Re: [RFC PATCH] asynchronous page fault.

[Balbir Singh]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2009-12-28 10:05:14]:

> On Mon, 28 Dec 2009 06:27:46 +0530
> Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > * Peter Zijlstra <peterz@xxxxxxxxxxxxx> [2009-12-27 12:19:56]:
> > 
> > > Your changelog states as much.
> > > 
> > > "Even if RB-tree rotation occurs while we walk tree for look-up, we just
> > > miss vma without oops."
> > > 
> > > However, since this is the case, do we still need the
> > > rcu_assign_pointer() conversion your patch does? All I can see it do is
> > > slow down all RB-tree users, without any gain.
> > 
> > Don't we need the rcu_assign_pointer() on the read side primarily to
> > make sure the pointer is still valid and assignments (writes) are not
> > re-ordered? Are you suggesting that the pointer assignment paths be
> > completely atomic?
> > 
> >From following reasons.
>   - What we have to avoid is not to touch unkonwn memory via broken pointer.
>     This is speculative look up and can miss vmas. So, even if tree is broken,
>     there is no problem. Broken pointer which points to places other than rb-tree
>     is problem.

Exactly!

>   - rb-tree's rb_left and rb_right don't points to memory other than
>     rb-tree. (or NULL)  And vmas are not freed/reused while rcu_read_lock().
>     Then, we don't dive into unknown memory.
>   - Then, we can skip rcu_assign_pointer().
>

We can, but the data being on read-side is going to be out-of-date
more than without the use of rcu_assign_pointer(). Do we need variants
like to rcu_rb_next() to avoid overheads for everyone?

-- 
	Balbir
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