Fix subtle race with slave allocation

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 572a8ac8b2e0b1fda34c6d1867d7069d9653a1ae..49a7b4da8883ddcd76cebcde2e15bab6a15b32ae 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1864,9 +1864,14 @@ void Thread::idle_loop(SplitPoint* sp_master) {
       {
           assert(!do_sleep && !do_exit);
 
-          // Copy split point position and search stack and call search()
-          Stack ss[MAX_PLY_PLUS_2];
+          lock_grab(Threads.splitLock);
+
+          assert(is_searching);
           SplitPoint* sp = splitPoint;
+
+          lock_release(Threads.splitLock);
+
+          Stack ss[MAX_PLY_PLUS_2];
           Position pos(*sp->pos, threadID);
 
           memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
@@ -1885,12 +1890,9 @@ void Thread::idle_loop(SplitPoint* sp_master) {
 
           assert(is_searching);
 
-          // We return from search with lock held
+          is_searching = false;
           sp->slavesMask &= ~(1ULL << threadID);
           sp->nodes += pos.nodes_searched();
-          lock_release(sp->lock);
-
-          is_searching = false;
 
           // Wake up master thread so to allow it to return from the idle loop in
           // case we are the last slave of the split point.
@@ -1898,8 +1900,16 @@ void Thread::idle_loop(SplitPoint* sp_master) {
               && threadID != sp->master
               && !Threads[sp->master].is_searching)
               Threads[sp->master].wake_up();
+
+          // After releasing the lock we cannot access anymore any SplitPoint
+          // related data in a reliably way becuase it could have been released
+          // under our feet by the sp master.
+          lock_release(sp->lock);
       }
   }
+  // In helpful master concept a master can help only a sub-tree of its split
+  // point, and because here is all finished is not possible master is booked.
+  assert(!is_searching);
 }