Re: Read the "real_parent" field of task_struct

Oct. 8, 2020


      Hi,

On Fri, Oct 02, 2020 at 06:59:22PM +0200, John Wood wrote:
...
static void brute_share_stats(struct brute_stats **src,
      	      struct brute_stats **dst)
{
  spin_lock(&(*src)->lock);
  refcount_inc(&(*src)->refc);
  *dst = *src;
  spin_unlock(&(*src)->lock);
}
static int brute_task_alloc(struct task_struct *task, unsigned long clone_flags)
{
  struct task_struct *p_task;
  struct brute_stats **stats, **p_stats;
p_task = task->real_parent;		/////////// <----
  if (unlikely(!p_task))			/////////// <----
      return -ESRCH;
stats = brute_stats_ptr(task);
  p_stats = brute_stats_ptr(p_task);	/////////// <----
if (likely(*p_stats)) {
      brute_share_stats(p_stats, stats);
      return 0;
  }
*stats = brute_new_stats();
  if (!*stats)
      return -ENOMEM;
brute_share_stats(stats, p_stats);
  return 0;
}
This code is very untested.
Now the code is tested.
...
And now my first question: how can I read the
real_parent field in a secure way. Do I need to use an rcu_read_lock()/
rcu_read_unlock() block? Do I need to use rcu_dereference? Do I need to
use a read_lock(&task_list_lock)/read_unlock(&task_list_lock) block?
The lines with the mark are not clear to me. Sorry.
Any help would be greatly appreciated. Thanks in advance.

Regards,
John Wood