iothread: Stop threads before main() quits

Right after main_loop ends, we release various things but keep iothread
alive. The latter is not prepared to the sudden change of resources.

Specifically, after bdrv_close_all(), virtio-scsi dataplane get a
surprise at the empty BlockBackend:

(gdb) bt
    at /usr/src/debug/qemu-2.6.0/hw/scsi/virtio-scsi.c:543
    at /usr/src/debug/qemu-2.6.0/hw/scsi/virtio-scsi.c:577

It is because the d->conf.blk->root is set to NULL, then
blk_get_aio_context() returns qemu_aio_context, whereas s->ctx is still
pointing to the iothread:

    hw/scsi/virtio-scsi.c:543:

    if (s->dataplane_started) {
        assert(blk_get_aio_context(d->conf.blk) == s->ctx);
    }

To fix this, let's stop iothreads before doing bdrv_close_all().

Cc: qemu-stable@nongnu.org
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1473326931-9699-1-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit dce8921)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

include/sysemu/iothread.h

@@ -35,5 +35,6 @@ typedef struct {
 
 char *iothread_get_id(IOThread *iothread);
 AioContext *iothread_get_aio_context(IOThread *iothread);
+void iothread_stop_all(void);
 
 #endif /* IOTHREAD_H */

iothread.c

@@ -54,16 +54,25 @@ static void *iothread_run(void *opaque)
     return NULL;
 }
 
-static void iothread_instance_finalize(Object *obj)
+static int iothread_stop(Object *object, void *opaque)
 {
-    IOThread *iothread = IOTHREAD(obj);
+    IOThread *iothread;
 
-    if (!iothread->ctx) {
-        return;
+    iothread = (IOThread *)object_dynamic_cast(object, TYPE_IOTHREAD);
+    if (!iothread || !iothread->ctx) {
+        return 0;
     }
     iothread->stopping = true;
     aio_notify(iothread->ctx);
     qemu_thread_join(&iothread->thread);
+    return 0;
+}
+
+static void iothread_instance_finalize(Object *obj)
+{
+    IOThread *iothread = IOTHREAD(obj);
+
+    iothread_stop(obj, NULL);
     qemu_cond_destroy(&iothread->init_done_cond);
     qemu_mutex_destroy(&iothread->init_done_lock);
     aio_context_unref(iothread->ctx);
@@ -174,3 +183,10 @@ IOThreadInfoList *qmp_query_iothreads(Error **errp)
     object_child_foreach(container, query_one_iothread, &prev);
     return head;
 }
+
+void iothread_stop_all(void)
+{
+    Object *container = object_get_objects_root();
+
+    object_child_foreach(container, iothread_stop, NULL);
+}

vl.c

@@ -121,6 +121,7 @@ int main(int argc, char **argv)
 #include "crypto/init.h"
 #include "sysemu/replay.h"
 #include "qapi/qmp/qerror.h"
+#include "sysemu/iothread.h"
 
 #define MAX_VIRTIO_CONSOLES 1
 #define MAX_SCLP_CONSOLES 1
@@ -4603,6 +4604,7 @@ int main(int argc, char **argv, char **envp)
     trace_init_vcpu_events();
     main_loop();
     replay_disable_events();
+    iothread_stop_all();
 
     bdrv_close_all();
     pause_all_vcpus();