diff --git a/src/ws/tiny_httpd_ws.ml b/src/ws/tiny_httpd_ws.ml
index 41a8cc67..a0cf01e1 100644
--- a/src/ws/tiny_httpd_ws.ml
+++ b/src/ws/tiny_httpd_ws.ml
@@ -1,5 +1,28 @@
 open Common_ws_
 
+module With_lock = struct
+  type t = { with_lock: 'a. (unit -> 'a) -> 'a }
+  type builder = unit -> t
+
+  let default_builder : builder =
+   fun () ->
+    let mutex = Mutex.create () in
+    {
+      with_lock =
+        (fun f ->
+          Mutex.lock mutex;
+          try
+            let x = f () in
+            Mutex.unlock mutex;
+            x
+          with e ->
+            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
 
 module Frame_type = struct
@@ -52,7 +75,7 @@ module Writer = struct
     mutable offset: int;  (** number of bytes already in [buf] *)
     oc: IO.Output.t;
     mutable closed: bool;
-    mutex: Mutex.t;
+    mutex: With_lock.t;
   }
 
   let create ?(buf_size = 16 * 1024) ~oc () : t =
@@ -63,19 +86,9 @@ module Writer = struct
       offset = 0;
       oc;
       closed = false;
-      mutex = Mutex.create ();
+      mutex = !With_lock.builder ();
     }
 
-  let[@inline] with_mutex_ (self : t) f =
-    Mutex.lock self.mutex;
-    try
-      let x = f () in
-      Mutex.unlock self.mutex;
-      x
-    with e ->
-      Mutex.unlock self.mutex;
-      raise e
-
   let[@inline] close self = self.closed <- true
   let int_of_bool : bool -> int = Obj.magic
 
@@ -142,7 +155,7 @@ module Writer = struct
     if self.offset = Bytes.length self.buf then really_output_buf_ self
 
   let send_pong (self : t) : unit =
-    let@ () = with_mutex_ self in
+    let@ () = self.mutex.with_lock in
     self.header.fin <- true;
     self.header.ty <- Frame_type.pong;
     self.header.payload_len <- 0;
@@ -151,7 +164,7 @@ module Writer = struct
     write_header_ self
 
   let output_char (self : t) c : unit =
-    let@ () = with_mutex_ self in
+    let@ () = self.mutex.with_lock in
     let cap = Bytes.length self.buf - self.offset in
     (* make room for [c] *)
     if cap = 0 then really_output_buf_ self;
@@ -161,7 +174,7 @@ module Writer = struct
     if cap = 1 then really_output_buf_ self
 
   let output (self : t) buf i len : unit =
-    let@ () = with_mutex_ self in
+    let@ () = self.mutex.with_lock in
     let i = ref i in
     let len = ref len in
     while !len > 0 do
@@ -179,7 +192,7 @@ module Writer = struct
     flush_if_full self
 
   let flush self : unit =
-    let@ () = with_mutex_ self in
+    let@ () = self.mutex.with_lock in
     flush_ self
 end
 
@@ -187,8 +200,8 @@ module Reader = struct
   type state =
     | Begin  (** At the beginning of a frame *)
     | Reading_frame of { mutable remaining_bytes: int; mutable num_read: int }
-        (** Currently reading the payload of a frame with [remaining_bytes]
-        left to read from the underlying [ic] *)
+        (** Currently reading the payload of a frame with [remaining_bytes] left
+            to read from the underlying [ic] *)
     | Close
 
   type t = {
@@ -266,7 +279,7 @@ module Reader = struct
   external apply_masking_ :
     key:bytes -> key_offset:int -> buf:bytes -> int -> int -> unit
     = "tiny_httpd_ws_apply_masking"
-    [@@noalloc]
+  [@@noalloc]
   (** Apply masking to the parsed data *)
 
   let[@inline] apply_masking ~mask_key ~mask_offset (buf : bytes) off len : unit
@@ -414,7 +427,8 @@ let upgrade ic oc : _ * _ =
   in
   ws_ic, ws_oc
 
-(** Turn a regular connection handler (provided by the user) into a websocket upgrade handler *)
+(** Turn a regular connection handler (provided by the user) into a websocket
+    upgrade handler *)
 module Make_upgrade_handler (X : sig
   val accept_ws_protocol : string -> bool
   val handler : handler
diff --git a/src/ws/tiny_httpd_ws.mli b/src/ws/tiny_httpd_ws.mli
index 10ce1fee..4066b2b4 100644
--- a/src/ws/tiny_httpd_ws.mli
+++ b/src/ws/tiny_httpd_ws.mli
@@ -1,8 +1,7 @@
 (** Websockets for Tiny_httpd.
 
-    This sub-library ([tiny_httpd.ws]) exports a small implementation
-    for a websocket server. It has no additional dependencies.
-    *)
+    This sub-library ([tiny_httpd.ws]) exports a small implementation for a
+    websocket server. It has no additional dependencies. *)
 
 type handler = unit Request.t -> IO.Input.t -> IO.Output.t -> unit
 (** Websocket handler *)
@@ -11,8 +10,8 @@ val upgrade : IO.Input.t -> IO.Output.t -> IO.Input.t * IO.Output.t
 (** Upgrade a byte stream to the websocket framing protocol. *)
 
 exception Close_connection
-(** Exception that can be raised from IOs inside the handler,
-    when the connection is closed from underneath. *)
+(** Exception that can be raised from IOs inside the handler, when the
+    connection is closed from underneath. *)
 
 val add_route_handler :
   ?accept:(unit Request.t -> (unit, int * string) result) ->
@@ -23,8 +22,9 @@ val add_route_handler :
   handler ->
   unit
 (** 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. *)
+    @param accept_ws_protocol
+      decides whether this endpoint accepts the websocket protocol sent by the
+      client. Default accepts everything. *)
 
 (**/**)
 
@@ -33,4 +33,15 @@ 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
+
 (**/**)