Merged in rmlarsen/eigen_threadpool (pull request PR-640)

Fix deadlocks in thread pool.

Approved-by: Eugene Zhulenev <ezhulenev@google.com>

unsupported/Eigen/CXX11/src/ThreadPool/EventCount.h

@@ -20,8 +20,7 @@ namespace Eigen {
 //   if (predicate)
 //     return act();
 //   EventCount::Waiter& w = waiters[my_index];
-//   if (!ec.Prewait(&w))
-//     return act();
+//   ec.Prewait(&w);
 //   if (predicate) {
 //     ec.CancelWait(&w);
 //     return act();
@@ -62,23 +61,17 @@ class EventCount {
   }
 
   // Prewait prepares for waiting.
-  // If Prewait returns true, the thread must re-check the wait predicate
+  // After calling Prewait, the thread must re-check the wait predicate
   // and then call either CancelWait or CommitWait.
-  // Otherwise, the thread should assume the predicate may be true
-  // and don't call CancelWait/CommitWait (there was a concurrent Notify call).
-  bool Prewait() {
+  void Prewait() {
     uint64_t state = state_.load(std::memory_order_relaxed);
     for (;;) {
       CheckState(state);
       uint64_t newstate = state + kWaiterInc;
-      if ((state & kSignalMask) != 0) {
-        // Consume the signal and cancel waiting.
-        newstate -= kSignalInc + kWaiterInc;
-      }
       CheckState(newstate);
       if (state_.compare_exchange_weak(state, newstate,
                                        std::memory_order_seq_cst))
-        return (state & kSignalMask) == 0;
+        return;
     }
   }
 
@@ -118,8 +111,13 @@ class EventCount {
     for (;;) {
       CheckState(state, true);
       uint64_t newstate = state - kWaiterInc;
-      // Also take away a signal if any.
-      if ((state & kSignalMask) != 0) newstate -= kSignalInc;
+      // We don't know if the thread was also notified or not,
+      // so we should not consume a signal unconditionaly.
+      // Only if number of waiters is equal to number of signals,
+      // we know that the thread was notified and we must take away the signal.
+      if (((state & kWaiterMask) >> kWaiterShift) ==
+          ((state & kSignalMask) >> kSignalShift))
+        newstate -= kSignalInc;
       CheckState(newstate);
       if (state_.compare_exchange_weak(state, newstate,
                                        std::memory_order_acq_rel))

unsupported/Eigen/CXX11/src/ThreadPool/NonBlockingThreadPool.h

@@ -379,7 +379,7 @@ class ThreadPoolTempl : public Eigen::ThreadPoolInterface {
     eigen_plain_assert(!t->f);
     // We already did best-effort emptiness check in Steal, so prepare for
     // blocking.
-    if (!ec_.Prewait()) return true;
+    ec_.Prewait();
     // Now do a reliable emptiness check.
     int victim = NonEmptyQueueIndex();
     if (victim != -1) {

unsupported/Eigen/CXX11/src/ThreadPool/RunQueue.h

@@ -97,11 +97,9 @@ class RunQueue {
   }
 
   // PopBack removes and returns the last elements in the queue.
-  // Can fail spuriously.
   Work PopBack() {
     if (Empty()) return Work();
-    std::unique_lock<std::mutex> lock(mutex_, std::try_to_lock);
-    if (!lock) return Work();
+    std::unique_lock<std::mutex> lock(mutex_);
     unsigned back = back_.load(std::memory_order_relaxed);
     Elem* e = &array_[back & kMask];
     uint8_t s = e->state.load(std::memory_order_relaxed);
@@ -115,11 +113,10 @@ class RunQueue {
   }
 
   // PopBackHalf removes and returns half last elements in the queue.
-  // Returns number of elements removed. But can also fail spuriously.
+  // Returns number of elements removed.
   unsigned PopBackHalf(std::vector<Work>* result) {
     if (Empty()) return 0;
-    std::unique_lock<std::mutex> lock(mutex_, std::try_to_lock);
-    if (!lock) return 0;
+    std::unique_lock<std::mutex> lock(mutex_);
     unsigned back = back_.load(std::memory_order_relaxed);
     unsigned size = Size();
     unsigned mid = back;

unsupported/test/cxx11_eventcount.cpp

@@ -30,10 +30,10 @@ static void test_basic_eventcount()
   EventCount ec(waiters);
   EventCount::Waiter& w = waiters[0];
   ec.Notify(false);
-  VERIFY(ec.Prewait());
+  ec.Prewait();
   ec.Notify(true);
   ec.CommitWait(&w);
-  VERIFY(ec.Prewait());
+  ec.Prewait();
   ec.CancelWait();
 }
 
@@ -112,7 +112,7 @@ static void test_stress_eventcount()
         unsigned idx = rand_reentrant(&rnd) % kQueues;
         if (queues[idx].Pop()) continue;
         j--;
-        if (!ec.Prewait()) continue;
+        ec.Prewait();
         bool empty = true;
         for (int q = 0; q < kQueues; q++) {
           if (!queues[q].Empty()) {