From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752306AbeC1JRt (ORCPT ); Wed, 28 Mar 2018 05:17:49 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:35834 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751070AbeC1JRr (ORCPT ); Wed, 28 Mar 2018 05:17:47 -0400 X-Google-Smtp-Source: AIpwx4/t7Oamea6Fh+tI89SAQ933rLKzjRH9PiEq714mf1LSibC2lhricPx1CPhqkpsDLwM0XyYIXg== Date: Wed, 28 Mar 2018 11:17:36 +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: <20180328091736.GA30906@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> <20180327131339.GA4278@andrea> <1522187495.7364.70.camel@kernel.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1522187495.7364.70.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 Wed, Mar 28, 2018 at 08:51:35AM +1100, Benjamin Herrenschmidt wrote: > On Tue, 2018-03-27 at 15:13 +0200, Andrea Parri wrote: > > > > > > 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 ;), > > In the fast path or the trap case ? Because the latter doesn't matter > at all... Both: you must execute the sync "before" issuing the load of lock.slock. > > > 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? > > Not really :-) Just expressing an opinion. I don't fully object to your > approach, just saying it's open for debate. Always (open for debate). I'm sending a v2 of this series shortly, integrating some feedback; I prefer to keep this patch in the series (feel free to ignore). Andrea > > At this point, I have too many other things to chase to follow up too > much on this. > > Cheers, > Ben. >