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tcp: fix a timewait refcnt race 

After TCP RCU conversion, tw->tw_refcnt should not be set to 1 in
inet_twsk_alloc(). It allows a RCU reader to get this timewait socket,
while we not yet stabilized it.

Only choice we have is to set tw_refcnt to 0 in inet_twsk_alloc(),
then atomic_add() it later, once everything is done.

Location of this atomic_add() is tricky, because we dont want another
writer to find this timewait in ehash, while tw_refcnt is still zero !

Thanks to Kapil Dakhane tests and reports.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2009-12-03 00:49:01 +00:00 committed by David S. Miller
parent 13475a30b6
commit 47e1c32306
1 changed files with 16 additions and 3 deletions
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19
net/ipv4/inet_timewait_sock.c
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@ -109,7 +109,6 @@ void __inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
tw->tw_tb = icsk->icsk_bind_hash;
WARN_ON(!icsk->icsk_bind_hash);
inet_twsk_add_bind_node(tw, &tw->tw_tb->owners);
atomic_inc(&tw->tw_refcnt);
spin_unlock(&bhead->lock);
spin_lock(lock);
@ -119,13 +118,22 @@ void __inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
* Should be done before removing sk from established chain
* because readers are lockless and search established first.
*/
atomic_inc(&tw->tw_refcnt);
inet_twsk_add_node_rcu(tw, &ehead->twchain);
/* Step 3: Remove SK from established hash. */
if (__sk_nulls_del_node_init_rcu(sk))
sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
/*
* Notes :
* - We initially set tw_refcnt to 0 in inet_twsk_alloc()
* - We add one reference for the bhash link
* - We add one reference for the ehash link
* - We want this refcnt update done before allowing other
* threads to find this tw in ehash chain.
*/
atomic_add(1 + 1 + 1, &tw->tw_refcnt);
spin_unlock(lock);
}
@ -157,7 +165,12 @@ struct inet_timewait_sock *inet_twsk_alloc(const struct sock *sk, const int stat
tw->tw_transparent = inet->transparent;
tw->tw_prot = sk->sk_prot_creator;
twsk_net_set(tw, hold_net(sock_net(sk)));
atomic_set(&tw->tw_refcnt, 1);
/*
* Because we use RCU lookups, we should not set tw_refcnt
* to a non null value before everything is setup for this
* timewait socket.
*/
atomic_set(&tw->tw_refcnt, 0);
inet_twsk_dead_node_init(tw);
__module_get(tw->tw_prot->owner);
}