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Use a counter based condition variable implementation #371

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Use a counter based condition variable implementation #371

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bef8855
Use a counter based condition variable implementation
polytypic Jun 13, 2025
2de717e
More complex counter logic
polytypic Jun 13, 2025
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53 changes: 40 additions & 13 deletions lib/picos_std.sync/conditions.ml
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Original file line number Diff line number Diff line change
@@ -1,15 +1,14 @@
open Picos
open Picos_std_awaitable

type t = unit Awaitable.t
type t = int Awaitable.t

let create ?padded () = Awaitable.make ?padded ()
let create ?padded () = Awaitable.make ?padded 0

let[@inline] wait t lock ~acquire ~release =
let trigger = Trigger.create () in
let awaiter = Awaitable.Awaiter.add t trigger in
let before = Awaitable.get t in
release lock;
let lock_forbidden lock =
let lock_forbidden lock ~acquire =
let fiber = Fiber.current () in
let forbid = Fiber.exchange fiber ~forbid:true in
match acquire lock with
Expand All @@ -20,12 +19,40 @@ let[@inline] wait t lock ~acquire ~release =
Fiber.set fiber ~forbid;
Printexc.raise_with_backtrace exn bt
in
match Trigger.await trigger with
| None -> lock_forbidden lock
| Some exn_bt ->
Awaitable.Awaiter.remove awaiter;
lock_forbidden lock;
Printexc.raise_with_backtrace (fst exn_bt) (snd exn_bt)
let await t lock ~acquire before =
match if Awaitable.get t == before then Awaitable.await t before with
| () ->
let before = Awaitable.get t in
if before land 1 = 0 then begin
let after = before lor 1 in
if
(not (Awaitable.compare_and_set t before after))
&& Awaitable.get t land 1 = 0
then Awaitable.signal t
end;
lock_forbidden lock ~acquire
| exception exn ->
let bt = Printexc.get_raw_backtrace () in
lock_forbidden lock ~acquire;
Printexc.raise_with_backtrace exn bt
in
if before land 1 = 0 then
let after = before lor 1 in
if Awaitable.compare_and_set t before after || Awaitable.get t == after then
await t lock ~acquire after
else lock_forbidden lock ~acquire
else await t lock ~acquire before

let signal t =
let prior = Awaitable.fetch_and_add t 2 in
if prior land 1 <> 0 then begin
if Awaitable.compare_and_set t (prior + 2) (prior + 1) then ();
Awaitable.signal t
end

let signal = Awaitable.signal
let broadcast = Awaitable.broadcast
let broadcast t =
let prior = Awaitable.fetch_and_add t 2 in
if prior land 1 <> 0 then begin
if Awaitable.compare_and_set t (prior + 2) (prior + 1) then ();
Awaitable.broadcast t
end