Re: [BUG] Use of probe_kernel_address() in task_rcu_dereference() without checking return value

[Oleg Nesterov]

On 08/30, Linus Torvalds wrote:
>
> But yes, your hack is I guess optimal for this particular case where
> you simply can depend on "we know the pointer was valid, we just don't
> know if it was freed".
>
> Hmm. Don't we RCU-free the task struct? Because then we don't even
> need to care about CONFIG_DEBUG_PAGEALLOC. We can just always access
> the pointer as long as we have the RCU read lock.

For example,

	rcu_read_lock();
	p = task_rcu_dereference(&cpu_rq(cpu)->curr);
	rcu_read_unlock();

->curr is not protected by RCU, the last schedule does put_task_struct()
in finish_task_switch().

Of course we can change this and add another call_rcu (actually we can do
better), and after that we do not need task_rcu_dereference() at all.

Oleg.