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 A5469109B47B for ; Tue, 31 Mar 2026 15:05:45 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1w7adZ-0003JT-UR; Tue, 31 Mar 2026 11:04:13 -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 1w7adY-0003Gy-1B for qemu-devel@nongnu.org; Tue, 31 Mar 2026 11:04:12 -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 1w7adW-0002ZF-Im for qemu-devel@nongnu.org; Tue, 31 Mar 2026 11:04:11 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1774969450; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=4/EQKTbtcH/MX2YkSmC6DrV9C1Uks/s3kAn+UlSgZnk=; b=J5PcoOpfEqzTpVBBcpCgGNPy+EBNdBXxldSYQNks2VuZsDjE0r2IkpWQjpgqBKtJ8mv5pk kHWb66ybGHTaFm/6bWhpiccfMSfAAJKifNbXhB6WErecAocbU1TlxWUhR/I55wu2Tz5O5n 0zAPjqFTau/jp3Q+kvHYv0m748VpA5s= Received: from mx-prod-mc-05.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-34-GN0xrhozMYCzet6TcjxOEw-1; Tue, 31 Mar 2026 11:04:06 -0400 X-MC-Unique: GN0xrhozMYCzet6TcjxOEw-1 X-Mimecast-MFC-AGG-ID: GN0xrhozMYCzet6TcjxOEw_1774969445 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (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-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 42032195609E; Tue, 31 Mar 2026 15:04:05 +0000 (UTC) Received: from merkur.redhat.com (unknown [10.44.50.38]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id DACDE1954102; Tue, 31 Mar 2026 15:04:03 +0000 (UTC) From: Kevin Wolf To: qemu-block@nongnu.org Cc: kwolf@redhat.com, qemu-devel@nongnu.org, peter.maydell@linaro.org Subject: [PULL 4/6] monitor: Fix deadlock in monitor_cleanup Date: Tue, 31 Mar 2026 17:03:50 +0200 Message-ID: <20260331150352.256332-5-kwolf@redhat.com> In-Reply-To: <20260331150352.256332-1-kwolf@redhat.com> References: <20260331150352.256332-1-kwolf@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 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: 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 From: hongmianquan 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. Suggested-by: Kevin Wolf Signed-off-by: hongmianquan Signed-off-by: wubo.bob Message-ID: <20260327131024.51947-1-hongmianquan@bytedance.com> Acked-by: Markus Armbruster Reviewed-by: Kevin Wolf Signed-off-by: Kevin Wolf --- qapi/qmp-dispatch.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/qapi/qmp-dispatch.c b/qapi/qmp-dispatch.c index 9bb1e6a9f4a..e3897d51977 100644 --- a/qapi/qmp-dispatch.c +++ b/qapi/qmp-dispatch.c @@ -128,6 +128,16 @@ 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); + + /* + * If the QMP dispatcher coroutine is waiting to be scheduled + * in iohandler_ctx, we must kick the main loop. This ensures + * that AIO_WAIT_WHILE_UNLOCKED() in monitor_cleanup() doesn't + * block indefinitely waiting for an event in qemu_aio_context, + * but actually gets the chance to poll iohandler_ctx and resume + * the coroutine. + */ + aio_wait_kick(); } /* -- 2.53.0