Re: [PATCH 1/2] rbtree_latch: quit searching when reaching to maximum depth

[Peter Zijlstra <peterz@infradead.org>]

On Fri, May 15, 2020 at 12:47:06PM +0000, Lai Jiangshan wrote:
> lib/rbtree.c has ensured that there is not possible to
> inadvertently cause (temporary) loops in the tree structure
> as seen in program order of the modifier. But loop is still
> possible to be seen in searcher due to CPU's reordering.
> 
> for example:
> modifier				searcher
> 
> left rotate at parent
> parent->rb_right is node
> 					search to parent
> 					parent->rb_right is node
> 				     +->see node->rb_left changed
> WRITE_ONCE(parent->rb_right, tmp);-+ |  node->rb_left is parennt
> no smp_wmb(), some arch can        | |
> reorder these two writes           | |  loop long between
> WRITE_ONCE(node->rb_left, parent);-+-+  parent and node
> 				   |
> 				   +--->finally see
> 					parent->rb_right
> 
> The long loop won't stop until the modifer's CPU flushes
> its writes. Too avoid it, we should limit the searching depth.

Cute, have you actually observed this? Did you have performance issues?

> There are no more than (1<<BITS_PER_LONG)-1 nodes in the tree.
> And the max_depth of a tree is no more than 2*lg(node_count+1),
> which is no mare than 2*BITS_PER_LONG.
> 
> So the serarch should stop when diving down up to
> 2*BITS_PER_LONG depth.

Arguably you can have a larger key space, but I think due to memory
constraints this limit still isn't wrong. But I do feel you need a
comment with that.