Simplify locking

Retire slavesMask. We don't need it. It si enough
'searching' and 'thinking' flags.

Further simplification is still possible, perhaps
we could use a single flag.

No functional change.

src/search.cpp

@@ -291,12 +291,13 @@ void Search::think() {
       {
           th->maxPly = 0;
           th->depth = DEPTH_ZERO;
+          th->searching = true;
           if (th != Threads.main())
           {
               th->pos = Position(rootPos, th);
               th->rootMoves = rootMoves;
+              th->notify_one(); // Wake up the thread and start searching
           }
-          th->notify_one(); // Wake up all the threads
       }
 
       Threads.timer->run = true;
@@ -306,10 +307,6 @@ void Search::think() {
 
       TT.new_search();
 
-      // Start the threads
-      for (Thread* th : Threads)
-          th->searching = true;
-
       Threads.main()->search();
 
       // Stop the threads and timer
@@ -317,7 +314,9 @@ void Search::think() {
       Threads.timer->run = false;
 
       // Wait until all threads have finished
-      while (Threads.main()->slavesMask != 0) {}
+      for (Thread* th : Threads)
+          if (th != Threads.main())
+              th->wait_while(th->searching);
 
       // Clear any candidate easy move that wasn't stable for the last search
       // iterations; the second condition prevents consecutive fast moves.
@@ -346,7 +345,7 @@ void Search::think() {
   if (!Signals.stop && (Limits.ponder || Limits.infinite))
   {
       Signals.stopOnPonderhit = true;
-      rootPos.this_thread()->wait_for(Signals.stop);
+      Threads.main()->wait_for(Signals.stop);
   }
 
   sync_cout << "bestmove " << UCI::move(rootMoves[0].pv[0], rootPos.is_chess960());
@@ -364,10 +363,6 @@ void Search::think() {
 
 void Thread::search() {
 
-  Threads.main()->mutex.lock();
-  Threads.main()->slavesMask.set(idx);
-  Threads.main()->mutex.unlock();
-
   Value bestValue, alpha, beta, delta;
 
   Move easyMove = MOVE_NONE;
@@ -560,10 +555,7 @@ void Thread::search() {
   }
 
   searching = false;
-
+  notify_one(); // Wake up main if is sleeping waiting for us
-  Threads.main()->mutex.lock();
-  Threads.main()->slavesMask.reset(idx);
-  Threads.main()->mutex.unlock();
 }
 
 

src/thread.cpp

@@ -75,6 +75,15 @@ void ThreadBase::wait_for(volatile const bool& condition) {
 }
 
 
+// ThreadBase::wait_while() set the thread to sleep until 'condition' turns false
+
+void ThreadBase::wait_while(volatile const bool& condition) {
+
+  std::unique_lock<Mutex> lk(mutex);
+  sleepCondition.wait(lk, [&]{ return !condition; });
+}
+
+
 // Thread c'tor makes some init but does not launch any execution thread that
 // will be started only when c'tor returns.
 
@@ -112,10 +121,6 @@ void Thread::idle_loop() {
 
   while (!exit)
   {
-      // If this thread has been assigned work, launch a search
-      if (searching)
-          this->search();
-
       // If search is finished then sleep
       if (!Threads.main()->thinking)
       {
@@ -123,6 +128,9 @@ void Thread::idle_loop() {
           while (!exit && !Threads.main()->thinking)
               sleepCondition.wait(lk);
       }
+
+      if (!exit && searching)
+          this->search();
   }
 }
 
@@ -147,11 +155,9 @@ void MainThread::idle_loop() {
       lk.unlock();
 
       if (!exit)
-      {
           Search::think();
   }
 }
-}
 
 
 // MainThread::join() waits for main thread to finish the search

src/thread.h

@@ -48,6 +48,7 @@ struct ThreadBase : public std::thread {
   virtual void idle_loop() = 0;
   void notify_one();
   void wait_for(volatile const bool& b);
+  void wait_while(volatile const bool& b);
 
   Mutex mutex;
   ConditionVariable sleepCondition;
@@ -89,7 +90,6 @@ struct MainThread : public Thread {
   virtual void idle_loop();
   void join();
   volatile bool thinking = true; // Avoid a race with start_thinking()
-  std::bitset<MAX_THREADS> slavesMask;
 };
 
 struct TimerThread : public ThreadBase {