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=-0.8 required=3.0 tests=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 8E0EFC43331 for ; Tue, 31 Mar 2020 09:37:14 +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 62D2420675 for ; Tue, 31 Mar 2020 09:37:14 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 62D2420675 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=proxmox.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:34704 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jJDKb-0006Ow-Jh for qemu-devel@archiver.kernel.org; Tue, 31 Mar 2020 05:37:13 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:37900) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jJDGz-0001bq-0U for qemu-devel@nongnu.org; Tue, 31 Mar 2020 05:33:29 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1jJDGy-000808-0t for qemu-devel@nongnu.org; Tue, 31 Mar 2020 05:33:28 -0400 Received: from proxmox-new.maurer-it.com ([212.186.127.180]:40443) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1jJDGx-0007zQ-Oe for qemu-devel@nongnu.org; Tue, 31 Mar 2020 05:33:27 -0400 Received: from proxmox-new.maurer-it.com (localhost.localdomain [127.0.0.1]) by proxmox-new.maurer-it.com (Proxmox) with ESMTP id 1F72743EE9; Tue, 31 Mar 2020 11:33:24 +0200 (CEST) Date: Tue, 31 Mar 2020 11:33:22 +0200 (CEST) From: Dietmar Maurer To: Sergio Lopez , Max Reitz , "jsnow@redhat.com" , Stefan Hajnoczi , "qemu-devel@nongnu.org" , "kwolf@redhat.com" , "vsementsov@virtuozzo.com" Message-ID: <1946378049.29.1585647202646@webmail.proxmox.com> In-Reply-To: <1005102592.28.1585646267717@webmail.proxmox.com> References: <658260883.24.1585644382441@webmail.proxmox.com> <1005102592.28.1585646267717@webmail.proxmox.com> Subject: Re: bdrv_drained_begin deadlock with io-threads MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Priority: 3 Importance: Normal X-Mailer: Open-Xchange Mailer v7.10.2-Rev23 X-Originating-Client: open-xchange-appsuite X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 212.186.127.180 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: , Reply-To: Dietmar Maurer Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" > Inside exec.c, there is a race: > > ------- > static bool prepare_mmio_access(MemoryRegion *mr) > { > bool unlocked = !qemu_mutex_iothread_locked(); > bool release_lock = false; > > if (unlocked && mr->global_locking) { > qemu_mutex_lock_iothread(); > ------ > > IMHO, checking for unlocked that way is wrong. Should use some kind > of _trylock instead? Answering this myself: above code is correct, because "iothread_locked" is a thread local var, and we want to lock the mutex if we did not already...