Re: [RFC PATCH] asynchronous page fault.

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2009-12-28 at 06:27 +0530, Balbir Singh wrote:
> * Peter Zijlstra <peterz@infradead.org> [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?

rcu_assign_pointer() is the write side, but if you need a barrier, you
can make do with a single smp_wmb() after doing the rb-tree op. There is
no need to add multiple in the tree-ops themselves.

You cannot make the assignment paths atomic (without locks) that's the
whole problem.