diff --git a/arrayjit/lib/backend_impl.ml b/arrayjit/lib/backend_impl.ml
index fdbc449..36f3783 100644
--- a/arrayjit/lib/backend_impl.ml
+++ b/arrayjit/lib/backend_impl.ml
@@ -77,7 +77,8 @@ end
 module Device_types (Device_config : Device_config) = struct
   include Device_config
 
-  type nonrec device = (Device_config.buffer_ptr, Device_config.dev, Device_config.event) device
+  type nonrec device =
+    (Device_config.buffer_ptr, Device_config.dev, Device_config.runner, Device_config.event) device
   [@@deriving sexp_of]
 
   type nonrec stream =
@@ -106,7 +107,8 @@ struct
       released = Atomic.make false;
       cross_stream_candidates = Hashtbl.create (module Tnode);
       owner_streams = Hashtbl.create (module Tnode);
-      stream_working_on = Hashtbl.create (module Tnode);
+      writer_stream = Hashtbl.create (module Tnode);
+      reader_streams = Hashtbl.create (module Tnode);
     }
 
   let make_stream device runner ~stream_id =
diff --git a/arrayjit/lib/backend_intf.ml b/arrayjit/lib/backend_intf.ml
index 32c9cc5..c59b13e 100644
--- a/arrayjit/lib/backend_intf.ml
+++ b/arrayjit/lib/backend_intf.ml
@@ -78,7 +78,7 @@ module type Device_config = sig
   val name : string
 end
 
-type ('buffer_ptr, 'dev, 'event) device = {
+type ('buffer_ptr, 'dev, 'runner, 'event) device = {
   dev : 'dev;
   ordinal : int;
   mutable shared_merge_buffer : 'buffer_ptr buffer option;
@@ -91,32 +91,41 @@ type ('buffer_ptr, 'dev, 'event) device = {
   cross_stream_candidates : 'buffer_ptr Hashtbl.M(Tnode).t;
       (** Freshly created arrays that might be shared across streams. The map can both grow and
           shrink. *)
-  owner_streams : int Hashtbl.M(Tnode).t;
-      (** The streams owning the given nodes. This map can only grow. *)
-  stream_working_on : (int * 'event) option Hashtbl.M(Tnode).t;
-      (** The stream that most recently has been updating the node, and the associated update
-          completion event. An entry for a tensor node is only populated when
-          {!field-queried_work_for} is also populated. *)
+  owner_streams : ('buffer_ptr, 'dev, 'runner, 'event) stream Hashtbl.M(Tnode).t;
+      (** The stream owning a given node. This map can only grow.
+
+          Currently, if the memory mode of a node is inferred, only this stream will modify a
+          cross-stream shared array. But memory modes can also be set manually. *)
+  writer_stream : (('buffer_ptr, 'dev, 'runner, 'event) stream * 'event option) Hashtbl.M(Tnode).t;
+      (** The stream that most recently has been updating (writing to) the node, and the associated
+          update completion event. *)
+  reader_streams : (('buffer_ptr, 'dev, 'runner, 'event) stream * 'event option) Hashtbl.M(Tnode).t;
+      (** The streams that most recently have been reading from the node, and the associated use
+          completion events. An entry is only populated for cross-stream shared nodes, and an event
+          only populated when multiple streams worked with the node. *)
 }
 [@@deriving sexp_of]
 
-type ('buffer_ptr, 'dev, 'runner, 'event) stream = {
-  device : ('buffer_ptr, 'dev, 'event) device;
+and ('buffer_ptr, 'dev, 'runner, 'event) stream = {
+  device : ('buffer_ptr, 'dev, 'runner, 'event) device;
   runner : 'runner;
   merge_buffer : 'buffer_ptr buffer option ref;
       (** Depending on backend implementations, either the currently used merge buffer, or the one
           most recently scheduled. *)
   mutable scheduled_merge_node : Tnode.t option;
       (** The tensor node that was most recently scheduled to be in the [stream]'s merge buffer. *)
-  stream_id : int;
+  stream_id : int;  (** An ID unique within the device. *)
   mutable allocated_buffer : 'buffer_ptr buffer option;
   queried_work_for : 'event option Hashtbl.M(Tnode).t;
-      (* The completion event for updating the node via this stream, if any. Only existing entries
-         are updated, and an entry is populated when {!work_for} is called for the first time on the
-         tensor node. *)
+      (* The completion event for updating (writing to) a node via this stream, if any. An entry
+         is populated when {!work_for} is called for the first time on the tensor node, or
+         {!field-writer_stream} needs to be updated with this stream. Otherwise, only existing
+         entries are udpated. *)
 }
 [@@deriving sexp_of]
 
+let equal_stream s1 s2 = s1.stream_id = s2.stream_id && s1.device.ordinal = s2.device.ordinal
+
 type ('buffer_ptr, 'stream) context = {
   stream : 'stream;
   parent : ('buffer_ptr, 'stream) context option;
@@ -130,7 +139,7 @@ type ('buffer_ptr, 'stream) context = {
 module type Device_types = sig
   include Device_config
 
-  type nonrec device = (buffer_ptr, dev, event) device [@@deriving sexp_of]
+  type nonrec device = (buffer_ptr, dev, runner, event) device [@@deriving sexp_of]
   type nonrec stream = (buffer_ptr, dev, runner, event) stream [@@deriving sexp_of]
   type nonrec context = (buffer_ptr, stream) context [@@deriving sexp_of]
 end
diff --git a/arrayjit/lib/backends.ml b/arrayjit/lib/backends.ml
index 1a90feb..b5a26c8 100644
--- a/arrayjit/lib/backends.ml
+++ b/arrayjit/lib/backends.ml
@@ -30,28 +30,49 @@ module Add_buffer_retrieval_and_syncing (Backend : No_buffer_retrieval_or_syncin
         | None | Some None -> default ()
         | Some (Some _ as event) -> event)
 
+  let wait_for_users ctx tn =
+    let s = ctx.stream in
+    let worked_multi_streams =
+      Hashtbl.find s.device.writer_stream tn
+      |> Option.map ~f:snd |> Option.join |> Option.is_some |> ref
+    in
+    Hashtbl.find s.device.writer_stream tn
+    |> Option.iter ~f:(fun (work_stream, e) ->
+           if not (equal_stream work_stream s) then (
+             worked_multi_streams := true;
+             Backend.will_wait_for ctx @@ Option.value e ~default:(Backend.all_work work_stream)));
+    !worked_multi_streams
+
+  let wait_for_writers ctx tn =
+    let s = ctx.stream in
+    Hashtbl.find s.device.writer_stream tn
+    |> Option.iter ~f:(fun (work_stream, e) ->
+           if not (equal_stream work_stream s) then
+             Backend.will_wait_for ctx @@ Option.value e ~default:(Backend.all_work work_stream))
+
+  let update_writer_event ~worked_multi_streams s tn =
+    if Hashtbl.mem s.queried_work_for tn || worked_multi_streams then (
+      let e = Backend.all_work s in
+      Hashtbl.update s.device.writer_stream tn ~f:(fun _ -> (s, Some e));
+      Hashtbl.update s.queried_work_for tn ~f:(Option.map ~f:(fun _ -> e)))
+    else Hashtbl.update s.device.writer_stream tn ~f:(fun _ -> (s, None))
+
   let%diagn2_l_sexp from_host (ctx : Backend.context) tn =
     match (tn, Map.find ctx.ctx_arrays tn) with
     | { Tn.array = (lazy (Some hosted)); _ }, Some dst ->
-        [%log "copying", Tn.debug_name tn, "to", (dst : Backend.buffer_ptr), "from host"];
-        let s = ctx.stream in
-        (* Wait for readers of the array before copying, if any are recorded. FIXME: this is
-           invalid. *)
-        Hashtbl.find s.device.stream_working_on tn
-        |> Option.join
-        |> Option.iter ~f:(fun (_work_stream_id, e) -> Backend.will_wait_for ctx e);
-        Backend.from_host ~dst_ptr:dst ~dst:ctx hosted;
-        (* Update the latest work event for the node. *)
-        if Hashtbl.mem s.queried_work_for tn then (
-          let e = Backend.all_work s in
-          Hashtbl.update s.device.stream_working_on tn ~f:(fun _ -> Some (s.stream_id, e));
-          Hashtbl.update s.queried_work_for tn ~f:(fun _ -> Some e));
+        ((* Wait for all users of the array before copying. *)
+         let worked_multi_streams = wait_for_users ctx tn in
+         [%log "copying", Tn.debug_name tn, "to", (dst : Backend.buffer_ptr), "from host"];
+         Backend.from_host ~dst_ptr:dst ~dst:ctx hosted;
+         update_writer_event ~worked_multi_streams ctx.stream tn);
         true
     | _ -> false
 
   let%diagn2_l_sexp to_host (ctx : Backend.context) (tn : Tn.t) =
     match (tn, Map.find ctx.ctx_arrays tn) with
     | { Tn.array = (lazy (Some hosted)); _ }, Some src ->
+        (* Only wait for writers of the array before copying. *)
+        wait_for_writers ctx tn;
         [%log "copying", Tn.debug_name tn, "at", (src : Backend.buffer_ptr), "to host"];
         Backend.to_host ~src_ptr:src ~src:ctx hosted;
         true
@@ -342,12 +363,12 @@ module Raise_backend (Device : Lowered_backend) : Backend = struct
           Map.add_exn ctx_arrays ~key ~data)
       else if Tn.known_shared_cross_stream key then (
         if Hashtbl.mem device.owner_streams key then (
-          if not (stream.stream_id = Hashtbl.find_exn device.owner_streams key) then
+          if not (equal_stream stream (Hashtbl.find_exn device.owner_streams key)) then
             raise
             @@ Utils.User_error
                  ("Cuda_backend.alloc_if_needed: node " ^ Tn.debug_name key
                 ^ " assumed to be cross-stream-shared but then written to on multiple devices"))
-        else Hashtbl.add_exn device.owner_streams ~key ~data:stream.stream_id;
+        else Hashtbl.add_exn device.owner_streams ~key ~data:stream;
         let data = Hashtbl.find_exn device.cross_stream_candidates key in
         Map.add_exn ctx_arrays ~key ~data)
       else (