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SubjectRe: [RFC PATCH] asynchronous page fault.
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On Fri, 2009-12-25 at 10:51 +0900, KAMEZAWA Hiroyuki wrote:
> Index: linux-2.6.33-rc2/lib/rbtree.c
> ===================================================================
> --- linux-2.6.33-rc2.orig/lib/rbtree.c
> +++ linux-2.6.33-rc2/lib/rbtree.c
> @@ -30,19 +30,19 @@ static void __rb_rotate_left(struct rb_n
>
> if ((node->rb_right = right->rb_left))
> rb_set_parent(right->rb_left, node);
> - right->rb_left = node;
> + rcu_assign_pointer(right->rb_left, node);
>
> rb_set_parent(right, parent);
>
> if (parent)
> {
> if (node == parent->rb_left)
> - parent->rb_left = right;
> + rcu_assign_pointer(parent->rb_left, right);
> else
> - parent->rb_right = right;
> + rcu_assign_pointer(parent->rb_right, right);
> }
> else
> - root->rb_node = right;
> + rcu_assign_pointer(root->rb_node, right);
> rb_set_parent(node, right);
> }
>
> @@ -53,19 +53,19 @@ static void __rb_rotate_right(struct rb_
>
> if ((node->rb_left = left->rb_right))
> rb_set_parent(left->rb_right, node);
> - left->rb_right = node;
> + rcu_assign_pointer(left->rb_right, node);
>
> rb_set_parent(left, parent);
>
> if (parent)
> {
> if (node == parent->rb_right)
> - parent->rb_right = left;
> + rcu_assign_pointer(parent->rb_right, left);
> else
> - parent->rb_left = left;
> + rcu_assign_pointer(parent->rb_left, left);
> }
> else
> - root->rb_node = left;
> + rcu_assign_pointer(root->rb_node, left);
> rb_set_parent(node, left);
> }


Consider the tree rotation:


Q P
/ \ / \
P C A Q
/ \ / \
A B B C


Since this comprises of 3 assignments (assuming right rotation):

Q.left = B
P.right = Q
parent = P

it is non-atomic. This in turn means that any lock-less decent into the
tree will be able to miss a whole subtree or worse (imagine us being at
Q, needing to go to A, then the rotation happens, and all we can choose
from is B or C).

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.



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