8115 parallel zfs mount

*** 20,30 ****
   */
  
  /*
   * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright (c) 2011 Pawel Jakub Dawidek. All rights reserved.
!  * Copyright (c) 2011, 2016 by Delphix. All rights reserved.
   */
  
  #ifndef _LIBZFS_IMPL_H
  #define _LIBZFS_IMPL_H
  
--- 20,30 ----
   */
  
  /*
   * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright (c) 2011 Pawel Jakub Dawidek. All rights reserved.
!  * Copyright (c) 2011, 2017 by Delphix. All rights reserved.
   */
  
  #ifndef _LIBZFS_IMPL_H
  #define _LIBZFS_IMPL_H
  
*** 31,40 ****
--- 31,41 ----
  #include <sys/fs/zfs.h>
  #include <sys/spa.h>
  #include <sys/nvpair.h>
  #include <sys/dmu.h>
  #include <sys/zfs_ioctl.h>
+ #include <synch.h>
  
  #include <libuutil.h>
  #include <libzfs.h>
  #include <libshare.h>
  #include <libzfs_core.h>
*** 71,80 ****
--- 72,88 ----
          char libzfs_desc[1024];
          int libzfs_printerr;
          int libzfs_storeerr; /* stuff error messages into buffer */
          void *libzfs_sharehdl; /* libshare handle */
          boolean_t libzfs_mnttab_enable;
+         /*
+          * We need a lock to handle the case where parallel mount threads
+          * are populating the mnttab cache simultaneously. The lock only
+          * protects the integrity of of the avl tree, and does not protect
+          * the contents of the mnttab entries themselves.
+          */
+         mutex_t libzfs_mnttab_cache_lock;
          avl_tree_t libzfs_mnttab_cache;
          int libzfs_pool_iter;
          topo_hdl_t *libzfs_topo_hdl;
          libzfs_fru_t **libzfs_fru_hash;
          libzfs_fru_t *libzfs_fru_list;