linux-kernel - Re: [RFC PATCH] asynchronous page fault.

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20091228090047.280f4565.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Mon, 28 Dec 2009 09:00:47 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>,
	cl@...ux-foundation.org
Subject: Re: [RFC PATCH] asynchronous page fault.

On Sun, 27 Dec 2009 12:19:56 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, 2009-12-25 at 10:51 +0900, KAMEZAWA Hiroyuki wrote:
> > Index: linux-2.6.33-rc2/lib/rbtree.c
> > ===================================================================
> > --- linux-2.6.33-rc2.orig/lib/rbtree.c
> > +++ linux-2.6.33-rc2/lib/rbtree.c
> > @@ -30,19 +30,19 @@ static void __rb_rotate_left(struct rb_n
> >  
> >         if ((node->rb_right = right->rb_left))
> >                 rb_set_parent(right->rb_left, node);
> > -       right->rb_left = node;
> > +       rcu_assign_pointer(right->rb_left, node);
> >  
> >         rb_set_parent(right, parent);
> >  
> >         if (parent)
> >         {
> >                 if (node == parent->rb_left)
> > -                       parent->rb_left = right;
> > +                       rcu_assign_pointer(parent->rb_left, right);
> >                 else
> > -                       parent->rb_right = right;
> > +                       rcu_assign_pointer(parent->rb_right, right);
> >         }
> >         else
> > -               root->rb_node = right;
> > +               rcu_assign_pointer(root->rb_node, right);
> >         rb_set_parent(node, right);
> >  }
> >  
> > @@ -53,19 +53,19 @@ static void __rb_rotate_right(struct rb_
> >  
> >         if ((node->rb_left = left->rb_right))
> >                 rb_set_parent(left->rb_right, node);
> > -       left->rb_right = node;
> > +       rcu_assign_pointer(left->rb_right, node);
> >  
> >         rb_set_parent(left, parent);
> >  
> >         if (parent)
> >         {
> >                 if (node == parent->rb_right)
> > -                       parent->rb_right = left;
> > +                       rcu_assign_pointer(parent->rb_right, left);
> >                 else
> > -                       parent->rb_left = left;
> > +                       rcu_assign_pointer(parent->rb_left, left);
> >         }
> >         else
> > -               root->rb_node = left;
> > +               rcu_assign_pointer(root->rb_node, left);
> >         rb_set_parent(node, left);
> >  }
> 
> 
> Consider the tree rotation:
> 
> 
>            Q                        P
>          /   \                    /   \
>        P       C                A       Q
>      /   \                            /   \
>    A       B                        B       C
> 
> 
> Since this comprises of 3 assignments (assuming right rotation):
> 
>   Q.left = B
>   P.right = Q
>   parent = P
> 
> it is non-atomic. This in turn means that any lock-less decent into the
> tree will be able to miss a whole subtree or worse (imagine us being at
> Q, needing to go to A, then the rotation happens, and all we can choose
> from is B or C).
> 
> Your changelog states as much.
> 
> "Even if RB-tree rotation occurs while we walk tree for look-up, we just
> miss vma without oops."
> 
> However, since this is the case, do we still need the
> rcu_assign_pointer() conversion your patch does? All I can see it do is
> slow down all RB-tree users, without any gain.
> 

Ok, I'll remove all.

Thanks,
-Kame



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/