Invert writers and readers to no_writers and no_readers

lib/picos_std.sync/rwlock.slim.ml

@@ -6,9 +6,9 @@ type t = int Awaitable.t
 
 let frozen = 1 lsl 0
 let poisoned = 1 lsl 1
-let readers = 1 lsl 2
-let writers = 1 lsl 3
-let awaiters = readers lor writers
+let no_writers = 1 lsl 2
+let no_readers = 1 lsl 3
+let no_awaiters = no_readers lor no_writers
 let locked = 1 lsl 4
 let exclusive = 1 lsl (Sys.int_size - 2)
 
@@ -20,13 +20,13 @@ exception Frozen
 let rec lock_awaiting t =
   let before = Awaitable.get t in
   if before < locked then begin
-    let after = before lor locked lor exclusive lor writers in
+    let after = before land lnot no_writers lor locked lor exclusive in
     if not (Awaitable.compare_and_set t before after) then lock_awaiting t
   end
   else if before land (poisoned lor frozen) <> 0 then
     raise (if before land poisoned <> 0 then Poisoned else Frozen)
   else
-    let after = before lor writers in
+    let after = before land lnot no_writers in
     if before = after || Awaitable.compare_and_set t before after then
       Awaitable.await t after;
     lock_awaiting t
@@ -43,20 +43,20 @@ let lock t =
 let rec lock_ro_awaiting t =
   let before = Awaitable.get t in
   if before < exclusive then begin
-    let after = (before + locked) lor readers in
+    let after = (before + locked) land lnot no_readers in
     if not (Awaitable.compare_and_set t before after) then lock_ro_awaiting t
   end
   else if before land poisoned <> 0 then raise Poisoned
   else
-    let after = before lor readers in
+    let after = before land lnot no_readers in
     if before = after || Awaitable.compare_and_set t before after then
       Awaitable.await t after;
     lock_ro_awaiting t
 
 let rec lock_ro_contended t =
   let before = Awaitable.get t in
   if locked * 2 <= before then
-    let after = (before - locked) lor readers in
+    let after = (before - locked) land lnot no_readers in
     if Awaitable.compare_and_set t before after then lock_ro_awaiting t
     else lock_ro_contended t
 
@@ -66,34 +66,33 @@ let lock_ro t =
 
 let rec signal_awaiters t =
   let before = Awaitable.get t in
-  if before < locked && before land awaiters <> 0 then
-    if Awaitable.compare_and_set t before (before land lnot awaiters) then
-      if before land readers <> 0 then Awaitable.broadcast t
-      else Awaitable.signal t
+  if before < no_awaiters then
+    if Awaitable.compare_and_set t before (before lor no_awaiters) then
+      if no_readers <= before then Awaitable.signal t else Awaitable.broadcast t
     else signal_awaiters t
 
 let unlock t =
   let before = Awaitable.get t in
   if exclusive <= before then begin
     let prior =
       Awaitable.fetch_and_add t
-        (-(locked lor exclusive) - (before land awaiters))
+        ((-(locked lor exclusive) lor no_awaiters) - (before land no_awaiters))
     in
-    if prior land awaiters <> 0 then
-      if prior land readers <> 0 then Awaitable.broadcast t
-      else Awaitable.signal t
+    if prior < locked lor exclusive lor no_awaiters then
+      if locked lor exclusive lor no_readers <= prior then Awaitable.signal t
+      else Awaitable.broadcast t
   end
   else
     let prior = Awaitable.fetch_and_add t (-locked) in
-    if prior < locked * 2 && locked + readers < prior then signal_awaiters t
+    if prior < locked lor no_awaiters then signal_awaiters t
 
 let poison t =
   let before = Awaitable.get t in
   if before land exclusive = 0 then invalid_arg "not write locked";
   (* Unfortunately we cannot check ownership at this point. *)
   if before land poisoned = 0 then
     let prior = Awaitable.fetch_and_add t poisoned in
-    if prior land awaiters <> 0 then Awaitable.broadcast t
+    if prior land no_awaiters <> no_awaiters then Awaitable.broadcast t
 
 let freeze t =
   lock_ro t;
@@ -108,7 +107,7 @@ let freeze t =
     before := Awaitable.get t
   done;
   (* We must wake up any writers waiting to obtain the lock. *)
-  if !before land awaiters <> 0 then Awaitable.broadcast t;
+  if !before land no_awaiters <> no_awaiters then Awaitable.broadcast t;
   unlock t
 
 let protect t thunk =
@@ -133,7 +132,9 @@ let protect_ro t thunk =
       unlock t;
       Printexc.raise_with_backtrace exn bt
 
-let[@inline] create ?padded () = Awaitable.make ?padded 0
+let[@inline] create ?padded () =
+  Awaitable.make ?padded (no_readers lor no_writers)
+
 let[@inline] is_locked t = exclusive <= Awaitable.get t
 let[@inline] is_poisoned t = Awaitable.get t land poisoned <> 0
 let[@inline] is_frozen t = Awaitable.get t land frozen <> 0