Background

Under certain conditions, it may be necessary to create a kernel thread whose stack lives in some custom memory.  This page provides and example of how that would be done:

Example

Here is the body of some function.  It expects to have the following inputs:

  1. taskname:  The name of the kernel thread to be started
  2. stacksize:  The size of the custom stack
  3. priority:  The priority of the kernel thread to be started
  4. entry_point:  The entry point of the kernel thread to be started
  5. argv:  An optional array of argument strings passed to the kernel thread


  /* Allocate a TCB for the new kernel thread.  kmm_zalloc() is
   * used to that all fields of the new TCB will be zeroed.
   */

  tcb = (FAR struct task_tcb_s *)kmm_zalloc(sizeof(struct task_tcb_s));
  if (tcb == NULL)
    {
      return -ENOMEM;
    }

  /* Indicate (1) that this is a kernel thread and that (2) a custom
   * stack will be used.
   */

  tcb->flags = TCB_FLAG_TTYPE_KERNEL | TCB_FLAG_CUSTOM_STACK;

  /* Allocate the custom stack for the new kernel thread.
   *
   * Do whatever it takes to get a reference to the custom stack.
   * Here custom_alloc() is used as a placeholder for whatever
   * that may be.
   */

  stack = (FAR uint32_t *)custom_alloc(stacksize);
  if (stack == NULL)
    {
      kmm_free(tcb);
      return -ENOMEM;
    }

  /* Initialize the TCB.  This will initialize all remaining
   * fields of the TCB, associate the stack to the TCB, allocate
   * any additional resources needed by the kernel thread, and
   * place the TCB in a list of inactive tasks.
   */

  ret = task_init((FAR struct tcb_s *)tcb, progname, priority,
                  stack, stacksize, entry_point, argv);
  if (ret < 0)
    {
      kmm_free(tcb);
      custom_free(stack);
      return ret;
    }
  /* Then activate the kernel thread at the provided priority */

  ret = task_activate((FAR struct tcb_s *)tcb);
  if (ret < 0)
    {
      /* nxtask_unit() will undo all of the operations of nxtask_init().
       * It also has the side-effect of freeing the TCB which it assumes
       * was allocated with one of the kmm_malloc()functions.
       */

      nxtask_uninit(tcb);
      custom_free(stack);
      return ret;
    }

  return OK;

Freeing the TCB

Prior to calling nxtask_init(), the TCB  can be freed using the kmm allocator, specifically the function kmm_free().  However, after nxtask_init() is called, additional resources will be associated with the TCB and you must then call nxtask_uninit() to free the TCB and all of its associated resources.  kmm_free() will be used internally by nxtask_uninit() to free the TCB.  Note that in any event, the TCB must be allocated with one of the kmm_malloc() allocation functions.

You must never free the TCB after nxtask_activate() returns successfully.

Freeing the Custom Stack Memory

The effect of the TCB_FLAG_CUSTOM_STACK flag is that the OS will not attempt to free the custom stack memory if the kernel thread exits, crashes, or is killed.  Does this matter in your implementation?  Could this result in some kind of memory leak?  If any kind of clean-up is required by your application to free the custom stack memory, you will probably want to use an on_exit() or atexit() function to get a callback when the kernel thread is terminated.

If TCB_FLAG_CUSTOM_STACK were not set in the TCB flags, the OS would attempt to free the stack using kmm_free() which is probably not what you want in this case.

The actual logic is a slightly more complex and somewhat redundant:

  • If TCB_FLAG_CUSTOM_STACK is set in the TCB flags, no attempt will be made to free the custom stack.
  • If TCB_FLAG_CUSTOM_STACK is not set in the TCB flags, the stack will be de-allocated for the kernel thread only if the stack lies in the kernel memory pool.

So in reality TCB_FLAG_CUSTOM_STACK may not be necessary.  But the safest option is to include it in all cases where you do not expect the custom stack to be de-allocated.

You must not free the custom stack after nxtask_activate() returns successfully and until the kernel thread is terminated.



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