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 E16F6EE14A8 for ; Wed, 6 Sep 2023 15:19:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242443AbjIFPUA (ORCPT ); Wed, 6 Sep 2023 11:20:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57592 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242441AbjIFPT7 (ORCPT ); Wed, 6 Sep 2023 11:19:59 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 786241717 for ; Wed, 6 Sep 2023 08:19:54 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 34FF322430; Wed, 6 Sep 2023 15:19:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1694013593; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=tJZEAiR0QsymgjBhF1Qm86Umi96cI7YPNL5miQ+w2Rg=; b=LkH398Ihug0qRpiC15f7pKPbm7AbtAEzDLf1EX2pI6Hl4tfwx4a5hY/ApvLKXVJ/vvHhyo 2/KQK6EiMZx0xHqeFrngiu2DmQgPFngr/Xi0m1b9to3cA6Uh5jd7vxkBwwx1G4MkXVjavc TMa8ZnvdnIaHYKdEcKKyK95EzaOYpQo= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id E7A161333E; Wed, 6 Sep 2023 15:19:52 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id D3ExNZiY+GRCXQAAMHmgww (envelope-from ); Wed, 06 Sep 2023 15:19:52 +0000 Date: Wed, 6 Sep 2023 17:19:52 +0200 From: Anthony Iliopoulos To: "Darrick J. Wong" Cc: linux-xfs@vger.kernel.org Subject: Re: [PATCH] libxfs: fix atomic64_t detection on x86 32-bit architectures Message-ID: 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: <20230905164250.GV28186@frogsfrogsfrogs> Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org 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. 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