[threading] Fix Fence for multiple waiting threads

Author: JoelLinn

src/xenia/base/testing/threading_test.cc

@@ -38,6 +38,13 @@ TEST_CASE("Fence") {
   pFence->Signal();
   pFence->Wait();
 
+  // Signal and wait two times
+  pFence = std::make_unique<threading::Fence>();
+  pFence->Signal();
+  pFence->Wait();
+  pFence->Signal();
+  pFence->Wait();
+
   // Test to synchronize multiple threads
   std::atomic<int> started(0);
   std::atomic<int> finished(0);
@@ -57,15 +64,10 @@ TEST_CASE("Fence") {
   });
 
   Sleep(100ms);
+  REQUIRE(started.load() == threads.size());
   REQUIRE(finished.load() == 0);
 
-  // TODO(bwrsandman): Check if this is correct behaviour: looping with Sleep
-  // is the only way to get fence to signal all threads on windows
-  for (int i = 0; i < threads.size(); ++i) {
-    Sleep(10ms);
-    pFence->Signal();
-  }
-  REQUIRE(started.load() == threads.size());
+  pFence->Signal();
 
   for (auto& t : threads) t.join();
   REQUIRE(finished.load() == threads.size());

src/xenia/base/threading.h

@@ -24,27 +24,56 @@
 #include <utility>
 #include <vector>
 
+#include "xenia/base/assert.h"
+
 namespace xe {
 namespace threading {
 
+// This is more like an Event with self-reset when returning from Wait()
 class Fence {
  public:
-  Fence() : signaled_(false) {}
+  Fence() : signal_state_(0) {}
+
   void Signal() {
     std::unique_lock<std::mutex> lock(mutex_);
-    signaled_ = true;
+    signal_state_ |= SIGMASK_;
     cond_.notify_all();
   }
+
+  // Wait for the Fence to be signaled. Clears the signal on return.
   void Wait() {
     std::unique_lock<std::mutex> lock(mutex_);
-    cond_.wait(lock, [this] { return signaled_; });
-    signaled_ = false;
+    assert_true((signal_state_ & ~SIGMASK_) < (SIGMASK_ - 1) &&
+                "Too many threads?");
+
+    // keep local copy to minimize loads
+    auto signal_state = ++signal_state_;
+    for (; !(signal_state & SIGMASK_); signal_state = signal_state_) {
+      cond_.wait(lock);
+    }
+
+    // We can't just clear the signal as other threads may not have read it yet
+    assert_true((signal_state & ~SIGMASK_) > 0);  // wait_count > 0
+    if (signal_state == (1 | SIGMASK_)) {         // wait_count == 1
+      // Last one out turn off the lights
+      signal_state_ = 0;
+    } else {
+      // Oops, another thread is still waiting, set the new count and keep the
+      // signal.
+      signal_state_ = --signal_state;
+    }
   }
 
  private:
+  using state_t_ = uint_fast32_t;
+  static constexpr state_t_ SIGMASK_ = state_t_(1)
+                                       << (sizeof(state_t_) * 8 - 1);
+
   std::mutex mutex_;
   std::condition_variable cond_;
-  bool signaled_;
+  // Use the highest bit (sign bit) as the signal flag and the rest to count
+  // waiting threads.
+  volatile state_t_ signal_state_;
 };
 
 // Returns the total number of logical processors in the host system.