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monitor/qmp: fix race on CHR_EVENT_CLOSED without OOB

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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.

monitor_resume does not need to be called from main context, so we can
call it immediately after popping a request from the queue, which allows
us to drop the qmp_queue_lock mutex before yielding.

Suggested-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
Message-Id: <20210322154024.15011-1-s.reiter@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
This commit is contained in:
Stefan Reiter 2021-03-22 16:40:24 +01:00 committed by Markus Armbruster
parent a5ccdccc97
commit a67b996e78
1 changed files with 22 additions and 18 deletions
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40
monitor/qmp.c
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@ -257,24 +257,6 @@ void coroutine_fn monitor_qmp_dispatcher_co(void *data)
trace_monitor_qmp_in_band_dequeue(req_obj,
req_obj->mon->qmp_requests->length);
if (qatomic_xchg(&qmp_dispatcher_co_busy, true) == true) {
/*
* Someone rescheduled us (probably because a new requests
* came in), but we didn't actually yield. Do that now,
* only to be immediately reentered and removed from the
* list of scheduled coroutines.
*/
qemu_coroutine_yield();
}
/*
* Move the coroutine from iohandler_ctx to qemu_aio_context for
* executing the command handler so that it can make progress if it
* involves an AIO_WAIT_WHILE().
*/
aio_co_schedule(qemu_get_aio_context(), qmp_dispatcher_co);
qemu_coroutine_yield();
/*
* @req_obj has a request, we hold req_obj->mon->qmp_queue_lock
*/
@ -298,8 +280,30 @@ void coroutine_fn monitor_qmp_dispatcher_co(void *data)
monitor_resume(&mon->common);
}
/*
* Drop the queue mutex now, before yielding, otherwise we might
* deadlock if the main thread tries to lock it.
*/
qemu_mutex_unlock(&mon->qmp_queue_lock);
if (qatomic_xchg(&qmp_dispatcher_co_busy, true) == true) {
/*
* Someone rescheduled us (probably because a new requests
* came in), but we didn't actually yield. Do that now,
* only to be immediately reentered and removed from the
* list of scheduled coroutines.
*/
qemu_coroutine_yield();
}
/*
* Move the coroutine from iohandler_ctx to qemu_aio_context for
* executing the command handler so that it can make progress if it
* involves an AIO_WAIT_WHILE().
*/
aio_co_schedule(qemu_get_aio_context(), qmp_dispatcher_co);
qemu_coroutine_yield();
/* Process request */
if (req_obj->req) {
if (trace_event_get_state(TRACE_MONITOR_QMP_CMD_IN_BAND)) {