fix freeze on @threads loop exit

Closes #45626, hopefully.

src/jl_uv.c

@@ -60,6 +60,7 @@ void JL_UV_LOCK(void)
     }
     else {
         jl_atomic_fetch_add_relaxed(&jl_uv_n_waiters, 1);
+        jl_fence(); // [^store_buffering_1]
         jl_wake_libuv();
         JL_LOCK(&jl_uv_mutex);
         jl_atomic_fetch_add_relaxed(&jl_uv_n_waiters, -1);

src/partr.c

@@ -38,7 +38,12 @@ static const int16_t sleeping = 1;
 // * Enqueuer:
 //   * 2: `jl_atomic_load_relaxed(&ptls->sleep_check_state)` in `jl_wakeup_thread` returns `not_sleeping`
 // i.e., the dequeuer misses the enqueue and enqueuer misses the sleep state transition.
-
+// and
+// * Enqueuer:
+//   * 1: `jl_atomic_store_relaxed(jl_uv_n_waiters, 1)` in `JL_UV_LOCK`
+// * Dequeuer:
+//   * 2: `jl_atomic_load_relaxed(jl_uv_n_waiters)` in `jl_task_get_next` returns `0`
+// i.e., the dequeuer misses the `n_waiters` is set and enqueuer misses the uv_stop flag (in signal_async) transition to cleared
 
 JULIA_DEBUG_SLEEPWAKE(
 uint64_t wakeup_enter;