You might also try to use valid, plain ISO C99 instead of perverted gcc extensions which only cause a lot of trouble in the long run. Ced On 26 September 2016 at 23:28, Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> wrote: > 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). -- Cedric Blancher <cedric.blancher@xxxxxxxxx> [https://plus.google.com/u/0/+CedricBlancher/] Institute Pasteur -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href