RE: [PATCH 2/2] radix-tree: Fix optimisation problem

From: Matthew Wilcox
Date: Mon Sep 26 2016 - 17:28:10 EST

From: linus971@xxxxxxxxx [mailto:linus971@xxxxxxxxx] On Behalf Of Linus Torvalds
> On Sun, Sep 25, 2016 at 12:04 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > It gets rid of
> > the ad-hoc arithmetic in radix_tree_descend(), and just makes all that
> > be inside the is_sibling_entry() logic instead. Which got renamed and
> > made to actually return the main sibling.
> Sadly, it looks like gcc generates bad code for this approach. Looks
> like it ends up testing the resulting sibling pointer twice (because
> we explicitly disable -fno-delete-null-pointer-checks in the kernel,
> and we have no way to say "look, I know this pointer I'm returning is
> non-null").
> So a smaller patch that keeps the old boolean "is_sibling_entry()" but
> then actually *uses* that inside radix_tree_descend() and then tries
> to make the nasty cast to "void **" more legible by making it use a
> temporary variable seems to be a reasonable balance.
> At least I feel like I can still read the code, but admittedly by now
> that may be because I've stared at those few lines so much that I feel
> like I know what's going on. So maybe the code isn't actually any more
> legible after all.
> .. and unlike my previous patch, it actually generates better code
> than the original (while still passing the fixed test-suite, of
> course). The reason seems to be exactly that temporary variable,
> allowing us to just do
> entry = rcu_dereference_raw(*sibentry);
> rather than doing
> entry = rcu_dereference_raw(parent->slots[offset]);
> with the re-computed offset.
> So I think I'll commit this unless somebody screams.

Acked-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>

I don't love it. But I think it's a reasonable fix for this point in the release cycle, and I have an idea for changing the representation of sibling slots that will make this moot.

(Basically adopting Konstantin's idea for using the *last* entry instead of the *first*, and then using entries of the form (offset << 2 | RADIX_TREE_INTERNAL_NODE), so we can identify sibling entries without knowing the parent pointer, and we can go straight from sibling entry to slot offset as a shift rather than as a pointer subtraction).