From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752543AbeC0NNt (ORCPT ); Tue, 27 Mar 2018 09:13:49 -0400 Received: from mail-wr0-f194.google.com ([209.85.128.194]:35873 "EHLO mail-wr0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752289AbeC0NNs (ORCPT ); Tue, 27 Mar 2018 09:13:48 -0400 X-Google-Smtp-Source: AG47ELso81ZkAem01332GHCV4LrkIfrXf+FJcM4pzVJyD0WdEusBrhK+quo45Y2NZfCEaE+YSE2UZQ== Date: Tue, 27 Mar 2018 15:13:39 +0200 From: Andrea Parri To: Benjamin Herrenschmidt Cc: Paul Mackerras , Michael Ellerman , Peter Zijlstra , Ingo Molnar , linux-kernel@vger.kernel.org, Linus Torvalds Subject: Re: [PATCH for-4.17 2/2] powerpc: Remove smp_mb() from arch_spin_is_locked() Message-ID: <20180327131339.GA4278@andrea> References: <1522060667-7034-1-git-send-email-andrea.parri@amarulasolutions.com> <1522109216.7364.30.camel@kernel.crashing.org> <20180327102521.GA7347@andrea> <1522150386.7364.53.camel@kernel.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1522150386.7364.53.camel@kernel.crashing.org> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 27, 2018 at 10:33:06PM +1100, Benjamin Herrenschmidt wrote: > On Tue, 2018-03-27 at 12:25 +0200, Andrea Parri wrote: > > > I would rather wait until it is properly documented. Debugging that IPC > > > problem took a *LOT* of time and energy, I wouldn't want these issues > > > to come and bite us again. > > > > I understand. And I'm grateful for this debugging as well as for the (IMO) > > excellent account of it you provided in 51d7d5205d338. > > > > Said this ;) I cannot except myself from saying that I would probably have > > resisted that solution (adding an smp_mb() in my arch_spin_is_locked), and > > instead "blamed"/suggested that caller to fix his memory ordering... > > This is debatable. I tend go for the conservative approach assuming > most people using fairly high level APIs such as spin_is_locked() are > not fully cognisant of the subtleties of our memory model. > > After all, it works on x86 ... > > The fact that the load can leak into the internals of spin_lock() > implementation and re-order with the lock word load itself is quite > hard to fathom for somebody who doesn't have years of experience > dealing with that sort of issue. > > So people will get it wrong. > > So unless it's very performance sensitive, I'd rather have things like > spin_is_locked be conservative by default and provide simpler ordering > semantics. Well, it might not be "very performance sensitive" but allow me to say that "40+ SYNCs in stuff like BUG_ON or such" is sadness to my eyes ;), especially when considered that our "high level API" provides means to avoid this situation (e.g., smp_mb__after_spinlock(); BTW, if you look at architectures for which this macro is "non-trivial", you can get an idea of the architectures which "wouldn't work"; of course, x86 is not among these). Yes, we do appear to have different views on what is to be considered the "simpler ordering semantics". I'm willing to change mine _as soon as_ this gets documented: would you be willing to send a patch (on the lines of my [1]) to describe/document such semantics? Andrea > > Cheers, > Ben. >