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 AD2F3C433F5 for ; Tue, 31 May 2022 13:05:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1343553AbiEaNFs (ORCPT ); Tue, 31 May 2022 09:05:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47216 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231451AbiEaNFq (ORCPT ); Tue, 31 May 2022 09:05:46 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9689185EFA for ; Tue, 31 May 2022 06:05:45 -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=2LaeE6TXNYbEbm2V2tXhLHBQx057Dlx91hkU5xAWauU=; b=NafnL0NLUqNnb1avAu6c+9ng/1 5oEC8BhrwdbQXoTtFrOmL2MLVaHyLtwHfmXTkELVG++FlFc9sSPWOS6nFx+laQZUh0TgANJJtw66h JkhC9DcvgMpuUWe1bAwWV4H6VNDmSqGdiZz4ql25ZaTcVuTWHWUwH3MswFN0D67GWhZ2O0p8pECC/ apRZsAt+ksza8domwjXUD0pLNPvJUaGxtPj0IxLHWzK7jg2MMCxI4mTH8HqmIToXEya+OHZ+jCZuE x1do3ZJd/8GgUeTGrGe/bqnjjdA1aHRata6VB1rQWA6OWWwLg2VnmL8yOSzAgb0+umvGrPh/kHu6/ p/UuxSNQ==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1nw1Yl-005OL0-Eb; Tue, 31 May 2022 13:05:19 +0000 Date: Tue, 31 May 2022 14:05:19 +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: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 31, 2022 at 01:58:31PM +0100, Matthew Wilcox wrote: > 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? Looking over this code some more, I think this may harm performance. ptrace_may_access() itself takes the rcu_read_lock(). So we currently have: rcu_read_lock() rcu_read_lock(); rcu_read_unlock(); rcu_read_unlock(); In at least one RCU configuration, rcu_read_lock() maps to preempt_disable(). Nested preempt_disable() just bump a counter, while that counter reaching zero incurs some actual work. So nested rcu_read_lock() can be better than sequential lock/unlock/lock/unlock. This needs far better justification.