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 2E93CC433E0 for ; Fri, 19 Mar 2021 22:11:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EF37E61981 for ; Fri, 19 Mar 2021 22:11:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230512AbhCSWLL (ORCPT ); Fri, 19 Mar 2021 18:11:11 -0400 Received: from mail.kernel.org ([198.145.29.99]:50346 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230285AbhCSWKn (ORCPT ); Fri, 19 Mar 2021 18:10:43 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 7EDC061979; Fri, 19 Mar 2021 22:10:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1616191843; bh=9dG8aL3BrKWbFS+Lqoql6t6iTEwSbP1Iz6LMq7AC1R0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Jeoy602ofP6YvVVzI5eZImUy7ztEa/wuHa7l81NYDYPvXqNi4R3ALCVMu2Iayw38c 3DXh7b1CYR3Hmg4MkRrXwNoxSINNzXNA5PQ8oAV21/q/X8/+wSSv0UK7fxFxZqlmwX nT6yOspGUmC1btzkJ/U78YYYfEXQzVv8pt0yZYWicd/InqwxCjEjT4T2KuZ9GhzADU 4HFqAtPssCaGZYkG6b9gHE05erUw9BR9W1S4fC2jjqH0C3EdRWsBvIhN6InRHPEohF 1P/tBvUH8PPTx7WPnrpXJTkndb8bhs0vWKhGIkvj6V/GK65mw7uXaODXKt0JelUQsL 4DowOYIAJh5sA== Date: Fri, 19 Mar 2021 23:10:40 +0100 From: Frederic Weisbecker To: "Paul E. McKenney" 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: <20210319221040.GC814853@lothringen> References: <20210304002605.GA23785@paulmck-ThinkPad-P72> <20210304002632.23870-1-paulmck@kernel.org> <20210319135854.GA814853@lothringen> <20210319175116.GO2696@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210319175116.GO2696@paulmck-ThinkPad-P72> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 19, 2021 at 10:51:16AM -0700, Paul E. McKenney wrote: > On Fri, Mar 19, 2021 at 02:58:54PM +0100, Frederic Weisbecker wrote: > > It's all a matter of personal taste but if I may suggest some namespace > > modifications: > > > > get_state_synchronize_rcu() -> synchronize_rcu_poll_start_raw() > > start_poll_synchronize_rcu() -> synchronize_rcu_poll_start() > > poll_state_synchronize_rcu() -> synchronize_rcu_poll() > > cond_synchronize_rcu() -> synchronize_rcu_cond() > > > > But it's up to you really. > > I am concerned about starting anything "synchronize_rcu" if that > thing doesn't unconditionally wait for a grace period. "What do > you mean that there was no grace period? Don't you see that call to > synchronize_rcu_poll_start_raw()???" I see, that could indeed be confusing. > > This objection doesn't apply to cond_synchronize_rcu(), but it is > already in use, so any name change should be worked with the users. > All two of them. ;-) Probably not worth it. We have cond_resched() as a schedule() counterpart for a reference after all. > > > /** > > > + * start_poll_state_synchronize_rcu - Snapshot and start RCU grace period > > > + * > > > + * Returns a cookie that is used by a later call to cond_synchronize_rcu() > > > > It may be worth noting that calling start_poll_synchronize_rcu() and then > > pass the cookie to cond_synchronize_rcu() soon after may end up waiting for > > one more grace period. > > You mean this sequence of events? > > 1. cookie = start_poll_synchronize_rcu() > > 2. The grace period corresponding to cookie is almost over... > > 3. cond_synchronize_rcu() checks the cookie and sees that the > grace period has not yet expired. > > 4. The grace period corresponding to cookie completes. > > 5. Someone else starts a grace period. > > 6. cond_synchronize_rcu() invokes synchronize_rcu(), which waits > for the just-started grace period plus another grace period. > Thus, there has been no fewer than three full grace periods > between the call to start_poll_synchronize_rcu() and the > return from cond_synchronize_rcu(). > > Yes, this can happen! And it can be worse, for example, it is quite > possible that cond_synchronize_rcu() would be preempted for multiple > grace periods at step 5, in which case it would still wait for almost > two additional grace periods. > > Or are you thinking of something else? I didn't even think that far. My scenario was: 1. cookie = start_poll_synchronize_rcu() 2. cond_synchronize_rcu() checks the cookie and sees that the grace period has not yet expired. So it calls synchronize_rcu() which queues a callback. 3. The grace period for the cookie eventually completes. 4. The callback queued in 2. gets assigned a new grace period number. That new grace period starts. 5. The new grace period completes and synchronize_rcu() returns. But I think this is due to some deep misunderstanding from my end. > > > + * 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. > > > > Rephrase suggestion for the last sentence: > > > > "In case of failure, it's up to the caller to try polling again later or > > invoke synchronize_rcu() to wait for a new full grace period to complete." > > How about like this? > > /** > * 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. > * If @false is returned, it is the caller's responsibilty to invoke this > * function later on until it does return @true. Alternatively, the caller > * can explicitly wait for a grace period, for example, by passing @oldstate > * to cond_synchronize_rcu() or by directly invoking synchronize_rcu(). Yes very nice! Thanks!