Re: [PATCH] rbtree: Add some necessary condition checks

[Davidlohr Bueso]

On Thu, 2013-09-05 at 01:22 +0800, Zhi Yong Wu wrote:
> On Mon, Sep 2, 2013 at 4:57 PM, Michel Lespinasse <walken@xxxxxxxxxx> wrote:
> > On Sun, Sep 1, 2013 at 11:30 PM, Zhi Yong Wu <zwu.kernel@xxxxxxxxx> wrote:
> >> In Tue, Aug 27, 2013 at 6:01 AM, Michel Lespinasse <walken@xxxxxxxxxx> wrote:
> >>> On Fri, Aug 23, 2013 at 7:45 AM,  <zwu.kernel@xxxxxxxxx> wrote:
> >>>> From: Zhi Yong Wu <wuzhy@xxxxxxxxxxxxxxxxxx>
> >>>>
> >>>> Signed-off-by: Zhi Yong Wu <wuzhy@xxxxxxxxxxxxxxxxxx>
> >>>> ---
> >>>>  include/linux/rbtree_augmented.h | 3 ++-
> >>>>  lib/rbtree.c                     | 5 +++--
> >>>>  2 files changed, 5 insertions(+), 3 deletions(-)
> >>>
> >>> So, you are saying that the checks are necessary, but you are not saying why.
> >>>
> >>> The way I see it, the checks are *not* necessary, because the rbtree
> >>> invariants guarantee them to be true. The only way for the checks to
> >>> fail would be if people directly manipulate the rbtrees without going
> >>> through the proper APIs, and if they do that then I think they're on
> >>> their own. So to me, I think it's the same situation as dereferencing
> >>> a pointer without checking if it's NULL, because you know it should
> >>> never be NULL - which in my eyes is perfectly acceptable.
> >> In my patchset, some rbtree APIs to be invoked, and I think that those
> >> rbtree APIs are used corrently, Below is the pointer of its code:
> >> https://github.com/wuzhy/kernel/compare/torvalds:master...hot_tracking
> >> But I hit some issues when using compilebench to do perf benchmark.
> >> compile dir kernel-7 691MB in 8.92 seconds (77.53 MB/s)
> >
> > Thanks for the link - I now better understand where you are coming
> > from with these fixes.
> >
> > Going back to the original message:
> >
> >> diff --git a/include/linux/rbtree_augmented.h b/include/linux/rbtree_augmented.h
> >> index fea49b5..7d19770 100644
> >> --- a/include/linux/rbtree_augmented.h
> >> +++ b/include/linux/rbtree_augmented.h
> >> @@ -199,7 +199,8 @@ __rb_erase_augmented(struct rb_node *node, struct rb_root *root,
> >>                 }
> >>
> >>                 successor->rb_left = tmp = node->rb_left;
> >> -               rb_set_parent(tmp, successor);
> >> +               if (tmp)
> >> +                       rb_set_parent(tmp, successor);
> >>
> >>                 pc = node->__rb_parent_color;
> >>                 tmp = __rb_parent(pc);
> >
> > Note that node->rb_left was already fetched at the top of
> > __rb_erase_augmented(), and was checked to be non-NULL at the time -
> > otherwise we would have executed 'Case 1' in that function. So, you
> > are not expected to find tmp == NULL here.
> >
> >> diff --git a/lib/rbtree.c b/lib/rbtree.c
> >> index c0e31fe..2cb01ba 100644
> >> --- a/lib/rbtree.c
> >> +++ b/lib/rbtree.c
> >> @@ -214,7 +214,7 @@ ____rb_erase_color(struct rb_node *parent, struct rb_root *root,
> >>                  */
> >>                 sibling = parent->rb_right;
> >>                 if (node != sibling) {  /* node == parent->rb_left */
> >> -                       if (rb_is_red(sibling)) {
> >> +                       if (sibling && rb_is_red(sibling)) {
> >>                                 /*
> >>                                  * Case 1 - left rotate at parent
> >>                                  *
> >
> > Note the loop invariants quoted just above:
> >
> >                 /*
> >                  * Loop invariants:
> >                  * - node is black (or NULL on first iteration)
> >                  * - node is not the root (parent is not NULL)
> >                  * - All leaf paths going through parent and node have a
> >                  *   black node count that is 1 lower than other leaf paths.
> >                  */
> >
> > Because of these, each path from sibling to a leaf must include at
> > least one black node, which implies that sibling can't be NULL - or to
> > put it another way, if sibling is null then the expected invariants
> > were violated before we even got there.
> >
> >> @@ -226,7 +226,8 @@ ____rb_erase_color(struct rb_node *parent, struct rb_root *root,
> >>                                  */
> >>                                 parent->rb_right = tmp1 = sibling->rb_left;
> >>                                 sibling->rb_left = parent;
> >> -                               rb_set_parent_color(tmp1, parent, RB_BLACK);
> >> +                               if (tmp1)
> >> +                                       rb_set_parent_color(tmp1, parent, RB_BLACK);
> >>                                 __rb_rotate_set_parents(parent, sibling, root,
> >>                                                         RB_RED);
> >>                                 augment_rotate(parent, sibling);
> >
> > This is actually the same invariant here - each path from sibling to a
> > leaf must include at least one black node, and sibling is now known to
> > be red, so it must have two black children.
> If sibling is red, it can be made sure to have two non-null black
> children? 

This is guaranteed by cases 1 and 2 in __rb_insert().

> but my patchset sometimes trigger red sibling to have no
> non-null black children. Do you know what reason usually cause this?
> You know rbtree code is very tricky.

I haven't looked at your code, but a good way of verifying the tree
integrity is running rbtree_test.

Thanks,
Davidlohr

> 
> >
> >
> > Now I had a quick look at your code and I couldn't tell at which point
> > the invariants are violated. However I did notice a couple suspicious
> > things in the very first patch
> > (f5c8f2b256d87ac0bf789a787e6b795ac0c736e8):
> >
> > 1- In both hot_range_tree_free() and and hot_tree_exit(), you try to
> > destroy rb trees by iterating on each node with rb_next() and then
> > freeing them. Note that rb_next() can reference prior nodes, which
> > have already been freed in your scheme, so that seems quite unsafe.
> >
> > The simplest fix would be to do a full rb_erase() on each node before
> > freeing it. (you may be able to avoid rebalancing the tree here as
> > you're going to destroy it all, but if you really have that need it
> > would be better to come up with a new API to cover it rather than
> > hardcode it where you need it - I think it's easiest to start with the
> > simple dumb fix of using rb_erase).
> >
> > 2- I did not look long enough to understand the locking, but it wasn't
> > clear to me if you lock the rbtrees when doing rb_erase() on them
> > (while I could more clearly see that you do it for insertions).
> >
> > I'm really not sure if either of these will fix the issues you're
> > seeing, though. What I would try next would be to add explicit rbtree
> > invariant checks before and after rbtree manipulations, like what the
> > check() function does in lib/rbtree_test.c, to see at which point do
> > they get broken.
> >
> > --
> > Michel "Walken" Lespinasse
> > A program is never fully debugged until the last user dies.
> 
> 
> 


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