldstate: return from call to get_state_synchronize_rcu() or start_poll_synchronize_rcu() > + * > + * If a full RCU grace period has elapsed since the earlier call from > + * which oldstate was obtained, return @true, otherwise return @false. > + * Otherwise, invoke synchronize_rcu() to wait for a full grace period. > + * > + * Yes, this function does not take counter wrap into account. > + * But counter wrap is harmless. If the counter wraps, we have waited for > + * more than 2 billion grace periods (and way more on a 64-bit system!). > + * Those needing to keep oldstate values for very long time periods > + * (many hours even on 32-bit systems) should check them occasionally > + * and either refresh them or set a flag indicating that the grace period > + * has completed. > + */ > +bool poll_state_synchronize_rcu(unsigned long oldstate) > +{ > + if (rcu_seq_done(&rcu_state.gp_seq, oldstate)) { > + smp_mb(); /* Ensure GP ends before subsequent accesses. */ Also as usual I'm a bit lost with the reason behind those memory barriers. So this is ordering the read on rcu_state.gp_seq against something (why not an smp_rmb() btw?). And what does it pair with? Thanks.