On Tue, Dec 05, 2017 at 01:51:50PM +0800, Peter Xu wrote:
> @@ -3956,12 +3968,122 @@ static void handle_qmp_command(JSONMessageParser
> *parser, GQueue *tokens,
> }
> }
>
> -err_out:
> - monitor_qmp_respond(mon, rsp, err, id);
> + /* Respond if necessary */
> + monitor_qmp_respond(mon, rsp, NULL, id);
> +
> + /* This pairs with the monitor_suspend() in handle_qmp_command(). */
> + if (!qmp_oob_enabled(mon)) {
> + monitor_resume(mon);
monitor_resume() does not work between threads: if the event loop is
currently blocked in poll() it won't notice that the monitor fd should
be watched again.
Please add aio_notify() to monitor_resume() and monitor_suspend(). That
way the event loop is forced to check can_read() again.
> + }
>
> qobject_decref(req);
> }
>
> +/*
> + * Pop one QMP request from monitor queues, return NULL if not found.
> + * We are using round-robin fasion to pop the request, to avoid
> + * processing command only on a very busy monitor. To achieve that,
> + * when we processed one request on specific monitor, we put that
> + * monitor to the end of mon_list queue.
> + */
> +static QMPRequest *monitor_qmp_requests_pop_one(void)
> +{
> + QMPRequest *req_obj = NULL;
> + Monitor *mon;
> +
> + qemu_mutex_lock(&monitor_lock);
> +
> + QTAILQ_FOREACH(mon, &mon_list, entry) {
> + qemu_mutex_lock(&mon->qmp.qmp_queue_lock);
Please add a doc comment about the monitor_lock < qmp_queue_lock lock
ordering in the qmp_queue_lock field declaration so that deadlocks can
be prevented.
> + req_obj = g_queue_pop_head(mon->qmp.qmp_requests);
> + qemu_mutex_unlock(&mon->qmp.qmp_queue_lock);
> + if (req_obj) {
> + break;
> + }
> + }
> +
> + if (req_obj) {
> + /*
> + * We found one request on the monitor. Degrade this monitor's
> + * priority to lowest by re-inserting it to end of queue.
> + */
> + QTAILQ_REMOVE(&mon_list, mon, entry);
> + QTAILQ_INSERT_TAIL(&mon_list, mon, entry);
> + }
> +
> + qemu_mutex_unlock(&monitor_lock);
> +
> + return req_obj;
> +}
> +
> +static void monitor_qmp_bh_dispatcher(void *data)
> +{
> + QMPRequest *req_obj;
> +
> + while (true) {
Previously QEMU could only dispatch 1 QMP command per main loop
iteration. Now multiple requests can be processed in a single main loop
iteration.
If a producer enqueues requests faster than the dispatcher can dequeue
them then this is infinite loop will prevent the caller (i.e. QEMU main
loop) from making progress.
The following keeps 1 QMP command per main loop iteration and avoids the
infinite loop:
static void monitor_qmp_bh_dispatcher(void *data)
{
QMPRequest *req_obj = monitor_qmp_requests_pop_one();
if (req_obj) {
monitor_qmp_dispatch_one(req_obj);
/* Reschedule instead of looping so the main loop stays responsive */
qemu_bh_schedule(mon_global.qmp_dispatcher_bh);
}
}
> + /*
> + * Put the request to the end of queue so that requests will be
> + * handled in time order. Ownership for req_obj, req, id,
> + * etc. will be delivered to the handler side.
> + */
> + qemu_mutex_lock(&mon->qmp.qmp_queue_lock);
> + g_queue_push_tail(mon->qmp.qmp_requests, req_obj);
> + qemu_mutex_unlock(&mon->qmp.qmp_queue_lock);
> +
> + /* Kick the dispatcher routine */
> + qemu_bh_schedule(mon_global.qmp_dispatcher_bh);
> +
> + /*
> + * If OOB is not enabled on current monitor, we'll emulate the old
> + * behavior that we won't process current monitor any more until
> + * it is responded. This helps make sure that as long as OOB is
> + * not enabled, the server will never drop any command.
> + */
> + if (!qmp_oob_enabled(mon)) {
> + monitor_suspend(mon);
> + }
monitor_suspend() must be called before g_queue_push_tail(). Otherwise
the other thread might complete the request and call monitor_resume()
before we call monitor_suspend().
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