netdev - Re: RCU problems in fib_table

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Date:	Sun, 21 Mar 2010 22:25:52 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	robert.olsson@....uu.se, netdev@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com
Subject: Re: RCU problems in fib_table_insert

Le dimanche 21 mars 2010 à 21:25 +0100, Andi Kleen a écrit :
> Hi,
> 
> I got the following warning at boot with a 2.6.34-rc2ish git kernel
> with RCU debugging and preemption enabled.
> 
> It seems the problem is that not all callers of fib_find_node
> call it with rcu_read_lock() to stabilize access to the fib. 
> 
> I tried to fix it, but especially for fib_table_insert() that's rather 
> tricky: it does a lot of memory allocations and also route flushing and 
> other blocking operations while assuming the original fa is RCU stable.
> 
> I first tried to move some allocations to the beginning and keep
> preemption disabled in the rest, but it's difficult with all of them.
> No patch because of that.
> 
> Does the fa need an additional reference count for this problem?
> Or perhaps some optimistic locking?
> 
> -Andi

No real changes needed, only a lockdep warning...

Probably a rcu_dereference() should be changed to
rcu_dereference_check() like we did for __in6_dev_get()

We hold RTNL or rcu_read_lock

[PATCH] net: fib_find_node() rcu check

We hold rcu read lock or RTNL when fib_find_node() is called.
Shutup lockdep complain.

Reported-by: Andi Kleen <andi@...stfloor.org>
Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
---
diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
index af5d897..471fe07 100644
--- a/net/ipv4/fib_trie.c
+++ b/net/ipv4/fib_trie.c
@@ -961,7 +961,8 @@ fib_find_node(struct trie *t, u32 key)
 	struct node *n;
 
 	pos = 0;
-	n = rcu_dereference(t->trie);
+	n = rcu_dereference_check(t->trie,
+				  rcu_read_lock_held() || lockdep_rtnl_is_held());
 
 	while (n != NULL &&  NODE_TYPE(n) == T_TNODE) {
 		tn = (struct tnode *) n;


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