diff --git a/lib/libwebsockets.h b/lib/libwebsockets.h
index c1285e86..ec175715 100644
--- a/lib/libwebsockets.h
+++ b/lib/libwebsockets.h
@@ -954,6 +954,14 @@ struct libwebsocket_extension;
  *	LWS_CALLBACK_UNLOCK_POLL: These allow the external poll changes driven
  *		by libwebsockets to participate in an external thread locking
  *		scheme around the changes, so the whole thing is threadsafe.
+ *		These are called around three activities in the library,
+ *			- inserting a new wsi in the wsi / fd table (len=1)
+ *			- deleting a wsi from the wsi / fd table (len=1)
+ *			- changing a wsi's POLLIN/OUT state (len=0)
+ *		Locking and unlocking external synchronization objects when
+ *		len == 1 allows external threads to be synchronized against
+ *		wsi lifecycle changes if it acquires the same lock for the
+ *		duration of wsi dereference from the other thread context.
  */
 LWS_VISIBLE LWS_EXTERN int callback(struct libwebsocket_context *context,
 			struct libwebsocket *wsi,
diff --git a/lib/pollfd.c b/lib/pollfd.c
index 08fc4058..b38df3d5 100644
--- a/lib/pollfd.c
+++ b/lib/pollfd.c
@@ -47,7 +47,7 @@ insert_wsi_socket_into_fds(struct libwebsocket_context *context,
 //					    wsi, wsi->sock, context->fds_count);
 
 	if (context->protocols[0].callback(context, wsi,
-	    LWS_CALLBACK_LOCK_POLL, wsi->user_space, (void *) &pa, 0))
+	    LWS_CALLBACK_LOCK_POLL, wsi->user_space, (void *) &pa, 1))
 		return -1;
 
 	insert_wsi(context, wsi);
@@ -63,7 +63,7 @@ insert_wsi_socket_into_fds(struct libwebsocket_context *context,
 		return -1;
 
 	if (context->protocols[0].callback(context, wsi,
-	    LWS_CALLBACK_UNLOCK_POLL, wsi->user_space, (void *)&pa, 0))
+	    LWS_CALLBACK_UNLOCK_POLL, wsi->user_space, (void *)&pa, 1))
 		return -1;
 
 	return 0;
@@ -92,7 +92,7 @@ remove_wsi_socket_from_fds(struct libwebsocket_context *context,
 				    wsi, wsi->sock, wsi->position_in_fds_table);
 
 	if (context->protocols[0].callback(context, wsi,
-	    LWS_CALLBACK_LOCK_POLL, wsi->user_space, (void *)&pa, 0))
+	    LWS_CALLBACK_LOCK_POLL, wsi->user_space, (void *)&pa, 1))
 		return -1;
 
 	m = wsi->position_in_fds_table; /* replace the contents for this */
@@ -123,7 +123,7 @@ remove_wsi_socket_from_fds(struct libwebsocket_context *context,
 	}
 	if (context->protocols[0].callback(context, wsi,
 				       LWS_CALLBACK_UNLOCK_POLL,
-				       wsi->user_space, (void *) &pa, 0))
+				       wsi->user_space, (void *) &pa, 1))
 		return -1;
 
 	return 0;