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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 166D2C43603 for ; Fri, 6 Dec 2019 03:11:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B4A3122525 for ; Fri, 6 Dec 2019 03:11:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1575601914; bh=ygisAbywNNJp+p3FW0CP0VYzP7nARfH6UtkN6FtRDHg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=PpsyHNNkXYZ6MfzOjrfcOJ1nRXYxSDzzoklFPtz5dF5Z3xxRCTJwSUuc6r7lpBqXy 1rIbzNlu072wYQ5+5wkFzITed9/EeNrbGiHNMxrmmn+8FPQ1OQwtchlYbt1PRA2Ab/ Dt5IkN+Yq1QaJYijOCukScP7jD3GEbdRq02PqReI= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726407AbfLFDLx (ORCPT ); Thu, 5 Dec 2019 22:11:53 -0500 Received: from mail.kernel.org ([198.145.29.99]:32996 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726160AbfLFDLx (ORCPT ); Thu, 5 Dec 2019 22:11:53 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id D214E22525; Fri, 6 Dec 2019 03:11:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1575601911; bh=ygisAbywNNJp+p3FW0CP0VYzP7nARfH6UtkN6FtRDHg=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=mZkV+FO1I33/YSDut2aEfBwRscQ9rLY/yoZ+dCXwUFdZ43/ofZohFbOzUBFlIwfQq jAiSa3MDBQgZijLH0gfg+nTIbQ18wjBEfKS7yilNHKkP5/sj4c8dNKjW9EoK6JMnn1 diLcoxHU2yGnmt0jLJiURRcazar2Q7SX1yNhGlhE= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id A18573522782; Thu, 5 Dec 2019 19:11:51 -0800 (PST) Date: Thu, 5 Dec 2019 19:11:51 -0800 From: "Paul E. McKenney" To: Joel Fernandes Cc: Ingo Molnar , Peter Zijlstra , Masami Hiramatsu , Anders Roxell , "Naveen N . Rao" , Anil S Keshavamurthy , David Miller , Linux Kernel Mailing List Subject: Re: [PATCH -tip] kprobes: Lock rcu_read_lock() while searching kprobe Message-ID: <20191206031151.GY2889@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <157527193358.11113.14859628506665612104.stgit@devnote2> <20191202210854.GD17234@google.com> <20191203071329.GC115767@gmail.com> <20191203175712.GI2889@paulmck-ThinkPad-P72> <20191204100549.GB114697@gmail.com> <20191204161239.GL2889@paulmck-ThinkPad-P72> <20191206011137.GB142442@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191206011137.GB142442@google.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 05, 2019 at 08:11:37PM -0500, Joel Fernandes wrote: > On Wed, Dec 04, 2019 at 08:12:39AM -0800, Paul E. McKenney wrote: > > On Wed, Dec 04, 2019 at 11:05:50AM +0100, Ingo Molnar wrote: > > > > > > * Paul E. McKenney wrote: > > > > > > > > * This list-traversal primitive may safely run concurrently with > > > > > * the _rcu list-mutation primitives such as hlist_add_head_rcu() > > > > > * as long as the traversal is guarded by rcu_read_lock(). > > > > > */ > > > > > #define hlist_for_each_entry_rcu(pos, head, member, cond...) \ > > > > > > > > > > is actively harmful. Why is it there? > > > > > > > > For cases where common code might be invoked both from the reader > > > > (with RCU protection) and from the updater (protected by some > > > > lock). This common code can then use the optional argument to > > > > hlist_for_each_entry_rcu() to truthfully tell lockdep that it might be > > > > called with either form of protection in place. > > > > > > > > This also combines with the __rcu tag used to mark RCU-protected > > > > pointers, in which case sparse complains when a non-RCU API is applied > > > > to these pointers, to get back to your earlier question about use of > > > > hlist_for_each_entry_rcu() within the update-side lock. > > > > > > > > But what are you seeing as actively harmful about all of this? > > > > What should we be doing instead? > > > > > > Yeah, so basically in the write-locked path hlist_for_each_entry() > > > generates (slightly) more efficient code than hlist_for_each_entry_rcu(), > > > correct? > > > > Potentially yes, if the READ_ONCE() constrains the compiler. Or not, > > depending of course on the compiler and the surrounding code. > > > > > Also, the principle of passing warning flags around is problematic - but > > > I can see the point in this specific case. > > > > Would it help to add an hlist_for_each_entry_protected() that expected > > RCU-protected pointers and write-side protection, analogous to > > rcu_dereference_protected()? Or would that expansion of the RCU API > > outweigh any benefits? > > Personally, I like keeping the same API and using the optional argument like > we did thus preventing too many APIs / new APIs. Would you be willing to put together a prototype patch so that people can see exactly how it would look? Thanx, Paul