lock.

Author: trivialfis

include/xgboost/cache.h

@@ -79,7 +79,7 @@ class DMatrixCache {
       auto it = container_.find(p_fmat);
       CHECK(it != container_.cend());
       // Re-new the cache if this has never been read.
-      if (it->second.ref.expired() || !it->second.ref->HasBeenRead()) {
+      if (it->second.ref.expired() || !it->second.ref.lock()->HasBeenRead()) {
         expired.push_back(it->first);
       } else {
         remained.push(it->first);