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GET /api/patches/2216966/?format=api
{ "id": 2216966, "url": "http://patchwork.ozlabs.org/api/patches/2216966/?format=api", "web_url": "http://patchwork.ozlabs.org/project/qemu-devel/patch/20260327131024.51947-1-hongmianquan@bytedance.com/", "project": { "id": 14, "url": "http://patchwork.ozlabs.org/api/projects/14/?format=api", "name": "QEMU Development", "link_name": "qemu-devel", "list_id": "qemu-devel.nongnu.org", "list_email": "qemu-devel@nongnu.org", "web_url": "", "scm_url": "", "webscm_url": "", "list_archive_url": "", "list_archive_url_format": "", "commit_url_format": "" }, "msgid": "<20260327131024.51947-1-hongmianquan@bytedance.com>", "list_archive_url": null, "date": "2026-03-27T13:10:24", "name": "[v2] monitor: Fix deadlock in monitor_cleanup", "commit_ref": null, "pull_url": null, "state": "new", "archived": false, "hash": "2aee69fffb4d78a91d968f5e043693107650495c", "submitter": { "id": 86895, "url": "http://patchwork.ozlabs.org/api/people/86895/?format=api", "name": "hongmianquan", "email": "hongmianquan@bytedance.com" }, "delegate": null, "mbox": "http://patchwork.ozlabs.org/project/qemu-devel/patch/20260327131024.51947-1-hongmianquan@bytedance.com/mbox/", "series": [ { "id": 497768, "url": "http://patchwork.ozlabs.org/api/series/497768/?format=api", "web_url": "http://patchwork.ozlabs.org/project/qemu-devel/list/?series=497768", "date": "2026-03-27T13:10:24", "name": "[v2] monitor: Fix deadlock in monitor_cleanup", "version": 2, "mbox": "http://patchwork.ozlabs.org/series/497768/mbox/" } ], "comments": "http://patchwork.ozlabs.org/api/patches/2216966/comments/", "check": "pending", "checks": "http://patchwork.ozlabs.org/api/patches/2216966/checks/", "tags": {}, "related": [], "headers": { "Return-Path": "<qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org>", "X-Original-To": "incoming@patchwork.ozlabs.org", "Delivered-To": "patchwork-incoming@legolas.ozlabs.org", "Authentication-Results": [ "legolas.ozlabs.org;\n\tdkim=pass (2048-bit key;\n unprotected) header.d=bytedance.com header.i=@bytedance.com\n header.a=rsa-sha256 header.s=2212171451 header.b=jfcNvF8t;\n\tdkim-atps=neutral", "legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org\n (client-ip=209.51.188.17; helo=lists.gnu.org;\n envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org;\n receiver=patchwork.ozlabs.org)" ], "Received": [ "from lists.gnu.org (lists.gnu.org [209.51.188.17])\n\t(using TLSv1.2 with cipher ECDHE-ECDSA-AES256-GCM-SHA384 (256/256 bits))\n\t(No client certificate requested)\n\tby legolas.ozlabs.org (Postfix) with ESMTPS id 4fj1Hs6Cmnz1y1x\n\tfor <incoming@patchwork.ozlabs.org>; Sat, 28 Mar 2026 00:11:37 +1100 (AEDT)", "from localhost ([::1] helo=lists1p.gnu.org)\n\tby lists.gnu.org with esmtp (Exim 4.90_1)\n\t(envelope-from <qemu-devel-bounces@nongnu.org>)\n\tid 1w66xr-0000ph-WA; Fri, 27 Mar 2026 09:11:04 -0400", "from eggs.gnu.org ([2001:470:142:3::10])\n by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256)\n (Exim 4.90_1) (envelope-from <hongmianquan@bytedance.com>)\n id 1w66xo-0000pY-Ue\n for qemu-devel@nongnu.org; Fri, 27 Mar 2026 09:11:01 -0400", "from va-1-111.ptr.blmpb.com ([209.127.230.111])\n by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128)\n (Exim 4.90_1) (envelope-from <hongmianquan@bytedance.com>)\n id 1w66xm-0005zZ-0o\n for qemu-devel@nongnu.org; Fri, 27 Mar 2026 09:11:00 -0400" ], "DKIM-Signature": "v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed;\n s=2212171451; d=bytedance.com; t=1774617040; h=from:subject:\n mime-version:from:date:message-id:subject:to:cc:reply-to:content-type:\n mime-version:in-reply-to:message-id;\n bh=l/lgnMIV8DNoZCBSRTWY7xYrOxj5DgN3eWBfLFhjboU=;\n b=jfcNvF8tFlaYJ1nkitWy70Ksy9cV2OqJRFntlKCrttASDwgPUYNQUlz88np3agWqK5mctP\n xJ6vD1oZRbgDNN01SsrQUvcwyWOmD8YkjZYiNXZKICHH3ECkO+f6EYU8f/a/k6pWX6CTrP\n VsC58JKx2k3aH/fpIPsxwCCLuysGdGZd/6M2A4YDWK5X8yL5CgMi/+/a0gtyPuSMBsXNG7\n kx1NuxSsnFF+6xygoGjK+nTS9Cr7KJ2JQnwTe306CVamsSyUqudhdlyzS9GDzpvCQUi2PP\n wkjdvp6Qt/0aaaMlUMOjVygMzfJYXUxr4IluzjsIMcdnLmAXjA5PrdoMZDn8lw==", "Date": "Fri, 27 Mar 2026 21:10:24 +0800", "X-Mailer": "git-send-email 2.32.1 (Apple Git-133)", "To": "<qemu-devel@nongnu.org>", "Cc": "<kwolf@redhat.com>, <armbru@redhat.com>, <michael.roth@amd.com>,\n <wubo.bob@bytedance.com>, \"hongmianquan\" <hongmianquan@bytedance.com>", "Message-Id": "<20260327131024.51947-1-hongmianquan@bytedance.com>", "Mime-Version": "1.0", "X-Original-From": "hongmianquan <hongmianquan@bytedance.com>", "X-Lms-Return-Path": "\n <lba+269c681ce+2f99e9+nongnu.org+hongmianquan@bytedance.com>", "From": "\"hongmianquan\" <hongmianquan@bytedance.com>", "Subject": "[PATCH v2] monitor: Fix deadlock in monitor_cleanup", "Content-Transfer-Encoding": "7bit", "Content-Type": "text/plain; charset=UTF-8", "Received-SPF": "pass client-ip=209.127.230.111;\n 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,\n DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1,\n RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001,\n 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 <qemu-devel.nongnu.org>", "List-Unsubscribe": "<https://lists.nongnu.org/mailman/options/qemu-devel>,\n <mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>", "List-Archive": "<https://lists.nongnu.org/archive/html/qemu-devel>", "List-Post": "<mailto:qemu-devel@nongnu.org>", "List-Help": "<mailto:qemu-devel-request@nongnu.org?subject=help>", "List-Subscribe": "<https://lists.nongnu.org/mailman/listinfo/qemu-devel>,\n <mailto:qemu-devel-request@nongnu.org?subject=subscribe>", "Errors-To": "qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org", "Sender": "qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org" }, "content": "During qemu_cleanup, if a non-coroutine QMP command (e.g., query-commands) is concurrently\nreceived and processed by the mon_iothread, it can lead to a deadlock in monitor_cleanup.\n\nThe 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.\nSubsequently, 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.\n\nThe execution sequence is illustrated below:\n\n IO Thread Main Thread (qemu_aio_context) qmp_dispatcher_co (iohandler_ctx)\n | | |\n |-- query-commands | |\n |-- qmp_dispatcher_co_wake() | |\n | (sets busy = true) | |\n | | <-- Wakes up in iohandler_ctx --> |\n | | |-- qmp_dispatch()\n | | |-- Schedules BH (do_qmp_dispatch_bh)\n | | |-- qemu_coroutine_yield()\n | | [State: Suspended, busy=true]\n | [ quit triggered ] |\n | |-- monitor_cleanup()\n | |-- qmp_dispatcher_co_shutdown = true\n | |-- qmp_dispatcher_co_wake()\n | | -> Checks busy flag. It's TRUE!\n | | -> Skips aio_co_wake().\n | |\n | |-- AIO_WAIT_WHILE_UNLOCKED:\n | | |-- aio_poll(iohandler_ctx, false)\n | | | -> Executes do_qmp_dispatch_bh\n | | | -> Schedules 'co_schedule_bh' in iohandler_ctx\n | | |\n | | |-- aio_poll(qemu_aio_context, true)\n | | | -> Blocks indefinitely! (Deadlock)\n | |\n | X (Main thread sleeping) X (Waiting for next iohandler_ctx poll)\n\nTo 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.\n\nSigned-off-by: hongmianquan <hongmianquan@bytedance.com>\nSigned-off-by: wubo.bob <wubo.bob@bytedance.com>\n---\n qapi/qmp-dispatch.c | 10 ++++++++++\n 1 file changed, 10 insertions(+)", "diff": "diff --git a/qapi/qmp-dispatch.c b/qapi/qmp-dispatch.c\nindex 9bb1e6a9f4..e3897d5197 100644\n--- a/qapi/qmp-dispatch.c\n+++ b/qapi/qmp-dispatch.c\n@@ -128,6 +128,16 @@ static void do_qmp_dispatch_bh(void *opaque)\n data->cmd->fn(data->args, data->ret, data->errp);\n monitor_set_cur(qemu_coroutine_self(), NULL);\n aio_co_wake(data->co);\n+\n+ /*\n+ * If the QMP dispatcher coroutine is waiting to be scheduled\n+ * in iohandler_ctx, we must kick the main loop. This ensures\n+ * that AIO_WAIT_WHILE_UNLOCKED() in monitor_cleanup() doesn't\n+ * block indefinitely waiting for an event in qemu_aio_context,\n+ * but actually gets the chance to poll iohandler_ctx and resume\n+ * the coroutine.\n+ */\n+ aio_wait_kick();\n }\n \n /*\n", "prefixes": [ "v2" ] }