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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 39650C433EF for ; Tue, 26 Oct 2021 13:38:05 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id E06E160E73 for ; Tue, 26 Oct 2021 13:38:04 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org E06E160E73 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=nongnu.org Received: from localhost ([::1]:50972 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mfMeR-00074u-Nr for qemu-devel@archiver.kernel.org; Tue, 26 Oct 2021 09:38:03 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:37688) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mfMdE-0005g6-6d for qemu-devel@nongnu.org; Tue, 26 Oct 2021 09:36:48 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:38039) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mfMdC-0006ld-9m for qemu-devel@nongnu.org; Tue, 26 Oct 2021 09:36:47 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1635255404; h=from:from:reply-to:subject:subject: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=uMLFos5AmBjNcqFCqJyw+e3Yif9YTS9O81wAYMzb38o=; b=MMj3kIjGwsLFolf2Rmq24+udscRXDZ1uWfuHBd31+5ZqgT4rpjnXn92o4AVKvhUUBhw+9B fHqCL33lESSovqW30Pjj7OhJXTJhHj0zyCdW/XKYZjWoYLVgolZcBi1/1roeKFbJVTNztp kVDaViNYHIiQSfzv+k+bjoxbEsznER4= 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-451-vyealuNwMeCgX4crl6Ojqg-1; Tue, 26 Oct 2021 09:36:41 -0400 X-MC-Unique: vyealuNwMeCgX4crl6Ojqg-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7E235196634E; Tue, 26 Oct 2021 13:36:27 +0000 (UTC) Received: from localhost (unknown [10.39.195.43]) by smtp.corp.redhat.com (Postfix) with ESMTP id F29CD100EBAD; Tue, 26 Oct 2021 13:36:26 +0000 (UTC) Date: Tue, 26 Oct 2021 14:36:25 +0100 From: Stefan Hajnoczi To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Subject: Re: [RFC 1/2] tls: add macros for coroutine-safe TLS variables Message-ID: References: <20211025140716.166971-1-stefanha@redhat.com> <20211025140716.166971-2-stefanha@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=stefanha@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="gvw4wkoZn73RYNFY" Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: fweimer@redhat.com, Kevin Wolf , thuth@redhat.com, qemu-block@nongnu.org, qemu-devel@nongnu.org, Fam Zheng , sguelton@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --gvw4wkoZn73RYNFY Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon, Oct 25, 2021 at 03:14:36PM +0100, Daniel P. Berrang=E9 wrote: > On Mon, Oct 25, 2021 at 03:07:15PM +0100, Stefan Hajnoczi wrote: > > Compiler optimizations can cache TLS values across coroutine yield > > points, resulting in stale values from the previous thread when a > > coroutine is re-entered by a new thread. > >=20 > > Serge Guelton developed an __attribute__((noinline)) wrapper and tested > > it with clang and gcc. I formatted his idea according to QEMU's coding > > style and wrote documentation. > >=20 > > These macros must be used instead of __thread from now on to prevent > > coroutine TLS bugs. >=20 > Does this apply to all __thread usage in the QEMU process that can > be used from coroutine context, or just certain __thread usage ? >=20 > Mostly I'm wondering if this is going to have implications on external > libraries we use. eg if block layer is using librbd.so APIs, is librbd.sp > safe to use __thread directly in any way it desires ? There is a theoretical problem but I'm not sure if it will occur in practice: If a __thread variable is in an extern function then there's little risk of the value being cached. The function executes and returns back to QEMU, never yielding. The only case I can think of is when the library accesses a __thread variable, invokes a callback into QEMU, and that callback yields. If the coroutine is re-entered from another thread and returns back to the library, then we have a problem. This seems like a very rare case though. It gets trickier if the library has the __thread variable in a header file, because then the compiler may optimize it into a QEMU coroutine function and cache its value. Stefan --gvw4wkoZn73RYNFY Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEhpWov9P5fNqsNXdanKSrs4Grc8gFAmF4BFkACgkQnKSrs4Gr c8jJkwf/XCrveDdFpxEi7OiulxnWN1IXuN3eHi7yt2K6SlklhYybEMOsL+nZoC4G ZsOtCRdJf6y8vUrCSaCT++snvJRkZpnvCc9sMJKlidZj4X+XTnZpglDpvms9FAcu zxELfSez3rv1+Vyygtm4IH5rkClXJMdSTD6v2Pt4/E5tuHhZrzt0pANA8sQcpvr6 FHfvDD0EaEJpqM30f6LVWw5T+Levp1sCVVjZjfuP22W3+qQ/WxiX+4zn9lEd4DDA 0U17psAOGHgXG6k3oMYWIEKakX9cTu2sNY+RxMsO3i94d5gQf0A4zfynurUNJM5h SkpROT9htrYSnER7roztCujnM3qepw== =C9/u -----END PGP SIGNATURE----- --gvw4wkoZn73RYNFY--