Re: introduce task_rcu_dereference?

[Oleg Nesterov <oleg@redhat.com>]

On 10/23, Kirill Tkhai wrote:
>
> I'm agree generic helper is better. But probe_slab_address() has a sence
> if we know that SDBR is worse in our subject area.

And I still think it is worse.

> Less of code is
> easier to support :)

Sure, but ignoring the comments, SDBR needs the same code in
task_rcu_dereference() ? Except, of course

	-	probe_slab_address(&task->sighand, sighand);
	+	sighand = task->sighand;

or how do you think we can simplify it?


> probe_slab_address() it's not a trivial logic.

But it already has a user. And probably it can have more.

To me the usage of SDBR is not trivial (and confusing) in this case.
Once again, ignoring the CONFIG_DEBUG_PAGEALLOC problems it does not
help at all.

With or without SDBR rq->curr can be reused and we need to avoid this
race. The fact that with SDBR it can be reused only as another instance
of task_struct is absolutely immaterial imo.

Not to mention that SDBR still adds some overhead while probe_slab()
is free unless CONFIG_DEBUG_PAGEALLOC, but this option adds a large
slowdown anyway.


But again, I can't really work today, perhaps I missed something.
Perhaps you can show a better code which relies on SDBR?

Oleg.