[Qemu-devel] [PATCH 2/2] monitor: delay monitor iothread creation

[Wolfgang Bumiller]

Commit d32749deb615 moved the call to monitor_init_globals()
to before os_daemonize(), making it an unsuitable place to
spawn the monitor iothread as it won't be inherited over the
fork() in os_daemonize().

We now spawn the thread the first time we instantiate a
monitor which actually has use_io_thread == true.
Instantiation of monitors happens only after os_daemonize().
We still need to create the qmp_dispatcher_bh when not using
iothreads, so this now still happens in
monitor_init_globals().

Signed-off-by: Wolfgang Bumiller <address@hidden>
Fixes: d32749deb615 ("monitor: move init global earlier")
---
 monitor.c | 38 +++++++++++++++++++++++---------------
 1 file changed, 23 insertions(+), 15 deletions(-)

diff --git a/monitor.c b/monitor.c
index 4802148c82..e9d44b443b 100644
--- a/monitor.c
+++ b/monitor.c
@@ -806,9 +806,14 @@ static void monitor_qapi_event_init(void)
 
 static void handle_hmp_command(Monitor *mon, const char *cmdline);
 
+static void monitor_iothread_init(void);
+
 static void monitor_data_init(Monitor *mon, bool skip_flush,
                               bool use_io_thread)
 {
+    if (use_io_thread && !mon_iothread) {
+        monitor_iothread_init();
+    }
     memset(mon, 0, sizeof(Monitor));
     qemu_mutex_init(&mon->mon_lock);
     qemu_mutex_init(&mon->qmp.qmp_queue_lock);
@@ -4551,15 +4556,6 @@ static void monitor_iothread_init(void)
     mon_iothread = iothread_create("mon_iothread", &error_abort);
 
     /*
-     * The dispatcher BH must run in the main loop thread, since we
-     * 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);
-
-    /*
      * The responder BH must be run in the monitor I/O thread, so that
      * monitors that are using the I/O thread have their output
      * written by the I/O thread.
@@ -4576,7 +4572,15 @@ void monitor_init_globals(void)
     sortcmdlist();
     qemu_mutex_init(&monitor_lock);
     qemu_mutex_init(&mon_fdsets_lock);
-    monitor_iothread_init();
+
+    /*
+     * The dispatcher BH must run in the main loop thread, since we
+     * 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);
 }
 
 /* These functions just adapt the readline interface in a typesafe way.  We
@@ -4717,7 +4721,9 @@ void monitor_cleanup(void)
      * we need to unregister from chardev below in
      * monitor_data_destroy(), and chardev is not thread-safe yet
      */
-    iothread_stop(mon_iothread);
+    if (mon_iothread) {
+        iothread_stop(mon_iothread);
+    }
 
     /*
      * Flush all response queues.  Note that even after this flush,
@@ -4738,11 +4744,13 @@ void monitor_cleanup(void)
     /* QEMUBHs needs to be deleted before destroying the I/O thread */
     qemu_bh_delete(qmp_dispatcher_bh);
     qmp_dispatcher_bh = NULL;
-    qemu_bh_delete(qmp_respond_bh);
-    qmp_respond_bh = NULL;
+    if (mon_iothread) {
+        qemu_bh_delete(qmp_respond_bh);
+        qmp_respond_bh = NULL;
 
-    iothread_destroy(mon_iothread);
-    mon_iothread = NULL;
+        iothread_destroy(mon_iothread);
+        mon_iothread = NULL;
+    }
 }
 
 QemuOptsList qemu_mon_opts = {
-- 
2.11.0