diff --git a/src/ws/tiny_httpd_ws.ml b/src/ws/tiny_httpd_ws.ml
index a0cf01e1..3c4fdf03 100644
--- a/src/ws/tiny_httpd_ws.ml
+++ b/src/ws/tiny_httpd_ws.ml
@@ -19,8 +19,6 @@ module With_lock = struct
             Mutex.unlock mutex;
             raise e);
     }
-
-  let builder : builder ref = ref default_builder
 end
 
 type handler = unit Request.t -> IO.Input.t -> IO.Output.t -> unit
@@ -78,7 +76,7 @@ module Writer = struct
     mutex: With_lock.t;
   }
 
-  let create ?(buf_size = 16 * 1024) ~oc () : t =
+  let create ?(buf_size = 16 * 1024) ~with_lock ~oc () : t =
     {
       header = Header.create ();
       header_buf = Bytes.create 16;
@@ -86,7 +84,7 @@ module Writer = struct
       offset = 0;
       oc;
       closed = false;
-      mutex = !With_lock.builder ();
+      mutex = with_lock;
     }
 
   let[@inline] close self = self.closed <- true
@@ -403,8 +401,8 @@ module Reader = struct
     )
 end
 
-let upgrade ic oc : _ * _ =
-  let writer = Writer.create ~oc () in
+let upgrade ?(with_lock = With_lock.default_builder ()) ic oc : _ * _ =
+  let writer = Writer.create ~with_lock ~oc () in
   let reader = Reader.create ~ic ~writer () in
   let ws_ic : IO.Input.t =
     object
@@ -431,6 +429,7 @@ let upgrade ic oc : _ * _ =
     upgrade handler *)
 module Make_upgrade_handler (X : sig
   val accept_ws_protocol : string -> bool
+  val with_lock : With_lock.builder
   val handler : handler
 end) : Server.UPGRADE_HANDLER with type handshake_state = unit Request.t =
 struct
@@ -475,7 +474,8 @@ struct
     try Ok (handshake_ req) with Bad_req s -> Error s
 
   let handle_connection req ic oc =
-    let ws_ic, ws_oc = upgrade ic oc in
+    let with_lock = X.with_lock () in
+    let ws_ic, ws_oc = upgrade ~with_lock ic oc in
     try X.handler req ws_ic ws_oc
     with Close_connection ->
       Log.debug (fun k -> k "websocket: requested to close the connection");
@@ -483,9 +483,11 @@ struct
 end
 
 let add_route_handler ?accept ?(accept_ws_protocol = fun _ -> true) ?middlewares
-    (server : Server.t) route (f : handler) : unit =
+    ?(with_lock = With_lock.default_builder) (server : Server.t) route
+    (f : handler) : unit =
   let module M = Make_upgrade_handler (struct
     let handler = f
+    let with_lock = with_lock
     let accept_ws_protocol = accept_ws_protocol
   end) in
   let up : Server.upgrade_handler = (module M) in
diff --git a/src/ws/tiny_httpd_ws.mli b/src/ws/tiny_httpd_ws.mli
index 4066b2b4..30120454 100644
--- a/src/ws/tiny_httpd_ws.mli
+++ b/src/ws/tiny_httpd_ws.mli
@@ -3,11 +3,36 @@
     This sub-library ([tiny_httpd.ws]) exports a small implementation for a
     websocket server. It has no additional dependencies. *)
 
+(** Synchronization primitive used to allow both the reader to reply to "ping",
+    and the handler to send messages, without stepping on each other's toes.
+
+    @since NEXT_RELEASE *)
+module With_lock : sig
+  type t = { with_lock: 'a. (unit -> 'a) -> 'a }
+  (** A primitive to run the callback in a critical section where others cannot
+      run at the same time.
+
+      The default is a mutex, but that works poorly with thread pools so it's
+      possible to use a semaphore or a cooperative mutex instead. *)
+
+  type builder = unit -> t
+
+  val default_builder : builder
+  (** Lock using [Mutex]. *)
+end
+
 type handler = unit Request.t -> IO.Input.t -> IO.Output.t -> unit
 (** Websocket handler *)
 
-val upgrade : IO.Input.t -> IO.Output.t -> IO.Input.t * IO.Output.t
-(** Upgrade a byte stream to the websocket framing protocol. *)
+val upgrade :
+  ?with_lock:With_lock.t ->
+  IO.Input.t ->
+  IO.Output.t ->
+  IO.Input.t * IO.Output.t
+(** Upgrade a byte stream to the websocket framing protocol.
+    @param with_lock
+      if provided, use this to prevent reader and writer to compete on sending
+      frames. since NEXT_RELEASE. *)
 
 exception Close_connection
 (** Exception that can be raised from IOs inside the handler, when the
@@ -17,6 +42,7 @@ val add_route_handler :
   ?accept:(unit Request.t -> (unit, int * string) result) ->
   ?accept_ws_protocol:(string -> bool) ->
   ?middlewares:Server.Head_middleware.t list ->
+  ?with_lock:With_lock.builder ->
   Server.t ->
   (Server.upgrade_handler, Server.upgrade_handler) Route.t ->
   handler ->
@@ -24,7 +50,11 @@ val add_route_handler :
 (** Add a route handler for a websocket endpoint.
     @param accept_ws_protocol
       decides whether this endpoint accepts the websocket protocol sent by the
-      client. Default accepts everything. *)
+      client. Default accepts everything.
+    @param with_lock
+      if provided, use this to synchronize writes between the frame reader
+      (replies "pong" to "ping") and the handler emitting writes. since
+      NEXT_RELEASE. *)
 
 (**/**)
 
@@ -33,15 +63,4 @@ module Private_ : sig
     mask_key:bytes -> mask_offset:int -> bytes -> int -> int -> unit
 end
 
-(** @since NEXT_RELEASE *)
-module With_lock : sig
-  type t = { with_lock: 'a. (unit -> 'a) -> 'a }
-  type builder = unit -> t
-
-  val default_builder : builder
-  (** Lock using [Mutex]. *)
-
-  val builder : builder ref
-end
-
 (**/**)