fix #755: a race condition cousing a deadlock

This reverts a JobSystem optimization that attempted to avoid signaling
a condition when there was no waiters. Unfortunately, there was a
race that caused the the signaling thread to miss that the waiter flag
was set, thus not signaling.

libs/utils/include/utils/JobSystem.h

@@ -70,8 +70,7 @@ class JobSystem {
         uint16_t parent;                                        //  2 |  2
         std::atomic<uint16_t> runningJobCount = { 1 };          //  2 |  2
         mutable std::atomic<uint16_t> refCount = { 1 };         //  2 |  2
-        std::atomic_bool hasWaiter = { false };                 //  1 |  1
-                                                                //  5 |  1 (padding)
+                                                                //  6 |  2 (padding)
                                                                 // 64 | 64
     };
 

libs/utils/src/JobSystem.cpp

@@ -321,7 +321,7 @@ void JobSystem::finish(Job* job) noexcept {
             std::atomic_thread_fence(std::memory_order_acquire);
 #endif
             // no more work, destroy this job and notify its the parent
-            notify = notify || job->hasWaiter.load();
+            notify = true;
             Job* const parent = job->parent == 0x7FFF ? nullptr : &storage[job->parent];
             decRef(job);
             job = parent;
@@ -427,7 +427,6 @@ void JobSystem::waitAndRelease(Job*& job) noexcept {
             // test if job has completed first, to possibly avoid taking the lock
             if (!hasJobCompleted(job)) {
                 std::unique_lock<Mutex> lock(mWaiterLock);
-                job->hasWaiter.store(true);
                 while (!hasJobCompleted(job) && !exitRequested()) {
                     mWaiterCondition.wait(lock);
                 }