From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on dcvr.yhbt.net X-Spam-Level: X-Spam-Status: No, score=-4.2 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,MAILING_LIST_MULTI, SPF_HELO_PASS,SPF_PASS shortcircuit=no autolearn=ham autolearn_force=no version=3.4.2 Received: from sourceware.org (server2.sourceware.org [8.43.85.97]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by dcvr.yhbt.net (Postfix) with ESMTPS id 5F9891F55B for ; Mon, 25 May 2020 17:36:38 +0000 (UTC) Received: from server2.sourceware.org (localhost [IPv6:::1]) by sourceware.org (Postfix) with ESMTP id EDBFA389043E; Mon, 25 May 2020 17:36:36 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org EDBFA389043E DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sourceware.org; s=default; t=1590428197; bh=YJSgtXw/2qFLeYoA0bx8GuMUqdVH5+3SyAR99jrIWYo=; h=Date:To:In-Reply-To:References:Subject:List-Id:List-Unsubscribe: List-Archive:List-Post:List-Help:List-Subscribe:From:Reply-To:Cc: From; b=g4f/SToTjz2U8SPTzlxNhlmxO0C2Q/RFBozq8OAR+CbNLgiDTaWF/0RwpC9BUB26H jF8YgB16V3aNqZsYJLr+pnQRj7Yw67DzG58uPLEDv5uBaVp3TMCRnq9/iGA/Rok2gn fOXltaF1HlIcd3ugVtt3thVuHToGFBvDMz3zDxwM= Received: from mail.efficios.com (mail.efficios.com [167.114.26.124]) by sourceware.org (Postfix) with ESMTPS id 74732383E80C for ; Mon, 25 May 2020 17:36:25 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 74732383E80C Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id 2C2622C438D; Mon, 25 May 2020 13:36:25 -0400 (EDT) Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id pof2oeGh7p-k; Mon, 25 May 2020 13:36:24 -0400 (EDT) Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id B18F92C3FE9; Mon, 25 May 2020 13:36:24 -0400 (EDT) DKIM-Filter: OpenDKIM Filter v2.10.3 mail.efficios.com B18F92C3FE9 X-Virus-Scanned: amavisd-new at efficios.com Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id zI1L57FqandZ; Mon, 25 May 2020 13:36:24 -0400 (EDT) Received: from mail03.efficios.com (mail03.efficios.com [167.114.26.124]) by mail.efficios.com (Postfix) with ESMTP id 9EDF52C3F76; Mon, 25 May 2020 13:36:24 -0400 (EDT) Date: Mon, 25 May 2020 13:36:24 -0400 (EDT) To: Florian Weimer Message-ID: <108939265.33525.1590428184533.JavaMail.zimbra@efficios.com> In-Reply-To: <87367ovy6k.fsf@oldenburg2.str.redhat.com> 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> Subject: Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.26.124] X-Mailer: Zimbra 8.8.15_GA_3928 (ZimbraWebClient - FF76 (Linux)/8.8.15_GA_3928) Thread-Topic: glibc: Perform rseq registration at C startup and thread creation (v19) Thread-Index: 6to34dl4ew/LNfoxYauNE9D4OCKBdw== 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: , From: Mathieu Desnoyers via Libc-alpha Reply-To: Mathieu Desnoyers Cc: Rich Felker , libc-alpha , Peter Zijlstra , linux-api , Boqun Feng , Will Deacon , linux-kernel , Ben Maurer , Dave Watson , Thomas Gleixner , Paul , Paul Turner , Joseph Myers Errors-To: libc-alpha-bounces@sourceware.org Sender: "Libc-alpha" ----- On May 25, 2020, at 11:20 AM, Florian Weimer fweimer@redhat.com wrote: > * Mathieu Desnoyers: > >> The larger question here is: considering that we re-implement the entire >> uapi header within glibc (which includes the uptr addition), do we still >> care about using the header provided by the Linux kernel ? > > We don't care, but our users do. Eventually, they want to include > and to get new constants that are not yet > known to glibc. Good point! > >> Having different definitions depending on whether a kernel header is >> installed or not when including a glibc header seems rather unexpected. > > Indeed. > >> *If* we want to use the uapi header, I think something is semantically >> missing. Here is the scheme I envision. We could rely on the kernel header >> version.h to figure out which of glibc or kernel uapi header is more >> recent. Any new concept we try to integrate into glibc (e.g. uptr) >> should go into the upstream Linux uapi header first. > > I think we should always prefer the uapi header. The Linux version > check does not tell you anything about backports. Fair enough. > >> For the coming glibc e.g. 2.32, we use the kernel uapi header if >> kernel version is >= 4.18.0. Within glibc, the fallback implements >> exactly the API exposed by the kernel rseq.h header. > > Agreed. > >> As we eventually introduce the uptr change into the Linux kernel, and >> say it gets merged for Linux 5.9.0, we mirror this change into glibc >> (e.g. release 2.33), and bump the Linux kernel version cutoff to 5.9.0. >> So starting from that version, we use the Linux kernel header only if >> version >= 5.9.0, else we fallback on glibc's own implementation. > > Fortunately, we don't need to settle this today. 8-) > > Let's stick to the 4.18 definitions for the fallback for now, and > discuss the incorporation of future changes later. OK > >>>> +/* Ensure the compiler supports __attribute__ ((aligned)). */ >>>> +_Static_assert (__alignof__ (struct rseq_cs) >= 32, "alignment"); >>>> +_Static_assert (__alignof__ (struct rseq) >= 32, "alignment"); >>> >>> This needs #ifndef __cplusplus or something like that. I'm surprised >>> that this passes the installed header tests. >> >> Would the following be ok ? >> >> #ifdef __cplusplus >> #define rseq_static_assert static_assert >> #else >> #define rseq_static_assert _Static_assert >> #endif >> >> /* Ensure the compiler supports __attribute__ ((aligned)). */ >> rseq_static_assert (__alignof__ (struct rseq_cs) >= 32, "alignment"); >> rseq_static_assert (__alignof__ (struct rseq) >= 32, "alignment"); > > Seems reasonable, yes. __alignof__ is still a GCC extension. C++11 has > alignof, C11 has _Alignof. So you could use something like this > (perhaps without indentation for the kernel header version): > > #ifdef __cplusplus > # if __cplusplus >= 201103L > # define rseq_static_assert(x) static_assert x; > # define rseq_alignof alignof > # endif > #elif __STDC_VERSION__ >= 201112L > # define rseq_static_assert(x) _Static_assert x; > # define rseq_alignof _Alignof > #endif > #ifndef rseq_static_assert > # define rseq_static_assert /* nothing */ > #endif > rseq_static_assert ((rseq_alignof__ (struct rseq_cs) >= 32, "alignment")) > rseq_static_assert ((rseq_alignof (struct rseq) >= 32, "alignment")) Something like this ? #ifdef __cplusplus # if __cplusplus >= 201103L # define rseq_static_assert (expr, diagnostic) static_assert (expr, diagnostic) # define rseq_alignof alignof # endif #elif __STDC_VERSION__ >= 201112L # define rseq_static_assert (expr, diagnostic) _Static_assert (expr, diagnostic) # define rseq_alignof _Alignof #endif #ifndef rseq_static_assert # define rseq_static_assert (expr, diagnostic) /* nothing */ #endif /* Ensure the compiler supports __attribute__ ((aligned)). */ rseq_static_assert ((rseq_alignof (struct rseq_cs) >= 32, "alignment")); rseq_static_assert ((rseq_alignof (struct rseq) >= 32, "alignment")); > 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 ? > with an error for > older standards and !__GNUC__ compilers (because neither the type nor > __thread can be represented there). By "type" you mean "struct rseq" here ? What does it contain that requires a __GNUC__ compiler ? About __thread, I recall other compilers have other means to declare it. In liburcu, I end up with the following: #if defined (__cplusplus) && (__cplusplus >= 201103L) # define URCU_TLS_STORAGE_CLASS thread_local #elif defined (__STDC_VERSION__) && (__STDC_VERSION__ >= 201112L) # define URCU_TLS_STORAGE_CLASS _Thread_local #elif defined (_MSC_VER) # define URCU_TLS_STORAGE_CLASS __declspec(thread) #else # define URCU_TLS_STORAGE_CLASS __thread #endif Would something along those lines be OK for libc ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com