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Date: Wed, 04 Sep 2013 18:12:20 -0700
From: Davidlohr Bueso <davidlohr@...com>
To: Zhi Yong Wu <zwu.kernel@...il.com>
Cc: Michel Lespinasse <walken@...gle.com>,
linux-kernel mlist <linux-kernel@...r.kernel.org>,
akpm@...ux-foundation.org, Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
Subject: Re: [PATCH] rbtree: Add some necessary condition checks
On Thu, 2013-09-05 at 08:37 +0800, Zhi Yong Wu wrote:
> On Thu, Sep 5, 2013 at 7:59 AM, Davidlohr Bueso <davidlohr@...com> wrote:
> > 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@...gle.com> wrote:
> >> > On Sun, Sep 1, 2013 at 11:30 PM, Zhi Yong Wu <zwu.kernel@...il.com> wrote:
> >> >> In Tue, Aug 27, 2013 at 6:01 AM, Michel Lespinasse <walken@...gle.com> wrote:
> >> >>> On Fri, Aug 23, 2013 at 7:45 AM, <zwu.kernel@...il.com> wrote:
> >> >>>> From: Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
> >> >>>>
> >> >>>> Signed-off-by: Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
> >> >>>> ---
> >> >>>> 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().
> ah, but this code is very tricky.
>
While it's not trivial, it is a lot more readable than a whole bunch of
red-black tree implementations out there - not to mention optimized.
AFAICT by the thread, you have yet to provide a case where, by properly
using the rbtree API, the tree implementation does not comply.
> >
> >> 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.
> rbtree_test seem to be not available for my patchset.
Why not? Is this an older kernel you're dealing with?
> my perf testing
> is super large scale, and it will try to create 1,0000,000 rb_nodes,
> while rbtree_test try to verify the rbtree when every rb_node is
> reserted or erased. This will cause my perf testing to be running very
> very slowly.
>
May I ask what you are attempting to do here? Are you trying to stress
the kernel's rbtree implementation?
Well, performance isn't a concern when doing this kind of testing. Yes,
the tree integrity verification (check() calls) is done when inserting
and deleting every node, which is the whole purpose of such tests. I
admit that the rbtree_test module is a bit limited as to user options -
ie: making the amount of nodes be a parameter is on my todo list. That
said, the check() function does properly test the rbtree properties, and
so far it complies.
Thanks,
Davidlohr
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