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 CAA43C433FE for ; Tue, 31 May 2022 12:59:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344429AbiEaM7S (ORCPT ); Tue, 31 May 2022 08:59:18 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33540 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S245085AbiEaM7Q (ORCPT ); Tue, 31 May 2022 08:59:16 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B4969165A2 for ; Tue, 31 May 2022 05:59:14 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=BUGBteA8JfJc0y+36tB67b05qFOjdvVc5xttIZ7wN/s=; b=pgO9wo+FHXzCz6tvx585lEwztt X6GBD59PGZXI0hlG4z61Q+5Ogny9aGHZsbtPHm9jSzubwFESORwhFP4Ydh+LWGt17e5jCCP1BR5t/ FW477VYiIpBYv3Xh3cHlbagKul15CCZpzjjNPVlkPhIUwvMUOPyuehCLfaqWeL3Qpo3PoOrWHivrP XL32olI79zvFkNvcD/e1dsrQoYzIonKuI/Lta3uTgOuM85YxMcDm7A6p3bpJifQBbh45y+Edec+gw /hDZRkiEfjBeOvxbQ6g+qDg/6RgpjghKLJ6QDyXVmzfu/4xhjJT+bVfj5JP/QT/7Cji2flZeZ6DuP 5l97hFfg==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1nw1SC-005O1Z-0G; Tue, 31 May 2022 12:58:32 +0000 Date: Tue, 31 May 2022 13:58:31 +0100 From: Matthew Wilcox To: Miaohe Lin Cc: akpm@linux-foundation.org, naoya.horiguchi@nec.com, peterx@redhat.com, apopple@nvidia.com, ying.huang@intel.com, osalvador@suse.de, mike.kravetz@oracle.com, songmuchun@bytedance.com, hch@lst.de, dhowells@redhat.com, cl@linux.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 1/4] mm: reduce the rcu lock duration Message-ID: References: <20220530113016.16663-1-linmiaohe@huawei.com> <20220530113016.16663-2-linmiaohe@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220530113016.16663-2-linmiaohe@huawei.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 30, 2022 at 07:30:13PM +0800, Miaohe Lin wrote: > Commit 3268c63eded4 ("mm: fix move/migrate_pages() race on task struct") > extends the period of the rcu_read_lock until after the permissions checks > are done to prevent the task pointed to from changing from under us. But > the task_struct refcount is also taken at that time, the reference to task > is guaranteed to be stable. So it's unnecessary to extend the period of > the rcu_read_lock. Release the rcu lock after task refcount is successfully > grabbed to reduce the rcu holding time. But why bother? You know the RCU read lock isn't a "real" lock, right?