Does Linux keep the kernel stack for zombie processes?

Question

I learned in a textbook that Linux keeps the process descriptors when the process becomes a zombie until future parent verifies the exit status. I understand that process descriptor comes in two structs: task_struct in slab, and thread_info in kernel stack (forget x86).

I am reading through <kernel/exit.c> do_exit() part of the source code, but I don't quite get where kernel stack is deallocated. I can find that exit_notify() changes the process' status to zombie. and the rest of the code looks like its about cleaning up mostly locks and stuff until schedule().

I can't seem to find the part for deallocating kernel stack? or am I not understanding how kernel stacks work? Or perhaps thread_info is not considered to be kept at all and was already discarded along with kernel stack before changing into zombie?

What is exactly going on?

Answer 1

After a bit of digging, I think I have finally found it...

void free_task(struct task_struct *tsk)
{
    prop_local_destroy_single(&tsk->dirties);
    account_kernel_stack(tsk->stack, -1);
    free_thread_info(tsk->stack);
    rt_mutex_debug_task_free(tsk);
    ftrace_graph_exit_task(tsk);
    free_task_struct(tsk);
}

on parent verifying the zombie,

put_task_struct()->__put_task_struct()->free_task() does free the kernel stack.

So, the answer is Yes. Zombie processes do keep the kernel stack.