block/export: wait for vhost-user-blk requests when draining

Each vhost-user-blk request runs in a coroutine. When the BlockBackend
enters a drained section we need to enter a quiescent state. Currently
any in-flight requests race with bdrv_drained_begin() because it is
unaware of vhost-user-blk requests.

When blk_co_preadv/pwritev()/etc returns it wakes the
bdrv_drained_begin() thread but vhost-user-blk request processing has
not yet finished. The request coroutine continues executing while the
main loop thread thinks it is in a drained section.

One example where this is unsafe is for blk_set_aio_context() where
bdrv_drained_begin() is called before .aio_context_detached() and
.aio_context_attach(). If request coroutines are still running after
bdrv_drained_begin(), then the AioContext could change underneath them
and they race with new requests processed in the new AioContext. This
could lead to virtqueue corruption, for example.

(This example is theoretical, I came across this while reading the
code and have not tried to reproduce it.)

It's easy to make bdrv_drained_begin() wait for in-flight requests: add
a .drained_poll() callback that checks the VuServer's in-flight counter.
VuServer just needs an API that returns true when there are requests in
flight. The in-flight counter needs to be atomic.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230516190238.8401-7-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 12 insertions, 6 deletions

diff --git a/util/vhost-user-server.c b/util/vhost-user-server.c
index 1622f8c..68c3bf1 100644
--- a/util/vhost-user-server.c
+++ b/util/vhost-user-server.c
@@ -78,17 +78,23 @@ static void panic_cb(VuDev *vu_dev, const char *buf)
 void vhost_user_server_inc_in_flight(VuServer *server)
 {
     assert(!server->wait_idle);
-    server->in_flight++;
+    qatomic_inc(&server->in_flight);
 }
 
 void vhost_user_server_dec_in_flight(VuServer *server)
 {
-    server->in_flight--;
-    if (server->wait_idle && !server->in_flight) {
-        aio_co_wake(server->co_trip);
+    if (qatomic_fetch_dec(&server->in_flight) == 1) {
+        if (server->wait_idle) {
+            aio_co_wake(server->co_trip);
+        }
     }
 }
 
+bool vhost_user_server_has_in_flight(VuServer *server)
+{
+    return qatomic_load_acquire(&server->in_flight) > 0;
+}
+
 static bool coroutine_fn
 vu_message_read(VuDev *vu_dev, int conn_fd, VhostUserMsg *vmsg)
 {
@@ -192,13 +198,13 @@ static coroutine_fn void vu_client_trip(void *opaque)
         /* Keep running */
     }
 
-    if (server->in_flight) {
+    if (vhost_user_server_has_in_flight(server)) {
         /* Wait for requests to complete before we can unmap the memory */
         server->wait_idle = true;
         qemu_coroutine_yield();
         server->wait_idle = false;
     }
-    assert(server->in_flight == 0);
+    assert(!vhost_user_server_has_in_flight(server));
 
     vu_deinit(vu_dev);