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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 C23E7C4727E for ; Mon, 21 Sep 2020 22:11:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7C930239EE for ; Mon, 21 Sep 2020 22:11:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728445AbgIUWLN (ORCPT ); Mon, 21 Sep 2020 18:11:13 -0400 Received: from helcar.hmeau.com ([216.24.177.18]:40618 "EHLO fornost.hmeau.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726457AbgIUWLM (ORCPT ); Mon, 21 Sep 2020 18:11:12 -0400 Received: from gwarestrin.arnor.me.apana.org.au ([192.168.0.7]) by fornost.hmeau.com with smtp (Exim 4.92 #5 (Debian)) id 1kKU1Y-00067N-Dc; Tue, 22 Sep 2020 08:11:05 +1000 Received: by gwarestrin.arnor.me.apana.org.au (sSMTP sendmail emulation); Tue, 22 Sep 2020 08:11:04 +1000 Date: Tue, 22 Sep 2020 08:11:04 +1000 From: Herbert Xu To: "Paul E. McKenney" Cc: Eric Biggers , tytso@mit.edu, linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, stable@vger.kernel.org, Linus Torvalds Subject: Re: [PATCH] random: use correct memory barriers for crng_node_pool Message-ID: <20200921221104.GA6556@gondor.apana.org.au> References: <20200916233042.51634-1-ebiggers@kernel.org> <20200917072644.GA5311@gondor.apana.org.au> <20200917165802.GC855@sol.localdomain> <20200921081939.GA4193@gondor.apana.org.au> <20200921152714.GC29330@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200921152714.GC29330@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 21, 2020 at 08:27:14AM -0700, Paul E. McKenney wrote: > On Mon, Sep 21, 2020 at 06:19:39PM +1000, Herbert Xu wrote: > > On Thu, Sep 17, 2020 at 09:58:02AM -0700, Eric Biggers wrote: > > > > > > smp_load_acquire() is obviously correct, whereas READ_ONCE() is an optimization > > > that is difficult to tell whether it's correct or not. For trivial data > > > structures it's "easy" to tell. But whenever there is a->b where b is an > > > internal implementation detail of another kernel subsystem, the use of which > > > could involve accesses to global or static data (for example, spin_lock() > > > accessing lockdep stuff), a control dependency can slip in. > > > > If we're going to follow this line of reasoning, surely you should > > be converting the RCU derference first and foremost, no? ... > And to Eric's point, it is also true that when you have pointers to > static data, and when the compiler can guess this, you do need something > like smp_load_acquire(). But this is a problem only when you are (1) > using feedback-driven compiler optimization or (2) when you compare the > pointer to the address of the static data. Let me restate what I think Eric is saying. He is concerned about the case where a->b and b is some opaque object that may in turn dereference a global data structure unconnected to a. The case in question here is crng_node_pool in drivers/char/random.c which in turn contains a spin lock. But this reasoning could apply to any data structure that contains a spin lock, in particular ones that are dereferenced through RCU. So my question if this reasoning is valid, then why aren't we first converting rcu_dereference to use smp_load_acquire? Cheers, -- Email: Herbert Xu Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt