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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id CC1F4C77B73 for ; Thu, 27 Apr 2023 11:04:06 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1przPS-0001nw-RI; Thu, 27 Apr 2023 07:03:34 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1przPR-0001nL-AN for qemu-devel@nongnu.org; Thu, 27 Apr 2023 07:03:33 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1przPN-0006X7-JP for qemu-devel@nongnu.org; Thu, 27 Apr 2023 07:03:33 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1682593406; 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=5GXBi7BMfgoQqlCZ0Xl4jWAkZuLphv05e22NvLnE0Hs=; b=OVUK+S9xUuweky16USJvZoYD8RAMhkv2Iqw4H5kXhpUevFJTJh9fkQyz1r22gzXMI8+eQW He7SEvkoEqI26eJwr8uh80mrI1JAr/OBntT5KtAmfADDEsWHV9rIoadTX42WEGBVoXwHhF MRzx1bP97fxexCb2cZHLsYfpFvpSBC8= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-462-iF8Mp5loNa-PihCYPZUY5A-1; Thu, 27 Apr 2023 07:03:23 -0400 X-MC-Unique: iF8Mp5loNa-PihCYPZUY5A-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id C920F29A9D3C; Thu, 27 Apr 2023 11:03:22 +0000 (UTC) Received: from redhat.com (unknown [10.39.193.241]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1B64B492B03; Thu, 27 Apr 2023 11:03:20 +0000 (UTC) Date: Thu, 27 Apr 2023 13:03:19 +0200 From: Kevin Wolf To: Fiona Ebner Cc: Paolo Bonzini , Markus Armbruster , QEMU Developers , "open list:Block layer core" , Michael Roth , Fam Zheng , Stefan Hajnoczi , Thomas Lamprecht , Peter Xu Subject: Re: QMP (without OOB) function running in thread different from the main thread as part of aio_poll Message-ID: References: <2a61b581-5a21-c945-bb98-b6863cac0c1f@proxmox.com> <877cu7gk1g.fsf@pond.sub.org> <3ba2f8b9-9818-6601-2247-7b0e20d7ab0d@proxmox.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <3ba2f8b9-9818-6601-2247-7b0e20d7ab0d@proxmox.com> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -22 X-Spam_score: -2.3 X-Spam_bar: -- X-Spam_report: (-2.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.171, 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_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 26.04.2023 um 16:31 hat Fiona Ebner geschrieben: > Am 20.04.23 um 08:55 schrieb Paolo Bonzini: > > > > > > Il gio 20 apr 2023, 08:11 Markus Armbruster > > ha scritto: > > > > So, splicing in a bottom half unmoored monitor commands from the main > > loop.  We weren't aware of that, as our commit messages show. > > > > I guess the commands themselves don't care; all they need is the BQL. > > > > However, did we unwittingly change what can get blocked?  Before, > > monitor commands could block only the main thread.  Now they can also > > block vCPU threads.  Impact? > > > > > > Monitor commands could always block vCPU threads through the BQL(*). > > However, aio_poll() only runs in the vCPU threads in very special cases; > > typically associated to resetting a device which causes a blk_drain() on > > the device's BlockBackend. So it is not a performance issue. > > > > AFAIU, all generated coroutine wrappers use aio_poll. In my backtrace > aio_poll happens via blk_pwrite for a pflash device. So a bit more > often than "very special cases" ;) Yes, it's a common thing for devices that start requests from the vcpu thread when handling I/O (as opposed to devices that use an eventfd or similar mechanisms). > > However, liberal reuse of the main block layer AioContext could indeed > > be a *correctness* issue. I need to re-read Fiona's report instead of > > stopping at the first three lines because it's the evening. :) > > For me, being called in a vCPU thread caused problems with a custom QMP > function patched in by Proxmox. The function uses a newly opened > BlockBackend and calls qemu_mutex_unlock_iothread() after which > qemu_get_current_aio_context() returns 0x0 (when running in the main > thread, it still returns the main thread's AioContext). It then calls > blk_pwritev which is also a generated coroutine wrapper and the > assert(qemu_get_current_aio_context() == qemu_get_aio_context()); > in the else branch of the AIO_WAIT_WHILE_INTERNAL macro fails. > > Sounds like there's room for improvement in our code :/ I'm not aware > of something similar in upstream QEMU. Yes, even if it didn't crash immediately, calling blk_*() without holding a lock is invalid. In many cases, this is the BQL. If you don't hold it while calling the function from a vcpu thread, you could run into races with the main thread, which would probably be very painful to debug. Kevin