+ /* There is potential deadlock race condition between object
+ * destroy and layout LFSCK. Consider the following scenario:
+ *
+ * 1) The LFSCK thread obtained the parent object firstly, at
+ * that time, the parent object has not been destroyed yet.
+ *
+ * 2) One RPC service thread destroyed the parent and all its
+ * children objects. Because the LFSCK is referencing the
+ * parent object, then the parent object will be marked as
+ * dying in RAM. On the other hand, the parent object is
+ * referencing all its children objects, then all children
+ * objects will be marked as dying in RAM also.
+ *
+ * 3) The LFSCK thread tries to find some child object with
+ * the parent object referenced. Then it will find that the
+ * child object is dying. According to the object visibility
+ * rules: the object with dying flag cannot be returned to
+ * others. So the LFSCK thread has to wait until the dying
+ * object has been purged from RAM, then it can allocate a
+ * new object (with the same FID) in RAM. Unfortunately, the
+ * LFSCK thread itself is referencing the parent object, and
+ * cause the parent object cannot be purged, then cause the
+ * child object cannot be purged also. So the LFSCK thread
+ * will fall into deadlock.
+ *
+ * We introduce non-blocked version lu_object_find() to allow
+ * the LFSCK thread to return failure immediately (instead of
+ * wait) when it finds dying (child) object, then the LFSCK
+ * thread can check whether the parent object is dying or not.
+ * So avoid above deadlock. LU-5395 */
+ cobj = lfsck_object_find_by_dev_nowait(env, tgt->ltd_tgt, fid);