Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762812Ab3IEBMY (ORCPT ); Wed, 4 Sep 2013 21:12:24 -0400 Received: from g5t0009.atlanta.hp.com ([15.192.0.46]:24201 "EHLO g5t0009.atlanta.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753750Ab3IEBMW (ORCPT ); Wed, 4 Sep 2013 21:12:22 -0400 Message-ID: <1378343540.2064.23.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 18:12:20 -0700 In-Reply-To: References: <1377269106-26468-1-git-send-email-zwu.kernel@gmail.com> <1378339153.2064.6.camel@buesod1.americas.hpqcorp.net> 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: 7295 Lines: 148 On Thu, 2013-09-05 at 08:37 +0800, Zhi Yong Wu wrote: > On Thu, Sep 5, 2013 at 7:59 AM, Davidlohr Bueso 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 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(). > 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 -- 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/