From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-1.mimecast.com (us-smtp-delivery-1.mimecast.com [207.211.31.120]) by sourceware.org (Postfix) with ESMTP id CC4F63851C0C for ; Tue, 26 May 2020 14:38:54 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org CC4F63851C0C Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-3-KcFqvKDmMqOGUKMO9JSqPg-1; Tue, 26 May 2020 10:38:52 -0400 X-MC-Unique: KcFqvKDmMqOGUKMO9JSqPg-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id EA5131005510; Tue, 26 May 2020 14:38:49 +0000 (UTC) Received: from oldenburg2.str.redhat.com (ovpn-112-180.ams2.redhat.com [10.36.112.180]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1F20A84648; Tue, 26 May 2020 14:38:46 +0000 (UTC) From: Florian Weimer To: Mathieu Desnoyers Cc: libc-alpha , Rich Felker , linux-api , Boqun Feng , Will Deacon , linux-kernel , Peter Zijlstra , Ben Maurer , Dave Watson , Thomas Gleixner , Paul , Paul Turner , Joseph Myers Subject: Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19) References: <20200501021439.2456-1-mathieu.desnoyers@efficios.com> <20200501021439.2456-2-mathieu.desnoyers@efficios.com> <87v9kqbzse.fsf@oldenburg2.str.redhat.com> <941087675.33347.1590418305398.JavaMail.zimbra@efficios.com> <87367ovy6k.fsf@oldenburg2.str.redhat.com> <108939265.33525.1590428184533.JavaMail.zimbra@efficios.com> <87lflerhqt.fsf@oldenburg2.str.redhat.com> <1701081361.34159.1590503556923.JavaMail.zimbra@efficios.com> Date: Tue, 26 May 2020 16:38:45 +0200 In-Reply-To: <1701081361.34159.1590503556923.JavaMail.zimbra@efficios.com> (Mathieu Desnoyers's message of "Tue, 26 May 2020 10:32:36 -0400 (EDT)") Message-ID: <87ftbmpxqi.fsf@oldenburg2.str.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Spam-Status: No, score=-8.6 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H3, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 26 May 2020 14:38:56 -0000 * Mathieu Desnoyers: > AFAIU, the only gain here would be to make sure we don't emit useless > ";" in the "/* nothing */" case. But does it matter ? I don't think C allows empty constructs like this at the top level. >>>> And something similar for _Alignas/attribute aligned, >>> >>> I don't see where _Alignas is needed here ? >>> >>> For attribute aligned, what would be the oldest supported C and C++ >>> standards ? >>=20 >> There are no standardized attributes for C, there is only _Alignas. >> C++11 has an alignas specifier; it's not an attribute either. I think >> these are syntactically similar. > > There appears to be an interesting difference between attribute aligned > and alignas. It seems like alignas cannot be used on a structure declarat= ion, > only on fields, e.g.: > > struct blah { > int a; > } _Alignas (16); > > o.c:3:1: warning: useless =E2=80=98_Alignas=E2=80=99 in empty declaration > } _Alignas (16); > > But > > struct blah { > int _Alignas (16) a; > }; Like the attribute, it needs to come right after the struct keyword, I think. (Trailing attributes can be ambiguous, but not in this case.) > is OK. So if I change e.g. struct rseq_cs to align > the first field: > > struct rseq_cs > { > /* Version of this structure. */ > uint32_t rseq_align (32) version; > /* enum rseq_cs_flags. */ > uint32_t flags; > uint64_t start_ip; > /* Offset from start_ip. */ > uint64_t post_commit_offset; > uint64_t abort_ip; > }; > > It should work. Indeed. > /* Rely on GNU extensions for older standards and tls model. */ > #ifdef __GNUC__ > # ifndef rseq_alignof > # define rseq_alignof(x) __alignof__ (x) > # endif > # ifndef rseq_alignas > # define rseq_alignas(x) __attribute__ ((aligned (x))) > # endif > # define rseq_tls_model_ie __attribute__ ((__tls_model__ ("initial-exec")= )) > #else > /* Specifying the TLS model on the declaration is optional. */ > # define rseq_tls_model_ie /* Nothing. */ > #endif > > /* Fall back to __thread for TLS storage class. */ > #ifndef rseq_tls_storage_class > # define rseq_tls_storage_class __thread > #endif If they are only used in the glibc headers, they should have __rseq prefixes, so that application code doesn't start using them (in case we have to change/fix them, or move the into later). Rest looks fine. Thanks, Florian