Avoid unnessary signal and broadcast calls

lib/picos_std.sync/rwlock.ml

@@ -45,11 +45,26 @@ end = struct
 
   let create ?padded () = Awaitable.make ?padded 0
   let is_zero t = Awaitable.get t == 0
-  let incr t = Awaitable.incr t
-  let decr t = Awaitable.fetch_and_add t (-1) = 1
+  let incr t = Awaitable.fetch_and_add t 2 |> ignore
+
+  let decr t =
+    let was = Awaitable.fetch_and_add t (-2) in
+    was = 3 && Awaitable.compare_and_set t 1 0
+
   let signal = Awaitable.signal
   let broadcast = Awaitable.broadcast
-  let await t = if Awaitable.get t <> 0 then Awaitable.await t Is 0
+
+  let rec await t before =
+    if before land 1 <> 0 then Awaitable.await t Is 0
+    else if Awaitable.compare_and_set t before (before lor 1) then
+      Awaitable.await t Is 0
+    else
+      let before = Awaitable.get t in
+      if before <> 0 then await t before
+
+  let[@inline] await t =
+    let before = Awaitable.get t in
+    if before <> 0 then await t before
 end
 
 module Tree_zero : sig