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=-15.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=ham 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 C3AE6C433E0 for ; Wed, 27 Jan 2021 19:14:35 +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 089A364DBE for ; Wed, 27 Jan 2021 19:14:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 089A364DBE 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]:39244 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l4qGv-0008MD-LJ for qemu-devel@archiver.kernel.org; Wed, 27 Jan 2021 14:14:33 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:48390) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l4qFs-0007vi-PV for qemu-devel@nongnu.org; Wed, 27 Jan 2021 14:13:28 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:60771) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l4qFq-0002l7-Ib for qemu-devel@nongnu.org; Wed, 27 Jan 2021 14:13:27 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1611774805; 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=7JRwJhNswRoKdi54MfP43v0Pqv8ffR1PlfrdEPGr0y4=; b=MhEYmptBkuVXLjpQsLy+df5Npx0djsR1OSZmn8AovLW3arGEjaLS/vJquHFjDYZ9EKrhp3 7huOAq5/juyHjO/Bj733cL8CP+cH+jVF3ohfHW6O3re3UqfLjgm8yCNbNdu0geTB+ckk9W jImKJYHNZL/gxGIXmdwVITPZH4whfBw= 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-553-dovmbmVhOMGicbURuRd1Dg-1; Wed, 27 Jan 2021 14:13:21 -0500 X-MC-Unique: dovmbmVhOMGicbURuRd1Dg-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 B0664190D341 for ; Wed, 27 Jan 2021 19:13:20 +0000 (UTC) Received: from [10.3.113.91] (ovpn-113-91.phx2.redhat.com [10.3.113.91]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 66CA760854; Wed, 27 Jan 2021 19:13:14 +0000 (UTC) Subject: Re: [PATCH] coroutine-sigaltstack: Add SIGUSR2 mutex To: Max Reitz , qemu-devel@nongnu.org References: <20210125120305.19520-1-mreitz@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Message-ID: <8c766d8d-4deb-ccef-1be1-a26a6654a88a@redhat.com> Date: Wed, 27 Jan 2021 13:13:13 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <20210125120305.19520-1-mreitz@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=eblake@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: 7bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.308, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=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 , =?UTF-8?B?TMOhc3psw7Mgw4lyc2Vr?= , Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 1/25/21 6:03 AM, Max Reitz wrote: > Disposition (action) for any given signal is global for the process. > When two threads run coroutine-sigaltstack's qemu_coroutine_new() > concurrently, they may interfere with each other: One of them may revert > the SIGUSR2 handler to SIG_DFL, between the other thread (a) setting up > coroutine_trampoline() as the handler and (b) raising SIGUSR2. That > SIGUSR2 will then terminate the QEMU process abnormally. > > We have to ensure that only one thread at a time can modify the > process-global SIGUSR2 handler. To do so, wrap the whole section where > that is done in a mutex. > > Alternatively, we could for example have the SIGUSR2 handler always be > coroutine_trampoline(), so there would be no need to invoke sigaction() > in qemu_coroutine_new(). Laszlo has posted a patch to do so here: > > https://lists.nongnu.org/archive/html/qemu-devel/2021-01/msg05962.html I indeed like that one, but also concur that simplicity trumps the uncertainty of a larger patch. Let's get things unbroken before we worry about optimizing things to avoid the mutex. > > However, given that coroutine-sigaltstack is more of a fallback > implementation for platforms that do not support ucontext, that change > may be a bit too invasive to be comfortable with it. The mutex proposed > here may negatively impact performance, but the change is much simpler. > > Signed-off-by: Max Reitz > --- > util/coroutine-sigaltstack.c | 9 +++++++++ > 1 file changed, 9 insertions(+) Reviewed-by: Eric Blake -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org