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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,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 1812AC43603 for ; Sun, 8 Dec 2019 00:08:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DCF80214D8 for ; Sun, 8 Dec 2019 00:08:46 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="FRa+gNKN" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726595AbfLHAIp (ORCPT ); Sat, 7 Dec 2019 19:08:45 -0500 Received: from mail-pj1-f66.google.com ([209.85.216.66]:35613 "EHLO mail-pj1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726418AbfLHAIp (ORCPT ); Sat, 7 Dec 2019 19:08:45 -0500 Received: by mail-pj1-f66.google.com with SMTP id w23so4298871pjd.2 for ; Sat, 07 Dec 2019 16:08:45 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=dXZlRb8cq1PmzSVNnBv1JsUvKa+HUrbvgeQMELIpBDY=; b=FRa+gNKNyGY+isoLzWTjdOUoV7Coi1XYDUCljTDMSvLtrd91MQWHH0kKBB/jA9GIab mCxNQdgbE72U4Pal6H6QutInxUi3w4ZziVwWc4lNTtg7d1JDFsFuV/ciNr0b9/xs6zQE n7KBAL8lxmzEGIDpbxIK2Iqpa7bNPi+7mzVzc= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=dXZlRb8cq1PmzSVNnBv1JsUvKa+HUrbvgeQMELIpBDY=; b=EFJdWIGDCTEerpIReQDb8d26quMG15OmY1NS2Ifck4kURqc5DP4wWgnX7W7xy+nS/q om+MCZVAoSE4yVyRSzSA8d0pU/h+BW+epTY3dK8Wfrcbrl7uywoLabGfVSO/or2FQp9M IsiKU5usaOnGOGzTRbtZTHFDEVhH6l3EFDT4K4txhuxvwDGCSr+qS9px1BnnJiZNyFtA 00e2ROkSDmLa2nyUCcIaRF4pybHz31/WdHwpHRBn8ghL5WZgto01Ae6mZ8W0OxLgmY3u qpB8WGodU5QWAbm7W9fS4SuC/lFzsucdlf0tkzwoTq/rwWo7z6PPEdRHXJBt8mT2KQ/6 CSsw== X-Gm-Message-State: APjAAAVjYgGXXTY/3QqpK5iLL+fAjyI88IIkFk4gPubOQDovxByZigRb GaRSFcAg7sWgu1g3x7K0ZVbN5Q== X-Google-Smtp-Source: APXvYqxt6pXSjNAdOsypElD3oLaCojPyxuXzDU1l+0WiqQFa4oNYi73z1sh8qTAJoWfpZEnX8gSoPA== X-Received: by 2002:a17:902:9a92:: with SMTP id w18mr22192805plp.91.1575763724541; Sat, 07 Dec 2019 16:08:44 -0800 (PST) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id z130sm20445219pgz.6.2019.12.07.16.08.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 07 Dec 2019 16:08:43 -0800 (PST) Date: Sat, 7 Dec 2019 19:08:42 -0500 From: Joel Fernandes To: "Paul E. McKenney" 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: <20191208000842.GA62607@google.com> 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> <20191206031151.GY2889@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191206031151.GY2889@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) 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 07:11:51PM -0800, Paul E. McKenney wrote: > 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? Hi Paul, I was referring to the same API we have at the moment (that is hlist_for_each_entry_rcu() with the additional cond parameter). I was saying let us keep that and not add a hlist_for_each_entry_protected() instead, so as to not proliferate the number of APIs. Or did I miss the point? thanks, - Joel