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=-4.0 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 6F399C54E4A for ; Tue, 12 May 2020 11:34:03 +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 3E5EF206F5 for ; Tue, 12 May 2020 11:34:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="BTTdpCkA" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3E5EF206F5 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]:59298 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jYTAg-0004Yx-FC for qemu-devel@archiver.kernel.org; Tue, 12 May 2020 07:34:02 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:33304) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jYT9p-0003Tr-6j for qemu-devel@nongnu.org; Tue, 12 May 2020 07:33:09 -0400 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:51894 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jYT9o-0006iI-97 for qemu-devel@nongnu.org; Tue, 12 May 2020 07:33:08 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1589283187; 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=humsqq0UQJSXbVwJpOATzrWoKVEJNugwaRhXcYJ/LX4=; b=BTTdpCkAKBCUvurGqQPkGNLHirvEp3vTnLKGApVvVwRpSxQMZpA+AZkjdyEdQMbZHkOuEp S6RrCblHgeMx43kUocE5yESICpcGEMvSHjIvKcM2nMbWya25D9Gk7pDdMxItZOK6nwxZni z4XXWMnR73j96V7yIDVjWocWt+HpmVc= 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-13-angsQK-eP3-wbzMvmQWB2g-1; Tue, 12 May 2020 07:33:05 -0400 X-MC-Unique: angsQK-eP3-wbzMvmQWB2g-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 87B8D872FE2; Tue, 12 May 2020 11:33:04 +0000 (UTC) Received: from linux.fritz.box (ovpn-114-74.ams2.redhat.com [10.36.114.74]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 39EEA5D9DD; Tue, 12 May 2020 11:32:57 +0000 (UTC) Date: Tue, 12 May 2020 13:32:55 +0200 From: Kevin Wolf To: Stefan Reiter Subject: Re: [RFC] bdrv_flush: only use fast path when in owned AioContext Message-ID: <20200512113255.GK5951@linux.fritz.box> References: <20200511165032.11384-1-s.reiter@proxmox.com> <20200512105719.GI5951@linux.fritz.box> MIME-Version: 1.0 In-Reply-To: <20200512105719.GI5951@linux.fritz.box> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 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=207.211.31.120; envelope-from=kwolf@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/12 02:20:15 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] 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_H2=-0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN 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: fam@euphon.net, qemu-block@nongnu.org, qemu-devel@nongnu.org, mreitz@redhat.com, stefanha@redhat.com, t.lamprecht@proxmox.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 12.05.2020 um 12:57 hat Kevin Wolf geschrieben: > Am 11.05.2020 um 18:50 hat Stefan Reiter geschrieben: > > Just because we're in a coroutine doesn't imply ownership of the context > > of the flushed drive. In such a case use the slow path which explicitly > > enters bdrv_flush_co_entry in the correct AioContext. > > > > Signed-off-by: Stefan Reiter > > --- > > > > We've experienced some lockups in this codepath when taking snapshots of VMs > > with drives that have IO-Threads enabled (we have an async 'savevm' > > implementation running from a coroutine). > > > > Currently no reproducer for upstream versions I could find, but in testing this > > patch fixes all issues we're seeing and I think the logic checks out. > > > > The fast path pattern is repeated a few times in this file, so if this change > > makes sense, it's probably worth evaluating the other occurences as well. > > What do you mean by "owning" the context? If it's about taking the > AioContext lock, isn't the problem more with calling bdrv_flush() from > code that doesn't take the locks? > > Though I think we have some code that doesn't only rely on holding the > AioContext locks, but that actually depends on running in the right > thread, so the change looks right anyway. Well, the idea is right, but the change itself isn't, of course. If we're already in coroutine context, we must not busy wait with BDRV_POLL_WHILE(). I'll see if I can put something together after lunch. Kevin