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Add tentative fix for QMP hang

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Not exactly as sent upstream[0] since we're missing a change in our
v5.2.0 branch (irrelevant for us), but functionally works the same.

[0] https://lists.gnu.org/archive/html/qemu-devel/2021-03/msg07590.html
This commit is contained in:
Stefan Reiter 2021-03-22 16:48:25 +01:00
parent 154a7c0f8d
commit bb751cab32
2 changed files with 49 additions and 0 deletions
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48
debian/patches/extra/0010-monitor-qmp-fix-race-on-CHR_EVENT_CLOSED-without-OOB.patch vendored Normal file
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@ -0,0 +1,48 @@
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Stefan Reiter <s.reiter@proxmox.com>
Date: Mon, 22 Mar 2021 15:20:04 +0100
Subject: [PATCH] monitor/qmp: fix race on CHR_EVENT_CLOSED without OOB
The QMP dispatcher coroutine holds the qmp_queue_lock over a yield
point, where it expects to be rescheduled from the main context. If a
CHR_EVENT_CLOSED event is received just then, it can race and block the
main thread on the mutex in monitor_qmp_cleanup_queue_and_resume.
Calculate need_resume immediately after we pop a request from the queue,
so that we can release the mutex before yielding.
Suggested-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
---
monitor/qmp.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/monitor/qmp.c b/monitor/qmp.c
index 2e37d11bd3..2aff833f7a 100644
--- a/monitor/qmp.c
+++ b/monitor/qmp.c
@@ -252,6 +252,12 @@ void coroutine_fn monitor_qmp_dispatcher_co(void *data)
}
}
+ mon = req_obj->mon;
+ /* qmp_oob_enabled() might change after "qmp_capabilities" */
+ need_resume = !qmp_oob_enabled(mon) ||
+ mon->qmp_requests->length == QMP_REQ_QUEUE_LEN_MAX - 1;
+ qemu_mutex_unlock(&mon->qmp_queue_lock);
+
if (qatomic_xchg(&qmp_dispatcher_co_busy, true) == true) {
/*
* Someone rescheduled us (probably because a new requests
@@ -270,11 +276,6 @@ void coroutine_fn monitor_qmp_dispatcher_co(void *data)
aio_co_schedule(qemu_get_aio_context(), qmp_dispatcher_co);
qemu_coroutine_yield();
- mon = req_obj->mon;
- /* qmp_oob_enabled() might change after "qmp_capabilities" */
- need_resume = !qmp_oob_enabled(mon) ||
- mon->qmp_requests->length == QMP_REQ_QUEUE_LEN_MAX - 1;
- qemu_mutex_unlock(&mon->qmp_queue_lock);
if (req_obj->req) {
QDict *qdict = qobject_to(QDict, req_obj->req);
QObject *id = qdict ? qdict_get(qdict, "id") : NULL;

1
debian/patches/series vendored
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@ -7,6 +7,7 @@ extra/0006-virtiofsd-prevent-opening-of-special-files-CVE-2020-.patch
extra/0007-virtiofsd-Add-_llseek-to-the-seccomp-whitelist.patch extra/0007-virtiofsd-Add-_llseek-to-the-seccomp-whitelist.patch
extra/0008-virtiofsd-Add-restart_syscall-to-the-seccomp-whiteli.patch extra/0008-virtiofsd-Add-restart_syscall-to-the-seccomp-whiteli.patch
extra/0009-i386-acpi-restore-device-paths-for-pre-5.1-vms.patch extra/0009-i386-acpi-restore-device-paths-for-pre-5.1-vms.patch
extra/0010-monitor-qmp-fix-race-on-CHR_EVENT_CLOSED-without-OOB.patch
bitmap-mirror/0001-drive-mirror-add-support-for-sync-bitmap-mode-never.patch bitmap-mirror/0001-drive-mirror-add-support-for-sync-bitmap-mode-never.patch
bitmap-mirror/0002-drive-mirror-add-support-for-conditional-and-always-.patch bitmap-mirror/0002-drive-mirror-add-support-for-conditional-and-always-.patch
bitmap-mirror/0003-mirror-add-check-for-bitmap-mode-without-bitmap.patch bitmap-mirror/0003-mirror-add-check-for-bitmap-mode-without-bitmap.patch