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=-11.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 67E4DC433E9 for ; Tue, 16 Mar 2021 14:48:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3958D65088 for ; Tue, 16 Mar 2021 14:48:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237363AbhCPOsN (ORCPT ); Tue, 16 Mar 2021 10:48:13 -0400 Received: from mail.kernel.org ([198.145.29.99]:52268 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236975AbhCPOrZ (ORCPT ); Tue, 16 Mar 2021 10:47:25 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 5048565081; Tue, 16 Mar 2021 14:47:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1615906044; bh=g9UVcTvuFGkk276bH2SawBy4olk3XdkrzRG3cge2yJE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=tKBUHkIT3nBE1LTPQtI8b3CafNZjYO4AvCQX0gSYi+71+07ccpql45MoT9NIOHHHN W6Qi+PImb/OHsd13S6878V1LO20wAh1tkh1rar/pZ/LGuQicq4iSu2MhjIzNXcnYKw +IIUXrXTsHvwAXaQaZJZtoS9WNLUHtkXh2Mjp8u1pqq8Fs6ykWhzNSCaWu7bTMBRTR BFF8ULiIj8Dr4vNY1oI+GBhG7OILAfmZBGsi+/FEq9uFYumD7pWyVjg2godFjZ1yey 264t8bBsg2DN5elaBNkzNbsck7LTPEMc+123cVX+0gbnxf0olBuIxvE0ofYH5kzGSw X1dqwcD0MqTaA== Date: Tue, 16 Mar 2021 15:47:22 +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: <20210316144722.GD639918@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. */ > + return true; > + } > + return false; > +} > +EXPORT_SYMBOL_GPL(poll_state_synchronize_rcu); > + > +/** > * cond_synchronize_rcu - Conditionally wait for an RCU grace period > * > * @oldstate: return value from earlier call to get_state_synchronize_rcu() > * > * If a full RCU grace period has elapsed since the earlier call to > - * get_state_synchronize_rcu(), just return. Otherwise, invoke > - * synchronize_rcu() to wait for a full grace period. > + * get_state_synchronize_rcu() or start_poll_synchronize_rcu(), just return. > + * 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 > @@ -3804,7 +3864,7 @@ EXPORT_SYMBOL_GPL(get_state_synchronize_rcu); > */ > void cond_synchronize_rcu(unsigned long oldstate) > { > - if (!rcu_seq_done(&rcu_state.gp_seq, oldstate)) > + if (!poll_state_synchronize_rcu(oldstate)) > synchronize_rcu(); > else > smp_mb(); /* Ensure GP ends before subsequent accesses. */ poll_state_synchronize_rcu() already does the full barrier. > -- > 2.9.5 >