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 X-Spam-Level: X-Spam-Status: No, score=-3.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 74EC4C433E0 for ; Sat, 23 Jan 2021 00:07:46 +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 EA97623B6A for ; Sat, 23 Jan 2021 00:07:45 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EA97623B6A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:47270 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l36Su-0004oh-RI for qemu-devel@archiver.kernel.org; Fri, 22 Jan 2021 19:07:44 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:36768) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l36S3-0004O5-UO for qemu-devel@nongnu.org; Fri, 22 Jan 2021 19:06:52 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:60163) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l36Rz-0001jo-Bl for qemu-devel@nongnu.org; Fri, 22 Jan 2021 19:06:51 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1611360405; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=QQ/lSwCa7sNQw0okvoEoyma9IPWziHaKlXmOCMI30V0=; b=i9/+wNsWZdzSkTbstMQBFjxihNjWL4ogKKkSKONG1xGJ3fBqCxaiSKhi2H6fpDxO8qeTlH rG1G5Du88sVQueaHsVLJZxi64USM7ziZ/K0kWeXKKOKUYvkhcI5KOtYnRgchjf537Ci0sn oUZZZFc8exOakv7pArlYgX7p6U0ltks= 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-354-YOHGSdGvNbCEa_8z1-7IeA-1; Fri, 22 Jan 2021 19:06:42 -0500 X-MC-Unique: YOHGSdGvNbCEa_8z1-7IeA-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7E99610054FF; Sat, 23 Jan 2021 00:06:41 +0000 (UTC) Received: from lacos-laptop-7.usersys.redhat.com (ovpn-113-81.ams2.redhat.com [10.36.113.81]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6EB815DA2D; Sat, 23 Jan 2021 00:06:36 +0000 (UTC) Subject: Re: Thread safety of coroutine-sigaltstack To: Peter Maydell , Max Reitz References: <7b8155ad-0942-dc1c-f43c-bb5eb518a278@redhat.com> <445268c9-d91f-af5a-3d7e-f4c6f014ca52@redhat.com> From: Laszlo Ersek Message-ID: Date: Sat, 23 Jan 2021 01:06:35 +0100 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=lersek@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=lersek@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.182, 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_H4=0.001, RCVD_IN_MSPIKE_WL=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: Kevin Wolf , "qemu-devel@nongnu.org" , Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 01/22/21 11:14, Peter Maydell wrote: > On Fri, 22 Jan 2021 at 08:50, Max Reitz wrote: >> >> On 20.01.21 18:25, Laszlo Ersek wrote: >> >> [...] >> >>> A simple grep for SIGUSR2 seems to indicate that SIGUSR2 is not used by >>> system emulation for anything else, in practice. Is it possible to >>> dedicate SIGUSR2 explicitly to coroutine-sigaltstack, and set up the >>> action beforehand, from some init function that executes on a "central" >>> thread, before qemu_coroutine_new() is ever called? >> >> I wrote a patch to that effect, but just before sending I wondered >> whether SIGUSR2 cannot be registered by the “guest” in user-mode >> emulation, and whether that would then break coroutines from there on. >> >> (I have no experience dealing with user-mode emulation, but it does look >> like the guest can just register handlers for any signal but SIGSEGV and >> SIGBUS.) > > Yes, SIGUSR2 is for the guest in user-emulation mode. OTOH do we > even use the coroutine code in user-emulation mode? Looking at > the meson.build files, we only add the coroutine_*.c to util_ss > if 'have_block', and we set have_block = have_system or have_tools. > I think (but have not checked) that that means we will build and > link the object file into the user-mode binaries if you happen > to build them in the same run as system-mode binaries, I did that, first running ./configure \ --enable-debug \ --target-list==x86_64-softmmu,x86_64-linux-user \ --with-coroutine=sigaltstack Then I checked the "qemu-system-x86_64" and "qemu-x86_64" binaries with "nm". Only the former contains "coroutine_init": 00000000009725e4 t coroutine_init So I believe the coroutine object file(s) are not even linked into the user-mode emulators. (coroutine_init() is a constructor function, so I think it would be preserved otherwise, even if it had no explicit caller.) I tried a different approach too: an #error in "coroutine-sigaltstack.c", if CONFIG_LINUX_USER were #defined. But that aborted the build, due to CONFIG_LINUX_USER being poisoned in the first place. Maybe that result was already enough to answer the question, but I wasn't sure, hence the check with "nm". Thanks, Laszlo > but won't > build them in if you built the user-mode binaries as a separate > build. Which is odd and probably worth fixing, but does mean we > know that we aren't actually using coroutines in user-mode. > (Also user-mode really means Linux or BSD and I think both of > those have working ucontext.) > > thanks > -- PMM >