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=-22.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, 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 8AC15C433DB for ; Wed, 20 Jan 2021 17:01:43 +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 0A6342137B for ; Wed, 20 Jan 2021 17:01:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0A6342137B 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]:45886 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l2GrV-0006ac-RT for qemu-devel@archiver.kernel.org; Wed, 20 Jan 2021 12:01:42 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:41766) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l2Gob-00052W-9p for qemu-devel@nongnu.org; Wed, 20 Jan 2021 11:58:41 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:30648) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l2GoZ-0001Gn-9M for qemu-devel@nongnu.org; Wed, 20 Jan 2021 11:58:41 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1611161918; 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=TLsrm1jtcoz/igpQMemUINau9y1VGRGmpZQdTaUip0c=; b=GYkUGzyb/cKVKJjJeo+9OIiioHdLD6TM2rajAIqWSVXxYJ1FVNdwabPn+s+l/GvYUKzMcG gjPGGelYrK6QD5m1A/6vIa1DRf0hm/8MFDBQsKnLAIc8w8R3AinHE5Zi40I/OJRy08T7BN IHVCLMh4DPoex1846W/l3NFEvMJguOY= 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-493-gmsG-FGWOnyYiogp7snzTw-1; Wed, 20 Jan 2021 11:58:35 -0500 X-MC-Unique: gmsG-FGWOnyYiogp7snzTw-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 73B6C8030A3 for ; Wed, 20 Jan 2021 16:58:34 +0000 (UTC) Received: from [10.3.113.116] (ovpn-113-116.phx2.redhat.com [10.3.113.116]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E70A71042A40; Wed, 20 Jan 2021 16:58:31 +0000 (UTC) To: Max Reitz , "qemu-devel@nongnu.org" References: <7b8155ad-0942-dc1c-f43c-bb5eb518a278@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Subject: Re: Thread safety of coroutine-sigaltstack Message-ID: Date: Wed, 20 Jan 2021 10:58:30 -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: <7b8155ad-0942-dc1c-f43c-bb5eb518a278@redhat.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 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: 8bit 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=-0.167, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.094, 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 , Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 1/20/21 10:26 AM, Max Reitz wrote: > Hi, > > I’ve run into trouble with Vladimir’s async backup series on MacOS, > namely that iotest 256 fails with qemu exiting because of a SIGUSR2. > > Turns out this is because MacOS (-xcode) uses coroutine-sigaltstack, > when I use this on Linux, I get the same error. > > (You can find the series applied on my block branch e.g. here: > > https://github.com/XanClic/qemu.git block > ) > > Some debugging later I found that the problem seems to be two threads > simultaneously creating a coroutine.  It makes sense that this case > would appear with Vladimir’s series and iotest 256, because 256 runs two > backup jobs in two different threads in a transaction, i.e. they’re > launched simultaneously.  The async backup series makes backup use many > concurrent coroutines and so by default launches 64+x coroutines when > the backup is started.  Thus, the case of two coroutines created > concurrently in two threads is very likely to occur. > > I think the problem is in coroutine-sigaltstack’s qemu_coroutine_new(). > It sets up a SIGUSR2 handler, then changes the signal handling stack, > then raises SIGUSR2, then reverts the signal handling stack and the > SIGUSR2 handler.  As far as I’m aware, setting up signal handlers and > changing the signal handling stack are both process-global operations, > and so if two threads do so concurrently, they will interfere with each > other. Yes, that is absolutely correct - messing with the signal handlers is process-wide. I guess we've been lucky that we haven't been trying to create coroutines in separate threads in the past. >  What usually happens is that one thread sets up everything, > while the other is already in the process of reverting its changes: So > the second thread reverts the SIGUSR2 handler to the default, and then > the first thread raises SIGUSR2, thus making qemu exit. > > (Could be worse though.  Both threads could set up the sigaltstack, then > both raise SIGUSR2, and then we get one coroutine_trampoline() > invocation in each thread, but both would use the same stack.  But I > don’t think I’ve ever seen that happen, presumably because the race time > window is much shorter.) > > Now, this all seems obvious to me, but I’m wondering...  If > coroutine-sigaltstack really couldn’t create coroutines concurrently, > why wouldn’t we have noticed before?  I mean, this new backup case is > kind of a stress test, yes, but surely we would have seen the problem > already, right?  That’s why I’m not sure whether my analysis is correct. I'm not sure if there is anything else going wrong, but you have definitely uncovered a latent problem, and I agree that a mutex is the right way to fix it. > > Anyway, I’ve attached a patch that wraps the whole SIGUSR2 handling > section in a mutex, and that makes 256 pass reliably with Vladimir’s > async backup series.  Besides being unsure whether the problem is really > in coroutine-sigaltstack, I also don’t know whether getting out the big > guns and wrapping everything in the mutex is the best solution.  So, > it’s an RFC, I guess. > > Max >>>From 08d4bb6a98fa731025683f20afe1381291d26031 Mon Sep 17 00:00:00 2001 > From: Max Reitz > Date: Wed, 20 Jan 2021 16:59:40 +0100 > Subject: [RFC] coroutine-sigaltstack: Add SIGUSR2 mutex > > Modifying signal handlers or the signal handling stack is a > process-global operation. When two threads run coroutine-sigaltstack's > qemu_coroutine_new() concurrently, thay may interfere with each other, they > e.g.: > > - One of the threads may revert the SIGUSR2 handler back to the default > between the other thread setting up coroutine_trampoline() as the > handler and raising SIGUSR2. That SIGUSR2 will then lead to the > process exiting. > > - Both threads may set up their coroutine stack with sigaltstack() > simultaneously, so that only one of them sticks. Both then raise > SIGUSR2, which goes to each of the threads separately, but both signal > handler invocations will then use the same stack, which cannot work. > > We have to ensure that only one thread at a time can modify the > process-global SIGUSR2 handler and the signal handling stack. To do so, > wrap the whole section where that is done in a mutex. > > Signed-off-by: Max Reitz > --- > util/coroutine-sigaltstack.c | 10 ++++++++++ > 1 file changed, 10 insertions(+) Reviewed-by: Eric Blake -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org