block/export: Fix vhost-user-blk shutdown with requests in flight

The vhost-user-blk export runs requests asynchronously in their own
coroutine. When the vhost connection goes away and we want to stop the
vhost-user server, we need to wait for these coroutines to stop before
we can unmap the shared memory. Otherwise, they would still access the
unmapped memory and crash.

This introduces a refcount to VuServer which is increased when spawning
a new request coroutine and decreased before the coroutine exits. The
memory is only unmapped when the refcount reaches zero.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20220125151435.48792-1-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 22 insertions, 0 deletions

diff --git a/util/vhost-user-server.c b/util/vhost-user-server.c
index f68287e..f66fbba 100644
--- a/util/vhost-user-server.c
+++ b/util/vhost-user-server.c
@@ -74,6 +74,20 @@ static void panic_cb(VuDev *vu_dev, const char *buf)
     error_report("vu_panic: %s", buf);
 }
 
+void vhost_user_server_ref(VuServer *server)
+{
+    assert(!server->wait_idle);
+    server->refcount++;
+}
+
+void vhost_user_server_unref(VuServer *server)
+{
+    server->refcount--;
+    if (server->wait_idle && !server->refcount) {
+        aio_co_wake(server->co_trip);
+    }
+}
+
 static bool coroutine_fn
 vu_message_read(VuDev *vu_dev, int conn_fd, VhostUserMsg *vmsg)
 {
@@ -177,6 +191,14 @@ static coroutine_fn void vu_client_trip(void *opaque)
         /* Keep running */
     }
 
+    if (server->refcount) {
+        /* Wait for requests to complete before we can unmap the memory */
+        server->wait_idle = true;
+        qemu_coroutine_yield();
+        server->wait_idle = false;
+    }
+    assert(server->refcount == 0);
+
     vu_deinit(vu_dev);
 
     /* vu_deinit() should have called remove_watch() */