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 EEDC310ED65A for ; Fri, 27 Mar 2026 10:42:18 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1w64dD-0002XN-K8; Fri, 27 Mar 2026 06:41:35 -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 1w64dC-0002XE-8s for qemu-devel@nongnu.org; Fri, 27 Mar 2026 06:41:34 -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 1w64dA-0006ap-JO for qemu-devel@nongnu.org; Fri, 27 Mar 2026 06:41:34 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1774608091; 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=7z+bJmCQvMsrSRnPawAuCqNTtylAFIRQidYbzNcsdME=; b=GEZfcVnJGBNrOCw+OyJyqDNZv0Iq3BNI9IDPZawJ0YPEr54TKby4cEedUDtFdx+Ev/6nOf yd4R1lIbWtvbtHHMFIQN0zpxNcYFsxKcMVY20eYCSVQfFIO7nEEHfNXkKQNq1dW3S4/WhL 1wLUlXfokhvbhrN7yWQvwALFQulvA7k= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-524-i4CobxYSMVy0pS3Py_f34g-1; Fri, 27 Mar 2026 06:41:27 -0400 X-MC-Unique: i4CobxYSMVy0pS3Py_f34g-1 X-Mimecast-MFC-AGG-ID: i4CobxYSMVy0pS3Py_f34g_1774608086 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 4D94B1956055; Fri, 27 Mar 2026 10:41:26 +0000 (UTC) Received: from redhat.com (unknown [10.45.225.239]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 7231C30001A1; Fri, 27 Mar 2026 10:41:24 +0000 (UTC) Date: Fri, 27 Mar 2026 11:41:22 +0100 From: Kevin Wolf To: hongmainquan Cc: qemu-devel@nongnu.org, dave@treblig.org, armbru@redhat.com, wubo.bob@bytedance.com Subject: Re: [RFC] monitor: Fix deadlock in monitor_cleanup Message-ID: References: <20260323133744.46493-1-hongmianquan@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 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: -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, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-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: 2.1.29 Precedence: list List-Id: qemu development 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 27.03.2026 um 08:56 hat hongmainquan geschrieben: > 在 2026/3/27 03:32, Kevin Wolf 写道: > > Am 23.03.2026 um 14:37 hat hongmianquan geschrieben: > >> During qemu_cleanup, if a non-coroutine QMP command (e.g., > >> query-commands) is concurrently received and processed by the > >> mon_iothread, it can lead to a deadlock in monitor_cleanup. > >> > >> The root cause is a race condition between the main thread's shutdown > >> sequence and the coroutine's dispatching mechanism. When handling a > >> non-coroutine QMP command, qmp_dispatcher_co schedules the actual > >> command execution as a bottom half in iohandler_ctx and then yields. > >> At this suspended point, qmp_dispatcher_co_busy remains true. > >> Subsequently, the main thread in monitor_cleanup(), sets > >> qmp_dispatcher_co_shutdown, and calls qmp_dispatcher_co_wake(). Since > >> qmp_dispatcher_co_busy is already true, the aio_co_wake is skipped. > >> The main thread then enters the AIO_WAIT_WHILE_UNLOCKED loop, it > >> executes the scheduled BH (do_qmp_dispatch_bh) via > >> aio_poll(iohandler_ctx, false), which attempts to wake up the > >> coroutine, aio_co_wake schedules a new wake-up BH in iohandler_ctx. > >> The main thread then blocks indefinitely in aio_poll(qemu_aio_context, > >> true), while the coroutine's wake-up BH is starved in iohandler_ctx, > >> qmp_dispatcher_co never reaches termination, resulting in a deadlock. > > > > If the real problem is that the aio_poll() in the main thread is never > > woken up, does this fix the problem? (Completely untested, and would > > need a comment if we commit it.) > > > After our reproduction tests, this change also resolves the issue and > proves to be a more appropriate solution. > Should we proceed with this commit based on our discussion, or do you > require further validation? If it works for you, let's move forward with this. I think we just should add a comment like "if we're in AIO_WAIT_WHILE_UNLOCKED() in monitor_cleanup(), make sure that it doesn't wait for an event in the main context, but iohandler_ctx is polled" in the real commit because it's not obvious why the aio_wait_kick() is needed here. > Thanks, > > diff --git a/qapi/qmp-dispatch.c b/qapi/qmp-dispatch.c > > index 9bb1e6a9f4a..0a0e9e07756 100644 > > --- a/qapi/qmp-dispatch.c > > +++ b/qapi/qmp-dispatch.c > > @@ -128,6 +128,7 @@ static void do_qmp_dispatch_bh(void *opaque) > > data->cmd->fn(data->args, data->ret, data->errp); > > monitor_set_cur(qemu_coroutine_self(), NULL); > > aio_co_wake(data->co); > > + aio_wait_kick(); > > } Do you want to send a v2 patch that takes this approach, or would you prefer if I send it as a proper patch? Kevin