From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-3.9 required=3.0 tests=AWL,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI shortcircuit=no autolearn=ham autolearn_force=no version=3.4.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id DE71D1F453 for ; Tue, 23 Oct 2018 20:28:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726360AbeJXExn (ORCPT ); Wed, 24 Oct 2018 00:53:43 -0400 Received: from cloud.peff.net ([104.130.231.41]:51676 "HELO cloud.peff.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1725948AbeJXExn (ORCPT ); Wed, 24 Oct 2018 00:53:43 -0400 Received: (qmail 7392 invoked by uid 109); 23 Oct 2018 20:28:45 -0000 Received: from Unknown (HELO peff.net) (10.0.1.2) by cloud.peff.net (qpsmtpd/0.94) with SMTP; Tue, 23 Oct 2018 20:28:45 +0000 Authentication-Results: cloud.peff.net; auth=none Received: (qmail 26401 invoked by uid 111); 23 Oct 2018 20:27:58 -0000 Received: from sigill.intra.peff.net (HELO sigill.intra.peff.net) (10.0.0.7) by peff.net (qpsmtpd/0.94) with (ECDHE-RSA-AES256-GCM-SHA384 encrypted) SMTP; Tue, 23 Oct 2018 16:27:58 -0400 Authentication-Results: peff.net; auth=none Received: by sigill.intra.peff.net (sSMTP sendmail emulation); Tue, 23 Oct 2018 16:28:43 -0400 Date: Tue, 23 Oct 2018 16:28:43 -0400 From: Jeff King To: =?utf-8?B?Tmd1eeG7hW4gVGjDoWkgTmfhu41j?= Duy Cc: git@vger.kernel.org Subject: Re: [PATCH/RFC] thread-utils: better wrapper to avoid #ifdef NO_PTHREADS Message-ID: <20181023202842.GA17371@sigill.intra.peff.net> References: <20181018170934.GA21138@sigill.intra.peff.net> <20181018180522.17642-1-pclouds@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20181018180522.17642-1-pclouds@gmail.com> Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Thu, Oct 18, 2018 at 08:05:22PM +0200, Nguyễn Thái Ngọc Duy wrote: > On Thu, Oct 18, 2018 at 7:09 PM Jeff King wrote: > > > In this particular case though I think we should be able to avoid so > > > much #if if we make a wrapper for pthread api that would return an > > > error or something when pthread is not available. But similar > > > situation may happen elsewhere too. > > > > Yeah, I think that is generally the preferred method anyway, just > > because of readability and simplicity. > > I've wanted to do this for a while, so let's test the water and see if > it's well received. > > This patch is a proof of concept that adds just enough macros so that > I can build index-pack.c on a single thread mode with zero #ifdef > related to NO_PTHREADS. > > Besides readability and simplicity, it reduces the chances of breaking > conditional builds (e.g. you rename a variable name but forgot that > the variable is in #if block that is not used by your > compiler/platform). Yes, I love this. We're already halfway there with things like read_lock() in index-pack and elsewhere, which are conditionally no-ops. The resulting code is much easier to read, I think. > Performance-wise I don't think there is any loss for single thread > mode. I rely on compilers recognizing HAVE_THREADS being a constant > and remove dead code or at least optimize in favor of non-dead code. > > Memory-wise, yes we use some more memory in single thread mode. But we > don't have zillions of mutexes or thread id, so a bit extra memory > does not worry me so much. Yeah, I don't think carrying around a handful of ints is going to be a big deal. I also think we may want to make a fundamental shift in our view of thread support. In the early days, it was "well, this is a thing that modern systems can take advantage of for certain commands". But these days I suspect it is more like "there are a handful of legacy systems that do not even support threads". I don't think we should break the build on those legacy systems, but it's probably OK to stop thinking of it as "non-threaded platforms are the default and must pay zero cost" and more as "threaded platforms are the default, and non-threaded ones are OK to pay a small cost as long as they still work". > @@ -74,4 +79,29 @@ int init_recursive_mutex(pthread_mutex_t *m) > pthread_mutexattr_destroy(&a); > } > return ret; > +#else > + return ENOSYS; > +#endif > +} I suspect some of these ENOSYS could just become a silent success. ("yep, I initialized your dummy mutex"). But it probably doesn't matter much either way, as we would not generally even bother checking this return. > +#ifdef NO_PTHREADS > +int dummy_pthread_create(pthread_t *pthread, const void *attr, > + void *(*fn)(void *), void *data) > +{ > + return ENOSYS; > } Whereas for this one, ENOSYS makes a lot of sense (we should avoid the threaded code-path anyway when we see that online_cpus()==1, and this would let us know when we mess that up). > +int dummy_pthread_init(void *data) > +{ > + /* > + * Do nothing. > + * > + * The main purpose of this function is to break compiler's > + * flow analysis or it may realize that functions like > + * pthread_mutex_init() is no-op, which means the (static) > + * variable is not used/initialized at all and trigger > + * -Wunused-variable > + */ > + return ENOSYS; > +} It might be worth marking the dummy variables as MAYBE_UNUSED, exactly to avoid this kind of compiler complaint. -Peff