netdev - Re: RCU lock bug in 3.0.21 (bisected to: 682cb56a, fix NULL dereferences in check_peer

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Date:	Tue, 27 Mar 2012 01:39:08 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	gregkh@...uxfoundation.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: RCU lock bug in 3.0.21 (bisected to: 682cb56a, fix NULL
 dereferences in check_peer_redir)

On Mon, 2012-03-26 at 16:06 -0700, Ben Greear wrote:
> On 03/26/2012 02:53 PM, Ben Greear wrote:
> > On 03/26/2012 02:49 PM, David Miller wrote:
> >>
> >> Looks like all of those strange undiagnosable reported Dave Jones
> >> has been feeding us. Something in one part of the kernel leaves
> >> a lock held, and this shows up as a warning elsewhere.
> >
> > Every (initial) bug printout fingers ipv6 and the 'ip' tool on my system.
> 
> I added a patch to convert rcu_read_lock/unlock to macros so
> that I could automatically grab the call site (_THIS_IP_)
> and pass it into the lockdep framework instead of the (useless)
> _THIS_IP_ in the old rcu_read_lock method which at best seems to
> only indicate which module the issue relates to...

Hi Ben

Is this problem also appears with current tree ?
(This could be a problem with the backport, as it was full of
dependencies)

Also, if you use a patch to better track rcu_read_lock()/unlock(), you
could add new macros as well to track that a particular unlock() matches
one given lock(). (maybe returning the rcu_preempt_depth at
rcu_read_lock() time , but maybe a more absolute ref would be better)

So we could have a warning if an unlock() doesnt match the lock()

inet6_dump_fib () was already a suspect but we could not find why.

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 5b27fbc..d1719e3 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -362,6 +362,7 @@ static int inet6_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
 	struct hlist_node *node;
 	struct hlist_head *head;
 	int res = 0;
+	int depth;
 
 	s_h = cb->args[0];
 	s_e = cb->args[1];
@@ -390,7 +391,7 @@ static int inet6_dump_fib(struct sk_buff *skb, struct netlink_callback *cb)
 	arg.net = net;
 	w->args = &arg;
 
-	rcu_read_lock();
+	depth = rcu_read_lock_return();
 	for (h = s_h; h < FIB6_TABLE_HASHSZ; h++, s_e = 0) {
 		e = 0;
 		head = &net->ipv6.fib_table_hash[h];
@@ -405,7 +406,7 @@ next:
 		}
 	}
 out:
-	rcu_read_unlock();
+	rcu_read_unlock_check(depth);
 	cb->args[1] = e;
 	cb->args[0] = h;
 


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