Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762532Ab3IDX7R (ORCPT ); Wed, 4 Sep 2013 19:59:17 -0400 Received: from g1t0029.austin.hp.com ([15.216.28.36]:5092 "EHLO g1t0029.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762462Ab3IDX7P (ORCPT ); Wed, 4 Sep 2013 19:59:15 -0400 Message-ID: <1378339153.2064.6.camel@buesod1.americas.hpqcorp.net> Subject: Re: [PATCH] rbtree: Add some necessary condition checks From: Davidlohr Bueso To: Zhi Yong Wu Cc: Michel Lespinasse , linux-kernel mlist , akpm@linux-foundation.org, Zhi Yong Wu Date: Wed, 04 Sep 2013 16:59:13 -0700 In-Reply-To: References: <1377269106-26468-1-git-send-email-zwu.kernel@gmail.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.4.4 (3.4.4-2.fc17) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7196 Lines: 154 On Thu, 2013-09-05 at 01:22 +0800, Zhi Yong Wu wrote: > On Mon, Sep 2, 2013 at 4:57 PM, Michel Lespinasse wrote: > > On Sun, Sep 1, 2013 at 11:30 PM, Zhi Yong Wu wrote: > >> In Tue, Aug 27, 2013 at 6:01 AM, Michel Lespinasse wrote: > >>> On Fri, Aug 23, 2013 at 7:45 AM, wrote: > >>>> From: Zhi Yong Wu > >>>> > >>>> Signed-off-by: Zhi Yong Wu > >>>> --- > >>>> 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. > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/