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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9A870C433F5 for ; Tue, 29 Mar 2022 23:23:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240150AbiC2XYm (ORCPT ); Tue, 29 Mar 2022 19:24:42 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47524 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236546AbiC2XYl (ORCPT ); Tue, 29 Mar 2022 19:24:41 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DDED3185452; Tue, 29 Mar 2022 16:22:57 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 76D5E60FA2; Tue, 29 Mar 2022 23:22:57 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id CACDBC2BBE4; Tue, 29 Mar 2022 23:22:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1648596176; bh=fyyI03YUsRLxwjzWMxbmnfhC7VJBY/raoIuviwH/S14=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=kdEr5XTK7Pj+Oiu0mRYwSRc4hLAw215MubZByZjF+HqXi3ZIaPeSGIuOYv2k3NRjA rmjGm4qkjSpcBoURvYA0r60KX7c8pcqUqO+TPenafgVFXp0QOsugCphqAoY+9M+BNt t9F9oIZCjSG/HVuj+SQsC66yj5rpVsS48qsUUSaJQlwB6P7JqDcFqNBu1t1R8Q4oeM WAJBi6FM/a9crU2EuXjd5IvDgJIpJp8+6TxqlQ/TnfH/xO+L8oV5V0u0uDwUtPHjWp V/29RHEX8zXe2VcnR235DF2Me6ftO643lMaFWAHIMO8pVf657qYGIFHNS1RlFEE2jV nKbL4P2NdzxNA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 609815C0A0D; Tue, 29 Mar 2022 16:22:56 -0700 (PDT) Date: Tue, 29 Mar 2022 16:22:56 -0700 From: "Paul E. McKenney" To: David Vernet Cc: frederic@kernel.org, jiangshanlai@gmail.com, joel@joelfernandes.org, josh@joshtriplett.org, linux-kernel@vger.kernel.org, mathieu.desnoyers@efficios.com, quic_neeraju@quicinc.com, rcu@vger.kernel.org, rostedt@goodmis.org Subject: Re: [PATCH] rcu_sync: Fix comment to properly reflect rcu_sync_exit() behavior Message-ID: <20220329232256.GE4285@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220329222612.1001586-1-void@manifault.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220329222612.1001586-1-void@manifault.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 29, 2022 at 03:26:13PM -0700, David Vernet wrote: > rcu_sync_enter() is an rcu_sync API used by updaters which forces RCU > readers (e.g. percpu-rwsem) to take a slow-path during an update. It does > this by setting the gp_state of the rcu_sync object to GP_ENTER (i.e. > 0). > In this state, readers will take a "slow path", such as having percpu-rwsem > readers wait on a semaphore rather than just incrementing a reader count. > When the updater has completed their work, they must invoke rcu_sync_exit() > to signal to readers that they may again take their fastpaths. > > rcu_sync_enter() currently has a comment that stipulates that a later call > to rcu_sync_exit() (by an updater) will re-enable reader "slowpaths". This > patch corrects the comment to properly reflect that rcu_sync_exit() > re-enables reader fastpaths. > > Signed-off-by: David Vernet Good eyes, thank you! As usual, I could not resist the urge to wordsmith. Please take a look at the following to see if I messed something up. Thanx, Paul ------------------------------------------------------------------------ commit b89e06a95c05009bcf31949814c42bc420f414a6 Author: David Vernet Date: Tue Mar 29 15:26:13 2022 -0700 rcu_sync: Fix comment to properly reflect rcu_sync_exit() behavior The rcu_sync_enter() function is used by updaters to force RCU readers (e.g. percpu-rwsem) to use their slow paths during an update. This is accomplished by setting the ->gp_state of the rcu_sync structure to GP_ENTER. In the case of percpu-rwsem, the readers' slow path waits on a semaphore instead of just incrementing a reader count. Each updater invokes the rcu_sync_exit() function to signal to readers that they may again take their fastpaths. The rcu_sync_exit() function sets the ->gp_state of the rcu_sync structure to GP_EXIT, and if all goes well, after a grace period the ->gp_state reverts back to GP_IDLE. Unfortunately, the rcu_sync_enter() function currently has a comment incorrectly stating that rcu_sync_exit() (by an updater) will re-enable reader "slowpaths". This patch changes the comment to state that this function re-enables reader fastpaths. Signed-off-by: David Vernet Signed-off-by: Paul E. McKenney diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c index 33d896d85902..5cefc702158f 100644 --- a/kernel/rcu/sync.c +++ b/kernel/rcu/sync.c @@ -111,7 +111,7 @@ static void rcu_sync_func(struct rcu_head *rhp) * a slowpath during the update. After this function returns, all * subsequent calls to rcu_sync_is_idle() will return false, which * tells readers to stay off their fastpaths. A later call to - * rcu_sync_exit() re-enables reader slowpaths. + * rcu_sync_exit() re-enables reader fastpaths. * * When called in isolation, rcu_sync_enter() must wait for a grace * period, however, closely spaced calls to rcu_sync_enter() can