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 EB91E109B469 for ; Tue, 31 Mar 2026 13:24:50 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1w7Z51-0005Q2-J9; Tue, 31 Mar 2026 09:24:27 -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 1w7Z50-0005PU-6G for qemu-devel@nongnu.org; Tue, 31 Mar 2026 09:24:26 -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 1w7Z4y-0005V2-FJ for qemu-devel@nongnu.org; Tue, 31 Mar 2026 09:24:25 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1774963463; 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=7MEtMnISvWBLfWW9UDfuN4obvQ4JSRJg0CWibd49zPY=; b=CpSfheUgM6wX9ckHNthLiHTGAS2z8ttxUGetd6sAuwQs88if6jZZIF0NMYPhKw1DYfOZAp D8sVKQAv1pPyeDNem7EqpAZuiQ+9Pn8gW3+E6ACDDMatMI0jNoubTep3kKQv5GvT6+7+PD PJExB237msPyoOzv3AomDaIVM3lJwuA= 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-654-NiynDcslPzq8Z6E7R6rbIA-1; Tue, 31 Mar 2026 09:24:19 -0400 X-MC-Unique: NiynDcslPzq8Z6E7R6rbIA-1 X-Mimecast-MFC-AGG-ID: NiynDcslPzq8Z6E7R6rbIA_1774963458 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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 7C7D81944F05; Tue, 31 Mar 2026 13:24:18 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.44.22.6]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E1A0719560AB; Tue, 31 Mar 2026 13:24:17 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 8C40921E6A28; Tue, 31 Mar 2026 15:24:15 +0200 (CEST) From: Markus Armbruster To: "hongmianquan" Cc: , , , , Subject: Re: [PATCH v2] monitor: Fix deadlock in monitor_cleanup In-Reply-To: <20260327131024.51947-1-hongmianquan@bytedance.com> (hongmianquan@bytedance.com's message of "Fri, 27 Mar 2026 21:10:24 +0800") References: <20260327131024.51947-1-hongmianquan@bytedance.com> Date: Tue, 31 Mar 2026 15:24:15 +0200 Message-ID: <875x6ci134.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 27 X-Spam_score: 2.7 X-Spam_bar: ++ X-Spam_report: (2.7 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.54, 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=-0.01, RCVD_IN_SBL_CSS=3.335, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=1, RCVD_IN_VALIDITY_RPBL_BLOCKED=1, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no 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 "hongmianquan" writes: > 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. > > The execution sequence is illustrated below: > > IO Thread Main Thread (qemu_aio_context) qmp_dispatcher_co (iohandler_ctx) > | | | > |-- query-commands | | > |-- qmp_dispatcher_co_wake() | | > | (sets busy = true) | | > | | <-- Wakes up in iohandler_ctx --> | > | | |-- qmp_dispatch() > | | |-- Schedules BH (do_qmp_dispatch_bh) > | | |-- qemu_coroutine_yield() > | | [State: Suspended, busy=true] > | [ quit triggered ] | > | |-- monitor_cleanup() > | |-- qmp_dispatcher_co_shutdown = true > | |-- qmp_dispatcher_co_wake() > | | -> Checks busy flag. It's TRUE! > | | -> Skips aio_co_wake(). > | | > | |-- AIO_WAIT_WHILE_UNLOCKED: > | | |-- aio_poll(iohandler_ctx, false) > | | | -> Executes do_qmp_dispatch_bh > | | | -> Schedules 'co_schedule_bh' in iohandler_ctx > | | | > | | |-- aio_poll(qemu_aio_context, true) > | | | -> Blocks indefinitely! (Deadlock) > | | > | X (Main thread sleeping) X (Waiting for next iohandler_ctx poll) > > To fix this, we add an explicit aio_wait_kick() in do_qmp_dispatch_bh() to break the main loop out of its blocking poll, allowing it to evaluate the loop condition and poll iohandler_ctx. > > Signed-off-by: hongmianquan > Signed-off-by: wubo.bob Please line-wrap your paragraphs at 70 columns or so. The maintainer accepting the patch may do that for you, to save you a respin. Suggested-by: Kevin Wolf Acked-by: Markus Armbruster