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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= X-MC-Unique: GN0xrhozMYCzet6TcjxOEw-1 X-Mimecast-MFC-AGG-ID: GN0xrhozMYCzet6TcjxOEw_1774969445 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: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; 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charset="utf-8" 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_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 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(); } =20 /* --=20 2.53.0