Re: introduce task_rcu_dereference?

[Oleg Nesterov <oleg@redhat.com>]

Damn.

On 10/22, Oleg Nesterov wrote:
>
> +struct task_struct *task_rcu_dereference(struct task_struct **ptask)
> +{
> +	struct task_struct *task;
> +	struct sighand_struct *sighand;
> +
> +	task = rcu_dereference(*ptask);
> +	if (!task)
> +		return NULL;
> +
> +	/* If it fails the check below must fail too */
> +	probe_slab_address(&task->sighand, sighand);
> +	/*
> +	 * Pairs with atomic_dec_and_test() in put_task_struct(task).
> +	 * If we have read the freed/reused memory, we must see that
> +	 * the pointer was updated. The caller might want to retry in
> +	 * this case.
> +	 */
> +	smp_rmb();
> +	if (unlikely(task != ACCESS_ONCE(*ptask)))
> +		return ERR_PTR(-EAGAIN);

This is not exactly right. task == *ptask can be false positive.

It can be freed, then resused (so that sighand != NULL can be false
positive), then freed again, and then reused again as task_struct.

This is not that bad, we still can safely use this task_struct, but
the comment should be updated. Plus -EINVAL below can be wrong in
this case although this minor.

Yeees, SLAB_DESTTROY_BY_RCU closes this race. Not sure why I'd like
to avoid it, but I do ;)

Oleg.