[PATCH v4 3/4] qmp: Move dispatcher to a coroutine

[Kevin Wolf]

This moves the QMP dispatcher to a coroutine and runs all QMP command
handlers that declare 'coroutine': true in coroutine context so they
can avoid blocking the main loop while doing I/O or waiting for other
events.

For commands that are not declared safe to run in a coroutine, the
dispatcher drops out of coroutine context by calling the QMP command
handler from a bottom half.

Signed-off-by: Kevin Wolf <address@hidden>
---
 include/qapi/qmp/dispatch.h |   1 +
 monitor/monitor-internal.h  |   6 +-
 monitor/monitor.c           |  33 ++++++++---
 monitor/qmp.c               | 110 ++++++++++++++++++++++++++----------
 qapi/qmp-dispatch.c         |  44 ++++++++++++++-
 qapi/qmp-registry.c         |   3 +
 util/aio-posix.c            |   7 ++-
 7 files changed, 162 insertions(+), 42 deletions(-)

diff --git a/include/qapi/qmp/dispatch.h b/include/qapi/qmp/dispatch.h
index d6ce9efc8e..6812e49b5f 100644
--- a/include/qapi/qmp/dispatch.h
+++ b/include/qapi/qmp/dispatch.h
@@ -30,6 +30,7 @@ typedef enum QmpCommandOptions
 typedef struct QmpCommand
 {
     const char *name;
+    /* Runs in coroutine context if QCO_COROUTINE is set */
     QmpCommandFunc *fn;
     QmpCommandOptions options;
     QTAILQ_ENTRY(QmpCommand) node;
diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h
index d78f5ca190..f180d03368 100644
--- a/monitor/monitor-internal.h
+++ b/monitor/monitor-internal.h
@@ -154,7 +154,9 @@ static inline bool monitor_is_qmp(const Monitor *mon)
 
 typedef QTAILQ_HEAD(MonitorList, Monitor) MonitorList;
 extern IOThread *mon_iothread;
-extern QEMUBH *qmp_dispatcher_bh;
+extern Coroutine *qmp_dispatcher_co;
+extern bool qmp_dispatcher_co_shutdown;
+extern bool qmp_dispatcher_co_busy;
 extern QmpCommandList qmp_commands, qmp_cap_negotiation_commands;
 extern QemuMutex monitor_lock;
 extern MonitorList mon_list;
@@ -172,7 +174,7 @@ void monitor_fdsets_cleanup(void);
 
 void qmp_send_response(MonitorQMP *mon, const QDict *rsp);
 void monitor_data_destroy_qmp(MonitorQMP *mon);
-void monitor_qmp_bh_dispatcher(void *data);
+void coroutine_fn monitor_qmp_dispatcher_co(void *data);
 
 int get_monitor_def(int64_t *pval, const char *name);
 void help_cmd(Monitor *mon, const char *name);
diff --git a/monitor/monitor.c b/monitor/monitor.c
index 12898b6448..e753fa435d 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -53,8 +53,18 @@ typedef struct {
 /* Shared monitor I/O thread */
 IOThread *mon_iothread;
 
-/* Bottom half to dispatch the requests received from I/O thread */
-QEMUBH *qmp_dispatcher_bh;
+/* Coroutine to dispatch the requests received from I/O thread */
+Coroutine *qmp_dispatcher_co;
+
+/* Set to true when the dispatcher coroutine should terminate */
+bool qmp_dispatcher_co_shutdown;
+
+/*
+ * true if the coroutine is active and processing requests. The coroutine may
+ * only be woken up externally (e.g. from the monitor thread) after changing
+ * qmp_dispatcher_co_busy from false to true (e.g. using atomic_xchg).
+ */
+bool qmp_dispatcher_co_busy;
 
 /* Protects mon_list, monitor_qapi_event_state, monitor_destroyed.  */
 QemuMutex monitor_lock;
@@ -579,9 +589,16 @@ void monitor_cleanup(void)
     }
     qemu_mutex_unlock(&monitor_lock);
 
-    /* QEMUBHs needs to be deleted before destroying the I/O thread */
-    qemu_bh_delete(qmp_dispatcher_bh);
-    qmp_dispatcher_bh = NULL;
+    /* The dispatcher needs to stop before destroying the I/O thread */
+    qmp_dispatcher_co_shutdown = true;
+    if (!atomic_xchg(&qmp_dispatcher_co_busy, true)) {
+        aio_co_wake(qmp_dispatcher_co);
+    }
+
+    AIO_WAIT_WHILE(qemu_get_aio_context(),
+                   (aio_poll(iohandler_get_aio_context(), false),
+                    atomic_mb_read(&qmp_dispatcher_co_busy)));
+
     if (mon_iothread) {
         iothread_destroy(mon_iothread);
         mon_iothread = NULL;
@@ -604,9 +621,9 @@ void monitor_init_globals_core(void)
      * have commands assuming that context.  It would be nice to get
      * rid of those assumptions.
      */
-    qmp_dispatcher_bh = aio_bh_new(iohandler_get_aio_context(),
-                                   monitor_qmp_bh_dispatcher,
-                                   NULL);
+    qmp_dispatcher_co = qemu_coroutine_create(monitor_qmp_dispatcher_co, NULL);
+    atomic_mb_set(&qmp_dispatcher_co_busy, true);
+    aio_co_schedule(iohandler_get_aio_context(), qmp_dispatcher_co);
 }
 
 QemuOptsList qemu_mon_opts = {
diff --git a/monitor/qmp.c b/monitor/qmp.c
index 54c06ba824..9444de9fcf 100644
--- a/monitor/qmp.c
+++ b/monitor/qmp.c
@@ -133,6 +133,10 @@ static void monitor_qmp_respond(MonitorQMP *mon, QDict 
*rsp)
     }
 }
 
+/*
+ * Runs outside of coroutine context for OOB commands, but in coroutine context
+ * for everything else.
+ */
 static void monitor_qmp_dispatch(MonitorQMP *mon, QObject *req)
 {
     Monitor *old_mon;
@@ -211,43 +215,87 @@ static QMPRequest 
*monitor_qmp_requests_pop_any_with_lock(void)
     return req_obj;
 }
 
-void monitor_qmp_bh_dispatcher(void *data)
+void coroutine_fn monitor_qmp_dispatcher_co(void *data)
 {
-    QMPRequest *req_obj = monitor_qmp_requests_pop_any_with_lock();
+    QMPRequest *req_obj = NULL;
     QDict *rsp;
     bool need_resume;
     MonitorQMP *mon;
 
-    if (!req_obj) {
-        return;
-    }
+    while (true) {
+        assert(atomic_mb_read(&qmp_dispatcher_co_busy) == true);
+
+        /* Mark the dispatcher as not busy already here so that we don't miss
+         * any new requests coming in the middle of our processing. */
+        atomic_mb_set(&qmp_dispatcher_co_busy, false);
+
+        while (!(req_obj = monitor_qmp_requests_pop_any_with_lock())) {
+            /* Wait to be reentered from handle_qmp_command, or terminate if
+             * qmp_dispatcher_co_shutdown is true*/
+            if (!qmp_dispatcher_co_shutdown) {
+                qemu_coroutine_yield();
+
+                /* busy must be set to true again by whoever rescheduled us to
+                 * avoid double scheduling */
+                assert(atomic_xchg(&qmp_dispatcher_co_busy, false) == true);
+            }
+
+            /* qmp_dispatcher_co_shutdown may have changed if we yielded and
+             * were reentered from monitor_cleanup() */
+            if (qmp_dispatcher_co_shutdown) {
+                return;
+            }
+        }
 
-    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;
-        trace_monitor_qmp_cmd_in_band(qobject_get_try_str(id) ?: "");
-        monitor_qmp_dispatch(mon, req_obj->req);
-    } else {
-        assert(req_obj->err);
-        rsp = qmp_error_response(req_obj->err);
-        req_obj->err = NULL;
-        monitor_qmp_respond(mon, rsp);
-        qobject_unref(rsp);
-    }
+        if (atomic_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();
+        }
 
-    if (need_resume) {
-        /* Pairs with the monitor_suspend() in handle_qmp_command() */
-        monitor_resume(&mon->common);
-    }
-    qmp_request_free(req_obj);
+        /*
+         * 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();
+
+        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;
+            trace_monitor_qmp_cmd_in_band(qobject_get_try_str(id) ?: "");
+            monitor_qmp_dispatch(mon, req_obj->req);
+        } else {
+            assert(req_obj->err);
+            rsp = qmp_error_response(req_obj->err);
+            req_obj->err = NULL;
+            monitor_qmp_respond(mon, rsp);
+            qobject_unref(rsp);
+        }
+
+        if (need_resume) {
+            /* Pairs with the monitor_suspend() in handle_qmp_command() */
+            monitor_resume(&mon->common);
+        }
+        qmp_request_free(req_obj);
 
-    /* Reschedule instead of looping so the main loop stays responsive */
-    qemu_bh_schedule(qmp_dispatcher_bh);
+        /*
+         * Yield and reschedule so the main loop stays responsive.
+         *
+         * Move back to iohandler_ctx so that nested event loops for
+         * qemu_aio_context don't start new monitor commands.
+         */
+        aio_co_schedule(iohandler_get_aio_context(), qmp_dispatcher_co);
+        qemu_coroutine_yield();
+    }
 }
 
 static void handle_qmp_command(void *opaque, QObject *req, Error *err)
@@ -308,7 +356,9 @@ static void handle_qmp_command(void *opaque, QObject *req, 
Error *err)
     qemu_mutex_unlock(&mon->qmp_queue_lock);
 
     /* Kick the dispatcher routine */
-    qemu_bh_schedule(qmp_dispatcher_bh);
+    if (!atomic_xchg(&qmp_dispatcher_co_busy, true)) {
+        aio_co_wake(qmp_dispatcher_co);
+    }
 }
 
 static void monitor_qmp_read(void *opaque, const uint8_t *buf, int size)
diff --git a/qapi/qmp-dispatch.c b/qapi/qmp-dispatch.c
index bc264b3c9b..eef09d15bc 100644
--- a/qapi/qmp-dispatch.c
+++ b/qapi/qmp-dispatch.c
@@ -12,12 +12,16 @@
  */
 
 #include "qemu/osdep.h"
+
+#include "block/aio.h"
 #include "qapi/error.h"
 #include "qapi/qmp/dispatch.h"
 #include "qapi/qmp/qdict.h"
 #include "qapi/qmp/qjson.h"
 #include "sysemu/runstate.h"
 #include "qapi/qmp/qbool.h"
+#include "qemu/coroutine.h"
+#include "qemu/main-loop.h"
 
 static QDict *qmp_dispatch_check_obj(const QObject *request, bool allow_oob,
                                      Error **errp)
@@ -75,6 +79,25 @@ static QDict *qmp_dispatch_check_obj(const QObject *request, 
bool allow_oob,
     return dict;
 }
 
+typedef struct QmpDispatchBH {
+    QmpCommand *cmd;
+    QDict *args;
+    QObject **ret;
+    Error **errp;
+    Coroutine *co;
+} QmpDispatchBH;
+
+static void do_qmp_dispatch_bh(void *opaque)
+{
+    QmpDispatchBH *data = opaque;
+    data->cmd->fn(data->args, data->ret, data->errp);
+    aio_co_wake(data->co);
+}
+
+/*
+ * Runs outside of coroutine context for OOB commands, but in coroutine context
+ * for everything else.
+ */
 static QObject *do_qmp_dispatch(QmpCommandList *cmds, QObject *request,
                                 bool allow_oob, Error **errp)
 {
@@ -129,7 +152,22 @@ static QObject *do_qmp_dispatch(QmpCommandList *cmds, 
QObject *request,
         qobject_ref(args);
     }
 
-    cmd->fn(args, &ret, &local_err);
+    assert(!(oob && qemu_in_coroutine()));
+    if ((cmd->options & QCO_COROUTINE) || !qemu_in_coroutine()) {
+        cmd->fn(args, &ret, &local_err);
+    } else {
+        /* Must drop out of coroutine context for this one */
+        QmpDispatchBH data = {
+            .cmd    = cmd,
+            .args   = args,
+            .ret    = &ret,
+            .errp   = &local_err,
+            .co     = qemu_coroutine_self(),
+        };
+        aio_bh_schedule_oneshot(qemu_get_aio_context(), do_qmp_dispatch_bh,
+                                &data);
+        qemu_coroutine_yield();
+    }
     if (local_err) {
         error_propagate(errp, local_err);
     } else if (cmd->options & QCO_NO_SUCCESS_RESP) {
@@ -164,6 +202,10 @@ bool qmp_is_oob(const QDict *dict)
         && !qdict_haskey(dict, "execute");
 }
 
+/*
+ * Runs outside of coroutine context for OOB commands, but in coroutine context
+ * for everything else.
+ */
 QDict *qmp_dispatch(QmpCommandList *cmds, QObject *request,
                     bool allow_oob)
 {
diff --git a/qapi/qmp-registry.c b/qapi/qmp-registry.c
index ca00f74795..3d896aedd8 100644
--- a/qapi/qmp-registry.c
+++ b/qapi/qmp-registry.c
@@ -20,6 +20,9 @@ void qmp_register_command(QmpCommandList *cmds, const char 
*name,
 {
     QmpCommand *cmd = g_malloc0(sizeof(*cmd));
 
+    /* QCO_COROUTINE and QCO_ALLOW_OOB are incompatible */
+    assert(!((options & QCO_COROUTINE) && (options & QCO_ALLOW_OOB)));
+
     cmd->name = name;
     cmd->fn = fn;
     cmd->enabled = true;
diff --git a/util/aio-posix.c b/util/aio-posix.c
index a4977f538e..223de08b91 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -15,6 +15,7 @@
 
 #include "qemu/osdep.h"
 #include "block/block.h"
+#include "qemu/main-loop.h"
 #include "qemu/rcu_queue.h"
 #include "qemu/sockets.h"
 #include "qemu/cutils.h"
@@ -616,7 +617,11 @@ bool aio_poll(AioContext *ctx, bool blocking)
     int64_t timeout;
     int64_t start = 0;
 
-    assert(in_aio_context_home_thread(ctx));
+    /* aio_poll() may only be called in the AioContext's thread. iohandler_ctx
+     * is special in that it runs in the main thread, but that thread's context
+     * is qemu_aio_context. */
+    assert(in_aio_context_home_thread(ctx == iohandler_get_aio_context() ?
+                                      qemu_get_aio_context() : ctx));
 
     /* aio_notify can avoid the expensive event_notifier_set if
      * everything (file descriptors, bottom halves, timers) will
-- 
2.20.1