OS-1988 Make ldi_ev_remove_callbacks safe to use in LDI callbacks

*** 20,35 ****
   */
  /*
   * Copyright 2007 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
   */
  
  #ifndef _SYS_SUNLDI_IMPL_H
  #define _SYS_SUNLDI_IMPL_H
  
- #pragma ident   "%Z%%M% %I%     %E% SMI"
- 
  #ifdef __cplusplus
  extern "C" {
  #endif
  
  #include <sys/dditypes.h>
--- 20,36 ----
   */
  /*
   * Copyright 2007 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
   */
+ /*
+  * Copyright (c) 2013, Joyent, Inc.  All rights reserved.
+  */
  
  #ifndef _SYS_SUNLDI_IMPL_H
  #define _SYS_SUNLDI_IMPL_H
  
  #ifdef __cplusplus
  extern "C" {
  #endif
  
  #include <sys/dditypes.h>
*** 139,154 ****
--- 140,172 ----
          void                    *lec_cookie;
          void                    *lec_id;
          list_node_t             lec_list;
  } ldi_ev_callback_impl_t;
  
+ /*
+  * Members of "struct ldi_ev_callback_list" are protected by their le_lock
+  * member.  The struct is currently only used once, as a file-level global,
+  * and the locking protocol is currently implemented in ldi_ev_lock() and
+  * ldi_ev_unlock().
+  *
+  * When delivering events to subscribers, ldi_invoke_notify() and
+  * ldi_invoke_finalize() will walk the list of callbacks: le_head.  It is
+  * possible that an invoked callback function will need to unregister an
+  * arbitrary number of callbacks from this list.
+  *
+  * To enable ldi_ev_remove_callbacks() to remove elements from the list
+  * without breaking the walk-in-progress, we store the next element in the
+  * walk direction on the struct as le_walker_next and le_walker_prev.
+  */
  struct ldi_ev_callback_list {
          kmutex_t                le_lock;
          kcondvar_t              le_cv;
          int                     le_busy;
          void                    *le_thread;
          list_t                  le_head;
+         ldi_ev_callback_impl_t  *le_walker_next;
+         ldi_ev_callback_impl_t  *le_walker_prev;
  };
  
  int ldi_invoke_notify(dev_info_t *dip, dev_t dev, int spec_type, char *event,
      void *ev_data);
  void ldi_invoke_finalize(dev_info_t *dip, dev_t dev, int spec_type, char *event,