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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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