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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id CC50EEEB57A for ; Fri, 8 Sep 2023 23:58:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345340AbjIHX6m (ORCPT ); Fri, 8 Sep 2023 19:58:42 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47510 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1345345AbjIHX6l (ORCPT ); Fri, 8 Sep 2023 19:58:41 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D0C202109 for ; Fri, 8 Sep 2023 16:58:21 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 70E36C433C8; Fri, 8 Sep 2023 23:58:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1694217501; bh=np8/0/Kh+pd2VdS3BH9sXTOsfAM+iX0qlIONUN0gB4k=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Z2SuCl1FPAC8rVETbvH6csm6GUHmWSF5j5xL0YFxrgl6LZB+hWTiHJhj8PooVox/9 4c+5MyICw141CyYeWjqXAeJDlG53xYdVvTG2WFLnNwQL2pA73hAbSTIqkm8XCMQUZG D4sQ6Ba7foIfsXag9cKej/X0QqpeRGBtaIBtdUBISa/nxZcbQvBZ4qFMOtTJw2l3wV MSDm/QEyUkaI4xpfO7ntqNY8jHybFxn4vfIcDq0u3EsxlLkHP/iW9Wd7M2PLUbEwWU DrkqpsGmSse5nTgFXA+aJNyLbqt3lRf1KW9K4a+XcraBjErlFVbtxWgZW/IruEvDwS XNrG1v1D/eZiQ== Date: Fri, 8 Sep 2023 16:58:20 -0700 From: "Darrick J. Wong" To: Anthony Iliopoulos Cc: linux-xfs@vger.kernel.org Subject: Re: [PATCH] libxfs: fix atomic64_t detection on x86 32-bit architectures Message-ID: <20230908235820.GQ28202@frogsfrogsfrogs> References: <20230905084623.24865-1-ailiop@suse.com> <20230905164250.GV28186@frogsfrogsfrogs> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Wed, Sep 06, 2023 at 05:19:52PM +0200, Anthony Iliopoulos wrote: > On Tue, Sep 05, 2023 at 09:42:50AM -0700, Darrick J. Wong wrote: > > On Tue, Sep 05, 2023 at 10:46:23AM +0200, Anthony Iliopoulos wrote: > > > xfsprogs during compilation tries to detect if liburcu supports atomic > > > 64-bit ops on the platform it is being compiled on, and if not it falls > > > back to using pthread mutex locks. > > > > > > The detection logic for that fallback relies on _uatomic_link_error() > > > which is a link-time trick used by liburcu that will cause compilation > > > errors on archs that lack the required support. That only works for the > > > generic liburcu code though, and it is not implemented for the > > > x86-specific code. > > > > > > In practice this means that when xfsprogs is compiled on 32-bit x86 > > > archs will successfully link to liburcu for atomic ops, but liburcu does > > > not support atomic64_t on those archs. It indicates this during runtime > > > by generating an illegal instruction that aborts execution, and thus > > > causes various xfsprogs utils to be segfaulting. > > > > > > Fix this by executing the liburcu atomic64_t detection code during > > > configure instead of only relying on the linker error, so that > > > compilation will properly fall back to pthread mutexes on those archs. > > > > > > Fixes: 7448af588a2e ("libxfs: fix atomic64_t poorly for 32-bit architectures") > > > > > > Signed-off-by: Anthony Iliopoulos > > > --- > > > m4/package_urcu.m4 | 8 ++++++-- > > > 1 file changed, 6 insertions(+), 2 deletions(-) > > > > > > diff --git a/m4/package_urcu.m4 b/m4/package_urcu.m4 > > > index ef116e0cda76..f26494a69718 100644 > > > --- a/m4/package_urcu.m4 > > > +++ b/m4/package_urcu.m4 > > > @@ -26,11 +26,15 @@ rcu_init(); > > > # > > > # Make sure that calling uatomic_inc on a 64-bit integer doesn't cause a link > > > # error on _uatomic_link_error, which is how liburcu signals that it doesn't > > > -# support atomic operations on 64-bit data types. > > > +# support atomic operations on 64-bit data types for its generic > > > +# implementation (which relies on compiler builtins). For certain archs > > > +# where liburcu carries its own implementation (such as x86_32), it > > > +# signals lack of support during runtime by emitting an illegal > > > +# instruction, so we also need to execute here to detect that. > > > # > > > AC_DEFUN([AC_HAVE_LIBURCU_ATOMIC64], > > > [ AC_MSG_CHECKING([for atomic64_t support in liburcu]) > > > - AC_LINK_IFELSE( > > > + AC_RUN_IFELSE( > > > > Unfortunately, this change breaks cross compiling: > > Of course.. I completely forgot about that. > > > checking for umode_t... no > > checking for atomic64_t support in liburcu... configure: error: in > > `.../xfsprogs/build-aarch64': > > configure: error: cannot run test program while cross compiling > > See `config.log' for more details > > > > (Note that this is an x64 host building aarch64) > > > > Seeing as we /do/ have a (slow) workaround for 32-bit machines, perhaps > > we should use it any time a long isn't 64-bits wide: > > > > diff --git a/m4/package_urcu.m4 b/m4/package_urcu.m4 > > index ef116e0cda7..2ad4179aca2 100644 > > --- a/m4/package_urcu.m4 > > +++ b/m4/package_urcu.m4 > > @@ -34,8 +34,11 @@ AC_DEFUN([AC_HAVE_LIBURCU_ATOMIC64], > > [ AC_LANG_PROGRAM([[ > > #define _GNU_SOURCE > > #include > > +#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)])) > > ]], [[ > > long long f = 3; > > + > > +BUILD_BUG_ON(CAA_BITS_PER_LONG < 64); > > uatomic_inc(&f); > > ]]) > > ], have_liburcu_atomic64=yes > > > > This will cause suboptimal performance on any 32-bit cpu that /does/ > > support atomic operations on a u64, but oh well. > > I am not sure there is atomic u64 liburcu support for any 32-bit cpu > (even if that cpu does actually support it). Everything is fenced behind > the same conditional (#if CAA_BITS_PER_LONG == 64) in urcu headers > already (e.g. ppc.h or pretty much anything else that falls back to > uatomic/generic.h). So your patch may be the best way forward. Yeah. These days 32-bit architectures are supported but not maximally performant. :/ --D > Honestly I am not sure why this isn't implemented at least for x86 (e.g. > via cmpxchg8b). There's a configure option enable-compiler-atomic-builtins > that makes this work, but it doesn't seem to be enabled in distros > (looks fairly new, liburcu commit 3afcf5a0407c). > > Regards, > Anthony