diff --git a/lib/xapi-fd-test/observations.ml b/lib/xapi-fd-test/observations.ml
index d52c6a3071..d81431fd28 100644
--- a/lib/xapi-fd-test/observations.ml
+++ b/lib/xapi-fd-test/observations.ml
@@ -288,13 +288,16 @@ let observe_rw read write ~f ~size kind expected =
   let written = Buffer.create 0 in
   let rd_buf = Buffer.create 0 in
   let gw fd =
-     let r = do_write write written expected 0 (as_writable_opt fd) in
-     close fd;
-     r
+     do_write write written expected 0 (as_writable_opt fd)
   and gr fd =
      do_read ~size read rd_buf (as_readable_opt fd)
   in
-  let res, thread_result = concurrently (wrap_measure f, concurrently (gr, gw)) (rw1, (rw2,rw2)) in
+  let g fd =
+    let r = concurrently (gr, gw) (fd, fd) in
+    close fd;
+    r
+  in
+  let res, thread_result = concurrently (wrap_measure f, g) (rw1, rw2) in
   let elapsed, res = unwrap_exn res in
   let read, write = unwrap_exn thread_result in
   let read =
diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-unix/test/unixext_test.ml b/ocaml/libs/xapi-stdext/lib/xapi-stdext-unix/test/unixext_test.ml
index 973cdb8b14..dc35a4f76b 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-unix/test/unixext_test.ml
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-unix/test/unixext_test.ml
@@ -168,11 +168,12 @@ let test_proxy =
       let fd2 = Xapi_fdcaps.Operations.For_test.unsafe_fd_exn wrapped_fd2 in
       Unixext.proxy (Unix.dup fd) (Unix.dup fd2)
     in
-    match Generate.run_ro behaviour buf ~f:test2 with
+    match Generate.run_rw behaviour buf ~f:test2 with
     | _, Error (`Exn_trap (e, bt)) -> Printexc.raise_with_backtrace e bt
     | obs, Ok () -> obs
   in
-  match Generate.run_wo behaviour ~f:test with
+  let buf' = String.init behaviour.size (fun i -> Char.chr ((30+i) mod 255)) in
+  match Generate.run_rw behaviour buf' ~f:test with
   | _, Error (`Exn_trap (e, bt)) -> Printexc.raise_with_backtrace e bt
   | {read=None;_}, Ok _ -> false
   | _, Ok {write=None;_} -> false