From nobody Sun Apr 5 13:08:00 2026 Delivered-To: importer@patchew.org Authentication-Results: mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=pass(p=quarantine dis=none) header.from=bytedance.com ARC-Seal: i=1; a=rsa-sha256; t=1774273132; cv=none; d=zohomail.com; s=zohoarc; b=F9J2iDQ2GwcbKJbaYPyj3srsPBSVTBoeT+JGNcGcDF55QovJd44OWD0pJNRSUAKbJC9SzYDz7NkSUQIpdQsY0d2J8J3BWHDfXMRDmsAAeOC4NC6gBpV5iY+BWAdEI80IUGBC2WMMtXFsrmAfZgjANlMhyAtVyR/Au+0n1xxZL98= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=zohomail.com; s=zohoarc; t=1774273132; h=Content-Type:Content-Transfer-Encoding:Cc:Cc:Date:Date:From:From:List-Subscribe:List-Post:List-Id:List-Archive:List-Help:List-Unsubscribe:MIME-Version:Message-ID:Sender:Subject:Subject:To:To:Message-Id:Reply-To; bh=P0N0Blz9eTWsx1Ygi7EPAoiTQsWDhfzDt4IseD6bdPA=; b=RxYg278pRxGGiNAOxYHLvdb87AcYw4qy67Km6B1zNLnMAa/UQKUc2TFO5r6e3GJmaWrwiNOLzZ0GoI+gQE+ysxHhOjg9dcLKCukFdG6oB7ERH8Pe0EHtZZyt4H1VrAk+J0F1Ue4GuL+kF1yaBh/TJSHRdS3X5vAJueAO3YAmSaE= ARC-Authentication-Results: i=1; mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=pass header.from= (p=quarantine dis=none) Return-Path: Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1774273132266795.4040329179591; Mon, 23 Mar 2026 06:38:52 -0700 (PDT) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1w4fU5-0004JB-Cg; Mon, 23 Mar 2026 09:38:21 -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 1w4fU2-0004IV-08 for qemu-devel@nongnu.org; Mon, 23 Mar 2026 09:38:18 -0400 Received: from va-1-111.ptr.blmpb.com ([209.127.230.111]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1w4fTr-00035M-RS for qemu-devel@nongnu.org; Mon, 23 Mar 2026 09:38:11 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; s=2212171451; d=bytedance.com; t=1774273077; h=from:subject: mime-version:from:date:message-id:subject:to:cc:reply-to:content-type: mime-version:in-reply-to:message-id; bh=P0N0Blz9eTWsx1Ygi7EPAoiTQsWDhfzDt4IseD6bdPA=; b=QwUyIkaK4OqwYg9YRZ94bA1bNNrtOIedwm9KR3m/Zf4bJommV/QaPGCtzRBFw0QDR0/y++ jm5Rh/nXJguetEQP8tlWfC6s2YUXDk14+fsyykkFGyUWewiHTj/MfwJ8txzh9jV7KAl/MN 8PQIf/NYkxuOC5DdD8XwhG1NEkrJInbPzxxGN3jUDTYuKyGB6oVFOyhJe1Ec6SEHkFq1Cv tLIqnuyqnA6qkvPGXJ3HjhZsfZO3ZdItnuY0SOBhY4la78q9rgElzRRzG1H1Sm03WbbuI0 BJ0U0awp6pB50s8J9FSOkxeyeNlNHZhH4Nn1eknqPVbLJlcx9nWzXTkZU8Onhw== Mime-Version: 1.0 X-Original-From: hongmianquan Cc: , , , "hongmianquan" From: "hongmianquan" Date: Mon, 23 Mar 2026 21:37:44 +0800 Message-Id: <20260323133744.46493-1-hongmianquan@bytedance.com> X-Lms-Return-Path: To: Subject: [RFC] monitor: Fix deadlock in monitor_cleanup X-Mailer: git-send-email 2.32.1 (Apple Git-133) Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=209.127.230.111; envelope-from=hongmianquan@bytedance.com; helo=va-1-111.ptr.blmpb.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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=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+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @bytedance.com) X-ZM-MESSAGEID: 1774273135507158500 Content-Type: text/plain; charset="utf-8" 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 mo= nitor_cleanup. The root cause is a race condition between the main thread's shutdown seque= nce and the coroutine's dispatching mechanism. When handling a non-coroutin= e QMP command, qmp_dispatcher_co schedules the actual command execution as = a bottom half in iohandler_ctx and then yields. At this suspended point, qm= p_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 th= e AIO_WAIT_WHILE_UNLOCKED loop, it executes the scheduled BH (do_qmp_dispat= ch_bh) via aio_poll(iohandler_ctx, false), which attempts to wake up the co= routine, 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 n= ever reaches termination, resulting in a deadlock. The execution sequence is illustrated below: IO Thread Main Thread (qemu_aio_context) qmp_dispat= cher_co (iohandler_ctx) | | = | |-- query-commands | = | |-- qmp_dispatcher_co_wake() | = | | (sets busy =3D true) | = | | | <-- Wakes up in iohandler_ctx -->= | | | = |-- qmp_dispatch() | | = |-- Schedules BH (do_qmp_dispatch_bh) | | = |-- qemu_coroutine_yield() | | = [State: Suspended, busy=3Dtrue] | [ quit triggered ] | | |-- monitor_cleanup() | |-- qmp_dispatcher_co_shutdown =3D tr= ue | |-- 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, tr= ue) | | | -> Blocks indefinitely! (Dea= dlock) | | | X (Main thread sleeping) = X (Waiting for next iohandler_ctx poll) To fix this, we should always wake up the dispatcher coroutine if shutdown = is requested, bypassing the qmp_dispatcher_co_busy check. This ensures that= the coroutine can process the shutdown signal and exit cleanly, breaking t= he deadlock. Signed-off-by: hongmianquan Signed-off-by: wubo.bob --- monitor/monitor-internal.h | 1 + monitor/monitor.c | 7 ++++++- monitor/qmp.c | 6 ++++++ 3 files changed, 13 insertions(+), 1 deletion(-) diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h index feca111ae3..943fc832f5 100644 --- a/monitor/monitor-internal.h +++ b/monitor/monitor-internal.h @@ -180,6 +180,7 @@ void qmp_send_response(MonitorQMP *mon, const QDict *rs= p); void monitor_data_destroy_qmp(MonitorQMP *mon); void coroutine_fn monitor_qmp_dispatcher_co(void *data); void qmp_dispatcher_co_wake(void); +void qmp_dispatcher_co_wake_force(void); =20 int get_monitor_def(Monitor *mon, int64_t *pval, const char *name); void handle_hmp_command(MonitorHMP *mon, const char *cmdline); diff --git a/monitor/monitor.c b/monitor/monitor.c index 1273eb7260..f6c7811cf4 100644 --- a/monitor/monitor.c +++ b/monitor/monitor.c @@ -671,7 +671,12 @@ void monitor_cleanup(void) WITH_QEMU_LOCK_GUARD(&monitor_lock) { qmp_dispatcher_co_shutdown =3D true; } - qmp_dispatcher_co_wake(); + + /* + * Force wake up the dispatcher coroutine to + * make sure it notices the shutdown flag. + */ + qmp_dispatcher_co_wake_force(); =20 AIO_WAIT_WHILE_UNLOCKED(NULL, (aio_poll(iohandler_get_aio_context(), false), diff --git a/monitor/qmp.c b/monitor/qmp.c index 687019811f..1b4a55cabf 100644 --- a/monitor/qmp.c +++ b/monitor/qmp.c @@ -362,6 +362,12 @@ void qmp_dispatcher_co_wake(void) } } =20 +void qmp_dispatcher_co_wake_force(void) +{ + qatomic_set_mb(&qmp_dispatcher_co_busy, true); + aio_co_wake(qmp_dispatcher_co); +} + static void handle_qmp_command(void *opaque, QObject *req, Error *err) { MonitorQMP *mon =3D opaque; --=20 2.32.1 (Apple Git-133)