Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932405Ab3IEAhV (ORCPT ); Wed, 4 Sep 2013 20:37:21 -0400 Received: from mail-qc0-f173.google.com ([209.85.216.173]:44099 "EHLO mail-qc0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760774Ab3IEAhU (ORCPT ); Wed, 4 Sep 2013 20:37:20 -0400 MIME-Version: 1.0 In-Reply-To: <1378339153.2064.6.camel@buesod1.americas.hpqcorp.net> References: <1377269106-26468-1-git-send-email-zwu.kernel@gmail.com> <1378339153.2064.6.camel@buesod1.americas.hpqcorp.net> Date: Thu, 5 Sep 2013 08:37:19 +0800 Message-ID: Subject: Re: [PATCH] rbtree: Add some necessary condition checks From: Zhi Yong Wu To: Davidlohr Bueso Cc: Michel Lespinasse , linux-kernel mlist , akpm@linux-foundation.org, Zhi Yong Wu Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7773 Lines: 170 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. > >> 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. 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. > > 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. >> >> >> > > -- Regards, Zhi Yong Wu -- 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/