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 358F8C43603 for ; Fri, 6 Dec 2019 01:11:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ED64C2077B for ; Fri, 6 Dec 2019 01:11:40 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="Jls1cMO1" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726134AbfLFBLk (ORCPT ); Thu, 5 Dec 2019 20:11:40 -0500 Received: from mail-pl1-f194.google.com ([209.85.214.194]:44485 "EHLO mail-pl1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725959AbfLFBLj (ORCPT ); Thu, 5 Dec 2019 20:11:39 -0500 Received: by mail-pl1-f194.google.com with SMTP id bh2so849315plb.11 for ; Thu, 05 Dec 2019 17:11:39 -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=afFgdaPpYNHvy5QgCmSyg24Hm+ltvoWQsuROrBrCOAA=; b=Jls1cMO18zwlNXp1RcxuRzsLNqRvDkKqZjbzo6ZngtVlM/X+QlWs13zMLYRwqXBRQo kBChDfpJSztwtKn52szanSvW1WWT2SCtmedbAOjZ1g6DwlufiYLEbc4xAjaotwjB4/NN N9zV3b+9CBxZXd52VTfe3/pWbW7+CyQeIN5q0= 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=afFgdaPpYNHvy5QgCmSyg24Hm+ltvoWQsuROrBrCOAA=; b=GHpSY1wEv7jTrXktwP7AttXWKe2mJ/JDzgvRxwpxvTe4H8ogHCvhiOIsd7KKW2a/4E 8bY/kAlQULoP11AV0LvrAngV1CqkeLYvM6n7eX9X0z9AvCRyxCexniB+TE9KhfVpm3hs sg2lUynwV5uRE3xvVeW8lDdbS9aerNMmBDLr+mCt8GLNqSPQQ27esqTysWJnBt5ckuUc UgwZet3K1ZsloHf2zu6jKe0e459ROqT9fUUacgDyiK0xhwAo51aXBlD3RhjD+2R+dNN1 xw2C1YUMdvnH+xkAx1ys+8TGP0stxpu3OYYHL79AASlY6/TsDQP/dr6axmrJwOWnhjTl eD/A== X-Gm-Message-State: APjAAAWFgbAPMIQidgUd/Nq8wqfVSa3hHbxlcofwNLUH6Cl+5glEbLua lUyP6/DSD4QuIS2YDiaB+W9P4g== X-Google-Smtp-Source: APXvYqwnyB3hgGzxp6a/jdh6wC9AyEDlREjAgJs5KYIUY5ylBbAWt0nWAe28Thxa5y10mEBEjTnvXg== X-Received: by 2002:a17:90a:c01:: with SMTP id 1mr12729171pjs.37.1575594698986; Thu, 05 Dec 2019 17:11:38 -0800 (PST) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id b2sm14031169pff.6.2019.12.05.17.11.37 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Dec 2019 17:11:38 -0800 (PST) Date: Thu, 5 Dec 2019 20:11:37 -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: <20191206011137.GB142442@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191204161239.GL2889@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 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. thanks, - Joel