Re: ipv4: crash at leaf_walk_rcu

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Jul 31, 2013 at 03:13:23PM +0200, Hannes Frederic Sowa wrote:
> On Wed, Jul 31, 2013 at 05:55:13AM -0700, Paul E. McKenney wrote:
> > On Wed, Jul 31, 2013 at 04:40:47PM +0530, vinayak menon wrote:
> > > Hi,
> > > 
> > > A crash was seen on 3.4.5 kernel during some random wlan operations.
> > > 
> > > CPU: Single core ARM Cortex A9.
> > > 
> > > fib_route_seq_next was called with second argument (void *v) as 0xd6e3e360
> > > which is a "freed" object of the "ip_fib_trie" cache. I confirmed that the
> > > object was freed with crash utility.
> > > 
> > > Sequence: fib_route_seq_next->trie_nextleaf->leaf_walk_rcu
> > > 
> > > As "v" was a freed object, inside trie_nextleaf(), node_parent_rcu()
> > > returned an invalid tnode. But as I had enabled slab poisoning and the
> > > object was already freed, the tnode was 0x6b6b6b6b. And this was passed to
> > > leaf_walk_rcu and resulted in the crash.
> > > 
> > > fib_route_seq_start, takes rcu_read_lock(), but free_leaf
> > > calls call_rcu_bh. Can this be the problem ?
> > > Should rcu_read_lock() in fib_route_seq_start be changed to rcu_read_lock_bh()
> > > ?
> > 
> > One way or the other, the RCU read-side primitives need to match the RCU
> > update-side primitives.  Adding netdev...
> 
> Already fixed by:
> 
> commit 0c03eca3d995e73d691edea8c787e25929ec156d
> Author: Eric Dumazet <edumazet@google.com>
> Date:   Tue Aug 7 00:47:11 2012 +0000
> 
>     net: fib: fix incorrect call_rcu_bh()
>     
>     After IP route cache removal, I believe rcu_bh() has very little use and
>     we should remove this RCU variant, since it adds some cycles in fast
>     path.
>     
>     Anyway, the call_rcu_bh() use in fib_true is obviously wrong, since
>     some users only assert rcu_read_lock().

Even better!  ;-)

							Thanx, Paul