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 F2712C56202 for ; Mon, 23 Nov 2020 10:12:26 +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 5815221D46 for ; Mon, 23 Nov 2020 10:12:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="HB4DnFgW" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5815221D46 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]:36400 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kh8pd-0003Cj-9Q for qemu-devel@archiver.kernel.org; Mon, 23 Nov 2020 05:12:25 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:35042) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kh8oU-0002J3-4x for qemu-devel@nongnu.org; Mon, 23 Nov 2020 05:11:14 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:38626) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kh8oR-000255-28 for qemu-devel@nongnu.org; Mon, 23 Nov 2020 05:11:13 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1606126269; 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=DD1Hds3pCPHmhCMPUKo7PDNIaMjUvgFOCNc3ZEy2dOI=; b=HB4DnFgWZ7xECncBgWZDJg9wfvPC2IS8XJDI8nrHrgiyU6AvYMBttnKngntQOPTTQWlkEA +jiJJVr6f22WWs5I0p21RHeIcetqZJBDU++cmwHIwIb6yX+9vx4K5eJa+e/xWwbB81w5hm 8L71lLhR6sRal+L+occxFZe0ZWKz7dI= 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-421-7XFX5IP5PRCso5ZzNFCYyg-1; Mon, 23 Nov 2020 05:11:08 -0500 X-MC-Unique: 7XFX5IP5PRCso5ZzNFCYyg-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 8AE3D1926DA4; Mon, 23 Nov 2020 10:11:06 +0000 (UTC) Received: from merkur.fritz.box (ovpn-113-153.ams2.redhat.com [10.36.113.153]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 5D3DD5D6DC; Mon, 23 Nov 2020 10:11:00 +0000 (UTC) Date: Mon, 23 Nov 2020 11:10:58 +0100 From: Kevin Wolf To: Vladimir Sementsov-Ogievskiy Subject: Re: [PATCH RFC 0/5] Fix accidental crash in iotest 30 Message-ID: <20201123101058.GA5317@merkur.fritz.box> References: <20201120161622.1537-1-vsementsov@virtuozzo.com> <20201120163627.GD5599@merkur.fritz.box> <20201120172251.GE5599@merkur.fritz.box> <6d6b17b9-80d6-aa90-6e1b-f8519ae181cc@virtuozzo.com> MIME-Version: 1.0 In-Reply-To: <6d6b17b9-80d6-aa90-6e1b-f8519ae181cc@virtuozzo.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=216.205.24.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, 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: peter.maydell@linaro.org, berto@igalia.com, qemu-block@nongnu.org, jsnow@redhat.com, qemu-devel@nongnu.org, mreitz@redhat.com, stefanha@redhat.com, den@openvz.org, pbonzini@redhat.com, philmd@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 20.11.2020 um 19:19 hat Vladimir Sementsov-Ogievskiy geschrieben: > 20.11.2020 20:22, Kevin Wolf wrote: > > Am 20.11.2020 um 17:43 hat Vladimir Sementsov-Ogievskiy geschrieben: > > > 20.11.2020 19:36, Kevin Wolf wrote: > > > > Am 20.11.2020 um 17:16 hat Vladimir Sementsov-Ogievskiy geschrieben: > > > > > Hi all! > > > > > > > > > > As Peter recently noted, iotest 30 accidentally fails. > > > > > > > > > > I found that Qemu crashes due to interleaving of graph-update > > > > > operations of parallel mirror and stream block-jobs. > > > > > > > > I haven't found the time yet to properly look into this or your other > > > > thread where you had a similar question, but there is one thing I'm > > > > wondering: Why can the nested job even make progress and run its > > > > completion handler? > > > > > > > > When we modify the graph, we should have drained the subtree in > > > > question, so in theory while one job finishes and modifies the graph, > > > > there should be no way for the other job to make progress and get > > > > interleaved - it shouldn't be able to start I/O requests and much less > > > > to run its completion handler and modify the graph. > > > > > > > > Are we missing drained sections somewhere or do they fail to achieve > > > > what I think they should achieve? > > > > > > > > > > It all looks like both jobs are reached their finish simultaneously. > > > So, all progress is done in both jobs. And they go concurrently to > > > completion procedures which interleaves. So, there no more io through > > > blk, which is restricted by drained sections. > > > > They can't be truly simultaneous because they run in the same thread. > > During job completion, this is the main thread. > > No, they not truly simultaneous, but completions may interleave > through nested aio_poll loops. > > > > > However as soon as job_is_completed() returns true, it seems we're not > > pausing the job any more when one of its nodes gets drained. > > > > Possibly also relevant: The job->busy = false in job_exit(). The comment > > there says it's a lie, but we might deadlock otherwise. > > > > This problem will probably affect other callers, too, which drain a > > subtree and then resonably expect that nobody will modify the graph > > until they end the drained section. So I think the problem that we need > > to address is that jobs run their completion handlers even though they > > are supposed to be paused by a drain. > > Hmm. I always thought about drained section as about thing that stops > IO requests, not other operations.. And we do graph modifications in > drained section to avoid in-flight IO requests during graph > modification. Is there any use for an operation that only stops I/O, but doesn't prevent graph changes? I always understood it as a request to have exclusive access to a subtree, so that nobody else would touch it. > > I'm not saying that your graph modification locks are a bad idea, but > > they are probably not a complete solution. > > > > Hmm. What do you mean? It's of course not complete, as I didn't > protect every graph modification procedure.. But if we do protect all > such things and do graph modifications always under this mutex, it > should work I think. What I mean is that not only graph modifications can conflict with each other, but most callers of drain_begin/end will probably not be prepared for the graph changing under their feet, even if they don't actively change the graph themselves. Kevin