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>

block/export/vhost-user-blk-server.c

@@ -172,6 +172,7 @@ vu_blk_discard_write_zeroes(VuBlkExport *vexp, struct iovec *iov,
     return VIRTIO_BLK_S_IOERR;
 }
 
+/* Called with server refcount increased, must decrease before returning */
 static void coroutine_fn vu_blk_virtio_process_req(void *opaque)
 {
     VuBlkReq *req = opaque;
@@ -286,10 +287,12 @@ static void coroutine_fn vu_blk_virtio_process_req(void *opaque)
     }
 
     vu_blk_req_complete(req);
+    vhost_user_server_unref(server);
     return;
 
 err:
     free(req);
+    vhost_user_server_unref(server);
 }
 
 static void vu_blk_process_vq(VuDev *vu_dev, int idx)
@@ -310,6 +313,8 @@ static void vu_blk_process_vq(VuDev *vu_dev, int idx)
 
         Coroutine *co =
             qemu_coroutine_create(vu_blk_virtio_process_req, req);
+
+        vhost_user_server_ref(server);
         qemu_coroutine_enter(co);
     }
 }

include/qemu/vhost-user-server.h

@@ -42,6 +42,8 @@ typedef struct {
     const VuDevIface *vu_iface;
 
     /* Protected by ctx lock */
+    unsigned int refcount;
+    bool wait_idle;
     VuDev vu_dev;
     QIOChannel *ioc; /* The I/O channel with the client */
     QIOChannelSocket *sioc; /* The underlying data channel with the client */
@@ -59,6 +61,9 @@ bool vhost_user_server_start(VuServer *server,
 
 void vhost_user_server_stop(VuServer *server);
 
+void vhost_user_server_ref(VuServer *server);
+void vhost_user_server_unref(VuServer *server);
+
 void vhost_user_server_attach_aio_context(VuServer *server, AioContext *ctx);
 void vhost_user_server_detach_aio_context(VuServer *server);
 

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() */