nbd/server: Fix race in draining the export

When draining an NBD export, nbd_drained_begin() first sets
client->quiescing so that nbd_client_receive_next_request() won't start
any new request coroutines. Then nbd_drained_poll() tries to makes sure
that we wait for any existing request coroutines by checking that
client->nb_requests has become 0.

However, there is a small window between creating a new request
coroutine and increasing client->nb_requests. If a coroutine is in this
state, it won't be waited for and drain returns too early.

In the context of switching to a different AioContext, this means that
blk_aio_attached() will see client->recv_coroutine != NULL and fail its
assertion.

Fix this by increasing client->nb_requests immediately when starting the
coroutine. Doing this after the checks if we should create a new
coroutine is okay because client->lock is held.

Cc: qemu-stable@nongnu.org
Fixes: fd6afc501a01 ("nbd/server: Use drained block ops to quiesce the server")
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20240314165825.40261-2-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

nbd/server.c

@@ -3007,8 +3007,8 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
 /* Owns a reference to the NBDClient passed as opaque.  */
 static coroutine_fn void nbd_trip(void *opaque)
 {
-    NBDClient *client = opaque;
-    NBDRequestData *req = NULL;
+    NBDRequestData *req = opaque;
+    NBDClient *client = req->client;
     NBDRequest request = { 0 };    /* GCC thinks it can be used uninitialized */
     int ret;
     Error *local_err = NULL;
@@ -3037,8 +3037,6 @@ static coroutine_fn void nbd_trip(void *opaque)
         goto done;
     }
 
-    req = nbd_request_get(client);
-
     /*
      * nbd_co_receive_request() returns -EAGAIN when nbd_drained_begin() has
      * set client->quiescing but by the time we get back nbd_drained_end() may
@@ -3112,9 +3110,7 @@ static coroutine_fn void nbd_trip(void *opaque)
     }
 
 done:
-    if (req) {
-        nbd_request_put(req);
-    }
+    nbd_request_put(req);
 
     qemu_mutex_unlock(&client->lock);
 
@@ -3143,10 +3139,13 @@ disconnect:
  */
 static void nbd_client_receive_next_request(NBDClient *client)
 {
+    NBDRequestData *req;
+
     if (!client->recv_coroutine && client->nb_requests < MAX_NBD_REQUESTS &&
         !client->quiescing) {
         nbd_client_get(client);
-        client->recv_coroutine = qemu_coroutine_create(nbd_trip, client);
+        req = nbd_request_get(client);
+        client->recv_coroutine = qemu_coroutine_create(nbd_trip, req);
         aio_co_schedule(client->exp->common.ctx, client->recv_coroutine);
     }
 }