diff --git a/src/kcas/kcas.ml b/src/kcas/kcas.ml
index 799e12cc..5bded5f4 100644
--- a/src/kcas/kcas.ml
+++ b/src/kcas/kcas.ml
@@ -671,97 +671,135 @@ module Xt = struct
     validate_one which node_r.loc node_r.state;
     validate_all_rec which node_r.gt
 
-  let[@inline] maybe_validate_log (Xt xt_r as xt : _ t) =
-    let c0 = xt_r.validate_counter in
-    let c1 = c0 + 1 in
-    xt_r.validate_counter <- c1;
-    (* Validate whenever counter reaches next power of 2. *)
-    if c0 land c1 = 0 then begin
-      Timeout.check xt_r.timeout;
-      validate_all_rec xt !(tree_as_ref xt)
-    end
-
   let[@inline] is_obstruction_free (Xt xt_r : _ t) loc =
     (* Fenceless is safe as we are accessing a private location. *)
     xt_r.mode == `Obstruction_free && 0 <= loc.id
 
-  let[@inline] update_new loc f xt lt gt =
-    (* Fenceless is safe inside transactions as each log update has a fence. *)
+  type (_, _) up =
+    | Compare_and_swap : ('a * 'a, 'a) up
+    | Fetch_and_add : (int, int) up
+    | Fn : ('a -> 'a, 'a) up
+    | Exchange : ('a, 'a) up
+    | Get : (unit, 'a) up
+
+  let update_new : type c a. _ -> a loc -> c -> (c, a) up -> _ -> _ -> a =
+   fun xt loc c up lt gt ->
     let state = fenceless_get (as_atomic loc) in
     let before = eval state in
-    match f before with
-    | after ->
-        let state =
-          if before == after && is_obstruction_free xt loc then state
-          else { before; after; which = W xt; awaiters = [] }
-        in
-        tree_as_ref xt := T (Node { loc; state; lt; gt; awaiters = [] });
-        before
-    | exception exn ->
-        tree_as_ref xt := T (Node { loc; state; lt; gt; awaiters = [] });
-        raise exn
+    let after : a =
+      match up with
+      | Compare_and_swap -> if fst c == before then snd c else before
+      | Fetch_and_add -> before + c
+      | Fn -> begin
+          let rot = !(tree_as_ref xt) in
+          match c before with
+          | after ->
+              assert (rot == !(tree_as_ref xt));
+              after
+          | exception exn ->
+              assert (rot == !(tree_as_ref xt));
+              tree_as_ref xt := T (Node { loc; state; lt; gt; awaiters = [] });
+              raise exn
+        end
+      | Exchange -> c
+      | Get -> before
+    in
+    let state =
+      if before == after && is_obstruction_free xt loc then state
+      else { before; after; which = W xt; awaiters = [] }
+    in
+    tree_as_ref xt := T (Node { loc; state; lt; gt; awaiters = [] });
+    before
+
+  let update_old : type c a. _ -> a loc -> c -> (c, a) up -> _ -> _ -> _ -> a =
+   fun (Xt xt_r as xt : _ t) loc c up lt gt state' ->
+    let c0 = xt_r.validate_counter in
+    let c1 = c0 + 1 in
+    xt_r.validate_counter <- c1;
+    (* Validate whenever counter reaches next power of 2.
 
-  let[@inline] update_top loc f xt state' lt gt =
-    let state = Obj.magic state' in
+       The assumption is that potentially infinite loops will repeatedly access
+       the same locations. *)
+    if c0 land c1 = 0 then begin
+      Timeout.check xt_r.timeout;
+      validate_all_rec xt !(tree_as_ref xt)
+    end;
+    let state : a state = Obj.magic state' in
     if is_cmp xt state then begin
-      let before = eval state in
-      let after = f before in
+      let current = eval state in
+      let after : a =
+        match up with
+        | Compare_and_swap -> if fst c == current then snd c else current
+        | Fetch_and_add -> current + c
+        | Fn ->
+            let rot = !(tree_as_ref xt) in
+            let after = c current in
+            assert (rot == !(tree_as_ref xt));
+            after
+        | Exchange -> c
+        | Get -> current
+      in
       let state =
-        if before == after then state
-        else { before; after; which = W xt; awaiters = [] }
+        if current == after then state
+        else { before = current; after; which = W xt; awaiters = [] }
       in
       tree_as_ref xt := T (Node { loc; state; lt; gt; awaiters = [] });
-      before
+      current
     end
     else
       let current = state.after in
-      let state = { state with after = f current } in
+      let after : a =
+        match up with
+        | Compare_and_swap -> if fst c == current then snd c else current
+        | Fetch_and_add -> current + c
+        | Fn ->
+            let rot = !(tree_as_ref xt) in
+            let after = c current in
+            assert (rot == !(tree_as_ref xt));
+            after
+        | Exchange -> c
+        | Get -> current
+      in
+      let state =
+        if current == after then state
+        else { before = state.before; after; which = W xt; awaiters = [] }
+      in
       tree_as_ref xt := T (Node { loc; state; lt; gt; awaiters = [] });
       current
 
-  let[@inline] unsafe_update ~xt loc f =
+  let update_as ~xt loc c up =
     let loc = Loc.to_loc loc in
-    maybe_validate_log xt;
     let x = loc.id in
     match !(tree_as_ref xt) with
-    | T Leaf -> update_new loc f xt (T Leaf) (T Leaf)
+    | T Leaf -> update_new xt loc c up (T Leaf) (T Leaf)
     | T (Node { loc = a; lt = T Leaf; _ }) as tree when x < a.id ->
-        update_new loc f xt (T Leaf) tree
+        update_new xt loc c up (T Leaf) tree
     | T (Node { loc = a; gt = T Leaf; _ }) as tree when a.id < x ->
-        update_new loc f xt tree (T Leaf)
+        update_new xt loc c up tree (T Leaf)
     | T (Node { loc = a; state; lt; gt; _ }) when Obj.magic a == loc ->
-        update_top loc f xt state lt gt
+        update_old xt loc c up lt gt state
     | tree -> begin
         match splay ~hit_parent:false x tree with
-        | l, T Leaf, r -> update_new loc f xt l r
-        | l, T (Node node_r), r -> update_top loc f xt node_r.state l r
+        | l, T Leaf, r -> update_new xt loc c up l r
+        | l, T (Node node_r), r -> update_old xt loc c up l r node_r.state
       end
 
-  let[@inline] protect xt f x =
-    let tree = !(tree_as_ref xt) in
-    let y = f x in
-    assert (!(tree_as_ref xt) == tree);
-    y
-
-  let get ~xt loc = unsafe_update ~xt loc Fun.id
-  let set ~xt loc after = unsafe_update ~xt loc (fun _ -> after) |> ignore
-  let modify ~xt loc f = unsafe_update ~xt loc (protect xt f) |> ignore
+  let get ~xt loc = update_as ~xt loc () Get
+  let set ~xt loc after = update_as ~xt loc after Exchange |> ignore
+  let modify ~xt loc f = update_as ~xt loc f Fn |> ignore
 
   let compare_and_swap ~xt loc before after =
-    unsafe_update ~xt loc (fun actual ->
-        if actual == before then after else actual)
+    update_as ~xt loc (before, after) Compare_and_swap
 
   let compare_and_set ~xt loc before after =
     compare_and_swap ~xt loc before after == before
 
-  let exchange ~xt loc after = unsafe_update ~xt loc (fun _ -> after)
-  let fetch_and_add ~xt loc n = unsafe_update ~xt loc (( + ) n)
-  let incr ~xt loc = unsafe_update ~xt loc inc |> ignore
-  let decr ~xt loc = unsafe_update ~xt loc dec |> ignore
-  let update ~xt loc f = unsafe_update ~xt loc (protect xt f)
+  let exchange ~xt loc after = update_as ~xt loc after Exchange
+  let fetch_and_add ~xt loc n = update_as ~xt loc n Fetch_and_add
+  let incr ~xt loc = update_as ~xt loc 1 Fetch_and_add |> ignore
+  let decr ~xt loc = update_as ~xt loc (-1) Fetch_and_add |> ignore
+  let update ~xt loc f = update_as ~xt loc f Fn
   let swap ~xt l1 l2 = set ~xt l1 @@ exchange ~xt l2 @@ get ~xt l1
-  let unsafe_modify ~xt loc f = unsafe_update ~xt loc f |> ignore
-  let unsafe_update ~xt loc f = unsafe_update ~xt loc f
 
   let[@inline] to_blocking ~xt tx =
     match tx ~xt with None -> Retry.later () | Some value -> value
@@ -889,7 +927,7 @@ module Xt = struct
         else stop
     | T (Node _) as stop -> stop
 
-  let initial_validate_period = 16
+  let initial_validate_period = 4
 
   let success (Xt xt_r : _ t) result =
     Timeout.cancel xt_r.timeout;
diff --git a/src/kcas/kcas.mli b/src/kcas/kcas.mli
index 2061c9ee..185a2c23 100644
--- a/src/kcas/kcas.mli
+++ b/src/kcas/kcas.mli
@@ -415,10 +415,11 @@ module Xt : sig
 
       To mitigate potential issues due to this read skew anomaly and due to very
       long running transactions, all of the access recording operations in this
-      section periodically validate the entire transaction log.  An important
-      guideline for writing transactions is that loops inside a transaction
-      should always include an access of some shared memory location through the
-      transaction log or should otherwise be guaranteed to be bounded. *)
+      section periodically validate the entire transaction log when a previously
+      accessed location is accessed again.  An important guideline for writing
+      transactions is that loops inside a transaction should always include an
+      access of some shared memory location through the transaction log or
+      should otherwise be guaranteed to be bounded. *)
 
   val get : xt:'x t -> 'a Loc.t -> 'a
   (** [get ~xt r] returns the current value of the shared memory location [r] in
@@ -558,14 +559,4 @@ module Xt : sig
       The default {{!Mode.t} [mode]} for [commit] is [`Obstruction_free].
       However, after enough attempts have failed during the verification step,
       [commit] switches to [`Lock_free]. *)
-
-  (**/**)
-
-  val unsafe_modify : xt:'x t -> 'a Loc.t -> ('a -> 'a) -> unit
-  (** [unsafe_modify ~xt r f] is equivalent to [modify ~xt r f], but does not
-      assert against misuse. *)
-
-  val unsafe_update : xt:'x t -> 'a Loc.t -> ('a -> 'a) -> 'a
-  (** [unsafe_update ~xt r f] is equivalent to [update ~xt r f], but does not
-      assert against misuse. *)
 end
diff --git a/src/kcas_data/hashtbl.ml b/src/kcas_data/hashtbl.ml
index 8cfa8f21..c094714a 100644
--- a/src/kcas_data/hashtbl.ml
+++ b/src/kcas_data/hashtbl.ml
@@ -219,7 +219,7 @@ module Xt = struct
             Array.unsafe_get old_buckets i
             |> Xt.get ~xt
             |> Assoc.iter_rev @@ fun k v ->
-               Xt.unsafe_modify ~xt
+               Xt.modify ~xt
                  (Array.unsafe_get new_buckets (hash k land mask))
                  (Assoc.cons k v)
           done
@@ -337,7 +337,7 @@ module Xt = struct
     let buckets = r.buckets in
     let mask = Array.length buckets - 1 in
     let bucket = Array.unsafe_get buckets (r.hash k land mask) in
-    match Xt.unsafe_modify ~xt bucket (Assoc.remove r.equal k) with
+    match Xt.modify ~xt bucket (Assoc.remove r.equal k) with
     | () ->
         Accumulator.Xt.decr ~xt r.length;
         if r.min_buckets <= mask && Random.bits () land mask = 0 then
@@ -353,7 +353,7 @@ module Xt = struct
     let buckets = r.buckets in
     let mask = Array.length buckets - 1 in
     let bucket = Array.unsafe_get buckets (r.hash k land mask) in
-    Xt.unsafe_modify ~xt bucket (Assoc.cons k v);
+    Xt.modify ~xt bucket (Assoc.cons k v);
     Accumulator.Xt.incr ~xt r.length;
     if mask + 1 < r.max_buckets && Random.bits () land mask = 0 then
       let capacity = mask + 1 in
@@ -367,7 +367,7 @@ module Xt = struct
     let mask = Array.length buckets - 1 in
     let bucket = Array.unsafe_get buckets (r.hash k land mask) in
     let change = ref Assoc.Nop in
-    Xt.unsafe_modify ~xt bucket (fun kvs ->
+    Xt.modify ~xt bucket (fun kvs ->
         let kvs' = Assoc.replace r.equal change k v kvs in
         if !change != Assoc.Nop then kvs' else kvs);
     if !change == Assoc.Added then begin
diff --git a/src/kcas_data/mvar.ml b/src/kcas_data/mvar.ml
index 7276d15c..ba31e192 100644
--- a/src/kcas_data/mvar.ml
+++ b/src/kcas_data/mvar.ml
@@ -11,14 +11,13 @@ module Xt = struct
     Magic_option.is_none
       (Xt.compare_and_swap ~xt mv Magic_option.none (Magic_option.some value))
 
-  let put ~xt mv value =
-    Xt.unsafe_modify ~xt mv (Magic_option.put_or_retry value)
+  let put ~xt mv value = Xt.modify ~xt mv (Magic_option.put_or_retry value)
 
   let take_opt ~xt mv =
     Magic_option.to_option (Xt.exchange ~xt mv Magic_option.none)
 
   let take ~xt mv =
-    Magic_option.get_unsafe (Xt.unsafe_update ~xt mv Magic_option.take_or_retry)
+    Magic_option.get_unsafe (Xt.update ~xt mv Magic_option.take_or_retry)
 
   let peek ~xt mv = Magic_option.get_or_retry (Xt.get ~xt mv)
   let peek_opt ~xt mv = Magic_option.to_option (Xt.get ~xt mv)
diff --git a/src/kcas_data/queue.ml b/src/kcas_data/queue.ml
index ec0fc8c1..543bb0c7 100644
--- a/src/kcas_data/queue.ml
+++ b/src/kcas_data/queue.ml
@@ -34,7 +34,7 @@ module Xt = struct
     + Elems.length (Xt.get ~xt middle)
     + Elems.length (Xt.get ~xt back)
 
-  let add ~xt x q = Xt.unsafe_modify ~xt q.back @@ Elems.cons x
+  let add ~xt x q = Xt.modify ~xt q.back @@ Elems.cons x
   let push = add
 
   (** Cooperative helper to move elems from back to middle. *)
@@ -53,7 +53,7 @@ module Xt = struct
 
   let take_opt ~xt t =
     let front = t.front in
-    let elems = Xt.unsafe_update ~xt front Elems.tl_safe in
+    let elems = Xt.update ~xt front Elems.tl_safe in
     if elems != Elems.empty then Elems.hd_opt elems
     else
       let middle = t.middle and back = t.back in
diff --git a/src/kcas_data/stack.ml b/src/kcas_data/stack.ml
index a42abe29..c4d0170a 100644
--- a/src/kcas_data/stack.ml
+++ b/src/kcas_data/stack.ml
@@ -9,13 +9,10 @@ let of_seq xs = Loc.make ~padded:true (Elems.of_seq_rev xs)
 module Xt = struct
   let length ~xt s = Xt.get ~xt s |> Elems.length
   let is_empty ~xt s = Xt.get ~xt s == Elems.empty
-  let push ~xt x s = Xt.unsafe_modify ~xt s @@ Elems.cons x
-  let pop_opt ~xt s = Xt.unsafe_update ~xt s Elems.tl_safe |> Elems.hd_opt
+  let push ~xt x s = Xt.modify ~xt s @@ Elems.cons x
+  let pop_opt ~xt s = Xt.update ~xt s Elems.tl_safe |> Elems.hd_opt
   let pop_all ~xt s = Elems.to_seq @@ Xt.exchange ~xt s Elems.empty
-
-  let pop_blocking ~xt s =
-    Xt.unsafe_update ~xt s Elems.tl_safe |> Elems.hd_or_retry
-
+  let pop_blocking ~xt s = Xt.update ~xt s Elems.tl_safe |> Elems.hd_or_retry
   let top_opt ~xt s = Xt.get ~xt s |> Elems.hd_opt
   let top_blocking ~xt s = Xt.get ~xt s |> Elems.hd_or_retry
   let clear ~xt s = Xt.set ~xt s Elems.empty