13902 Fix for 13717 may break 8-disk raidz2
13915 installctx() blocking allocate causes problems
Portions contributed by: Jerry Jelinek <gjelinek@gmail.com>
Change-Id: I934d69946cec42630fc541fa8c7385b862b69ca2

*** 19,29 ****
   * CDDL HEADER END
   */
  
  /*
   * Copyright (c) 1991, 2010, Oracle and/or its affiliates. All rights reserved.
!  * Copyright 2019 Joyent, Inc.
   */
  
  #include <sys/types.h>
  #include <sys/param.h>
  #include <sys/sysmacros.h>
--- 19,29 ----
   * CDDL HEADER END
   */
  
  /*
   * Copyright (c) 1991, 2010, Oracle and/or its affiliates. All rights reserved.
!  * Copyright 2021 Joyent, Inc.
   */
  
  #include <sys/types.h>
  #include <sys/param.h>
  #include <sys/sysmacros.h>
*** 1028,1038 ****
--- 1028,1059 ----
  
          mutex_exit(&reaplock);
  }
  
  /*
+  * Provide an allocation function for callers of installctx() that, for
+  * reasons of incomplete context-op initialization, must call installctx()
+  * in a kpreempt_disable() block.  The caller, therefore, must call this
+  * without being in such a block.
+  */
+ struct ctxop *
+ installctx_preallocate(void)
+ {
+         /*
+          * NOTE: We could ASSERT/VERIFY that we are not in a place where
+          * a KM_SLEEP allocation could block indefinitely.
+          *
+          * ASSERT(curthread->t_preempt == 0);
+          */
+ 
+         return (kmem_alloc(sizeof (struct ctxop), KM_SLEEP));
+ }
+ 
+ /*
   * Install thread context ops for the current thread.
+  * The caller can pass in a preallocated struct ctxop, eliminating the need
+  * for the requirement of entering with kernel preemption still enabled.
   */
  void
  installctx(
          kthread_t *t,
          void    *arg,
*** 1039,1053 ****
          void    (*save)(void *),
          void    (*restore)(void *),
          void    (*fork)(void *, void *),
          void    (*lwp_create)(void *, void *),
          void    (*exit)(void *),
!         void    (*free)(void *, int))
  {
!         struct ctxop *ctx;
! 
          ctx = kmem_alloc(sizeof (struct ctxop), KM_SLEEP);
          ctx->save_op = save;
          ctx->restore_op = restore;
          ctx->fork_op = fork;
          ctx->lwp_create_op = lwp_create;
          ctx->exit_op = exit;
--- 1060,1075 ----
          void    (*save)(void *),
          void    (*restore)(void *),
          void    (*fork)(void *, void *),
          void    (*lwp_create)(void *, void *),
          void    (*exit)(void *),
!         void    (*free)(void *, int),
!         struct ctxop *ctx)
  {
!         if (ctx == NULL)
                  ctx = kmem_alloc(sizeof (struct ctxop), KM_SLEEP);
+ 
          ctx->save_op = save;
          ctx->restore_op = restore;
          ctx->fork_op = fork;
          ctx->lwp_create_op = lwp_create;
          ctx->exit_op = exit;