Add seq_cst fences in parkThread/wakeThread to fix lost wakeup

src/fibers/fiber.cpp

@@ -703,6 +703,12 @@ void FiberScheduler::ProcessorState::profileEvent(ProfileEventKind kind, uint8_t
 
 void FiberScheduler::ProcessorState::wakeThread() noexcept
 {
+    // seq_cst fence pairs with the one in parkThread to prevent the store-buffering race on weak
+    // memory models (e.g. arm64): the producer's prior queue store and this sleeping load must not
+    // reorder, so either we observe sleeping=true here (and post the eventfd) or parkThread's
+    // hasWork() re-check observes our enqueued work. Same pattern as enqueueWaiter / releaseWaiters.
+    std::atomic_thread_fence(std::memory_order_seq_cst);
+
     if (sleeping.load(std::memory_order_acquire))
     {
         Perf::getSimpleCounter(simpleCounters[SCHEDULER_THREAD_WAKED], number).increment();
@@ -714,22 +720,25 @@ void FiberScheduler::ProcessorState::wakeThread() noexcept
 
 void FiberScheduler::ProcessorState::parkThread(uint64_t waitNs, CpuTimer * timer) noexcept
 {
-    __kernel_timespec ts;
-    ts.tv_sec = static_cast<int64_t>(waitNs / 1'000'000'000);
-    ts.tv_nsec = static_cast<int64_t>(waitNs % 1'000'000'000);
-
-    io_uring_getevents_arg arg{};
-    arg.ts = reinterpret_cast<uint64_t>(&ts);
-
-    // Announce that we are about to park.  Release ordering ensures that any
-    // concurrent wakeThread() that observes parked=true also observes all prior
-    // queue stores, guaranteeing the eventfd_write reaches us.
+    // Announce that we are about to park, then a seq_cst fence pairing with the one in wakeThread:
+    // release alone is not a StoreLoad barrier, so without this the store could reorder past the
+    // hasWork() re-check below while a concurrent wakeThread reads sleeping=false - both miss, and
+    // the wakeup is lost on weak memory models (e.g. arm64). Same pattern as enqueueWaiter.
     sleeping.store(true, std::memory_order_release);
 
+    std::atomic_thread_fence(std::memory_order_seq_cst);
+
     // Double-check: work may have arrived between the last drain and here.
     // If so, skip the park entirely so that work is not delayed by waitNs.
     if (!hasWork())
     {
+        __kernel_timespec ts;
+        ts.tv_sec = static_cast<int64_t>(waitNs / 1'000'000'000);
+        ts.tv_nsec = static_cast<int64_t>(waitNs % 1'000'000'000);
+
+        io_uring_getevents_arg arg{};
+        arg.ts = reinterpret_cast<uint64_t>(&ts);
+
         Perf::getSimpleCounter(simpleCounters[SCHEDULER_THREAD_PARKED], number).increment();
 
         timer->reset(simpleCounters[SCHEDULER_IDLE_TIME], number);