From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751337AbeEEJYo (ORCPT ); Sat, 5 May 2018 05:24:44 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:39930 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751057AbeEEJYn (ORCPT ); Sat, 5 May 2018 05:24:43 -0400 Date: Sat, 5 May 2018 11:24:38 +0200 From: Peter Zijlstra To: Ingo Molnar Cc: Mark Rutland , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, aryabinin@virtuozzo.com, boqun.feng@gmail.com, catalin.marinas@arm.com, dvyukov@google.com, will.deacon@arm.com Subject: Re: [PATCH] locking/atomics: Clean up the atomic.h maze of #defines Message-ID: <20180505092438.GB12217@hirez.programming.kicks-ass.net> References: <20180504173937.25300-1-mark.rutland@arm.com> <20180504173937.25300-2-mark.rutland@arm.com> <20180504180105.GS12217@hirez.programming.kicks-ass.net> <20180504180909.dnhfflibjwywnm4l@lakrids.cambridge.arm.com> <20180505081100.nsyrqrpzq2vd27bk@gmail.com> <20180505084721.GA32344@noisy.programming.kicks-ass.net> <20180505090403.p2ywuen42rnlwizq@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180505090403.p2ywuen42rnlwizq@gmail.com> User-Agent: Mutt/1.9.5 (2018-04-13) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, May 05, 2018 at 11:04:03AM +0200, Ingo Molnar wrote: > > * Peter Zijlstra wrote: > > > > So we could do the following simplification on top of that: > > > > > > #ifndef atomic_fetch_dec_relaxed > > > # ifndef atomic_fetch_dec > > > # define atomic_fetch_dec(v) atomic_fetch_sub(1, (v)) > > > # define atomic_fetch_dec_relaxed(v) atomic_fetch_sub_relaxed(1, (v)) > > > # define atomic_fetch_dec_acquire(v) atomic_fetch_sub_acquire(1, (v)) > > > # define atomic_fetch_dec_release(v) atomic_fetch_sub_release(1, (v)) > > > # else > > > # define atomic_fetch_dec_relaxed atomic_fetch_dec > > > # define atomic_fetch_dec_acquire atomic_fetch_dec > > > # define atomic_fetch_dec_release atomic_fetch_dec > > > # endif > > > #else > > > # ifndef atomic_fetch_dec > > > # define atomic_fetch_dec(...) __atomic_op_fence(atomic_fetch_dec, __VA_ARGS__) > > > # define atomic_fetch_dec_acquire(...) __atomic_op_acquire(atomic_fetch_dec, __VA_ARGS__) > > > # define atomic_fetch_dec_release(...) __atomic_op_release(atomic_fetch_dec, __VA_ARGS__) > > > # endif > > > #endif > > > > This would disallow an architecture to override just fetch_dec_release for > > instance. > > Couldn't such a crazy arch just define _all_ the 3 APIs in this group? > That's really a small price and makes the place pay the complexity > price that does the weirdness... I would expect the pattern where it can do all 'release' and/or all 'acquire' variants special but cannot use the __atomic_op_*() wrappery. > > I don't think there currently is any architecture that does that, but the > > intent was to allow it to override anything and only provide defaults where it > > does not. > > I'd argue that if a new arch only defines one of these APIs that's probably a bug. > If they absolutely want to do it, they still can - by defining all 3 APIs. > > So there's no loss in arch flexibility. Ideally we'd generate the whole mess.. and then allowing these extra few overrides is not a problem at all. > > None of this takes away the giant trainwreck that is the annotated atomic stuff > > though. > > > > And I seriously hate this one: > > > > ba1c9f83f633 ("locking/atomic/x86: Un-macro-ify atomic ops implementation") > > > > and will likely undo that the moment I need to change anything there. > > If it makes the code more readable then I don't object - the problem was that the > instrumentation indirection made all that code much harder to follow. Thing is, it is all the exact same loop, and bitrot mandates they drift over time. When I cleaned up all the architectures I found plenty cases where there were spurious differences between things.