[PULL v2 07/19] vhost-user: Monitor slave channel in vhost_user_read()

["Michael S. Tsirkin" <mst@redhat.com>]

From: Greg Kurz <groug@kaod.org>

Now that everything is in place, have the nested event loop to monitor
the slave channel. The source in the main event loop is destroyed and
recreated to ensure any pending even for the slave channel that was
previously detected is purged. This guarantees that the main loop
wont invoke slave_read() based on an event that was already handled
by the nested loop.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <20210312092212.782255-7-groug@kaod.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/virtio/vhost-user.c | 35 ++++++++++++++++++++++++++++++++---
 1 file changed, 32 insertions(+), 3 deletions(-)

diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index 00256fa318..ded0c10453 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -345,6 +345,35 @@ end:
     return G_SOURCE_REMOVE;
 }
 
+static gboolean slave_read(QIOChannel *ioc, GIOCondition condition,
+                           gpointer opaque);
+
+/*
+ * This updates the read handler to use a new event loop context.
+ * Event sources are removed from the previous context : this ensures
+ * that events detected in the previous context are purged. They will
+ * be re-detected and processed in the new context.
+ */
+static void slave_update_read_handler(struct vhost_dev *dev,
+                                      GMainContext *ctxt)
+{
+    struct vhost_user *u = dev->opaque;
+
+    if (!u->slave_ioc) {
+        return;
+    }
+
+    if (u->slave_src) {
+        g_source_destroy(u->slave_src);
+        g_source_unref(u->slave_src);
+    }
+
+    u->slave_src = qio_channel_add_watch_source(u->slave_ioc,
+                                                G_IO_IN | G_IO_HUP,
+                                                slave_read, dev, NULL,
+                                                ctxt);
+}
+
 static int vhost_user_read(struct vhost_dev *dev, VhostUserMsg *msg)
 {
     struct vhost_user *u = dev->opaque;
@@ -366,6 +395,7 @@ static int vhost_user_read(struct vhost_dev *dev, VhostUserMsg *msg)
      * be prepared for re-entrancy. So we create a new one and switch chr
      * to use it.
      */
+    slave_update_read_handler(dev, ctxt);
     qemu_chr_be_update_read_handlers(chr->chr, ctxt);
     qemu_chr_fe_add_watch(chr, G_IO_IN | G_IO_HUP, vhost_user_read_cb, &data);
 
@@ -377,6 +407,7 @@ static int vhost_user_read(struct vhost_dev *dev, VhostUserMsg *msg)
      * context that have been processed by the nested loop are purged.
      */
     qemu_chr_be_update_read_handlers(chr->chr, prev_ctxt);
+    slave_update_read_handler(dev, NULL);
 
     g_main_loop_unref(loop);
     g_main_context_unref(ctxt);
@@ -1580,9 +1611,7 @@ static int vhost_setup_slave_channel(struct vhost_dev *dev)
         return -1;
     }
     u->slave_ioc = ioc;
-    u->slave_src = qio_channel_add_watch_source(u->slave_ioc,
-                                                G_IO_IN | G_IO_HUP,
-                                                slave_read, dev, NULL, NULL);
+    slave_update_read_handler(dev, NULL);
 
     if (reply_supported) {
         msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
-- 
MST