Re: [PATCH -mm 2/2] is_current_single_threaded: don't use ->mmap_sem

[Oleg Nesterov <oleg@redhat.com>]

On 07/09, Oleg Nesterov wrote:
>
> is_current_single_threaded() can safely miss a freshly forked CLONE_VM
> task, but in this case it must not miss its parent. That is why we take
> mm->mmap_sem for writing to make sure a thread/task with the same ->mm
> can't pass exit_mm() and disappear.
>
> However we can avoid ->mmap_sem and rely on rcu/barriers:
>
> 	- if we do not see the exiting parent on thread/process list
> 	  we see the result of list_del_rcu(), in this case we must
> 	  also see the result of list_add_rcu() which does wmb().
>
> 	- if we do see the parent but its ->mm == NULL, we need rmb()
> 	  to make sure we can't miss the child.

To simplify the review, below is the code after the patch.

Oleg.

/*
 * Returns true if the task does not share ->mm with another thread/process.
 */
bool is_current_single_threaded(void)
{
	struct task_struct *task = current;
	struct mm_struct *mm = task->mm;
	struct task_struct *p, *t;
	bool ret;

	if (atomic_read(&task->signal->live) != 1)
		return false;

	if (atomic_read(&mm->mm_users) == 1)
		return true;

	ret = false;
	rcu_read_lock();
	for_each_process(p) {
		if (unlikely(p->flags & PF_KTHREAD))
			continue;
		if (unlikely(p == task->group_leader))
			continue;

		t = p;
		do {
			if (unlikely(t->mm == mm))
				goto found;
			if (likely(t->mm))
				break;
			/*
			 * t->mm == NULL. Make sure next_thread/next_task
			 * will see other CLONE_VM tasks which might be
			 * forked before exiting.
			 */
			smp_rmb();
		} while_each_thread(p, t);
	}
	ret = true;
found:
	rcu_read_unlock();

	return ret;
}