From: Xuanqiang Luo Since ehash lookups are lockless, if another CPU is converting sk to tw concurrently, fetching the newly inserted tw with tw->tw_refcnt == 0 cause lookup failure. The call trace map is drawn as follows: CPU 0 CPU 1 ----- ----- inet_twsk_hashdance_schedule() spin_lock() inet_twsk_add_node_rcu(tw, ...) __inet_lookup_established() (find tw, failure due to tw_refcnt = 0) __sk_nulls_del_node_init_rcu(sk) refcount_set(&tw->tw_refcnt, 3) spin_unlock() By replacing sk with tw atomically via hlist_nulls_replace_init_rcu() after setting tw_refcnt, we ensure that tw is either fully initialized or not visible to other CPUs, eliminating the race. It's worth noting that we replace under lock_sock(), so no need to check if sk is hashed. Thanks to Kuniyuki Iwashima! Fixes: 3ab5aee7fe84 ("net: Convert TCP & DCCP hash tables to use RCU / hlist_nulls") Suggested-by: Kuniyuki Iwashima Signed-off-by: Xuanqiang Luo --- net/ipv4/inet_timewait_sock.c | 13 ++++--------- 1 file changed, 4 insertions(+), 9 deletions(-) diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c index 5b5426b8ee92..bb98888584a8 100644 --- a/net/ipv4/inet_timewait_sock.c +++ b/net/ipv4/inet_timewait_sock.c @@ -116,7 +116,7 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw, spinlock_t *lock = inet_ehash_lockp(hashinfo, sk->sk_hash); struct inet_bind_hashbucket *bhead, *bhead2; - /* Step 1: Put TW into bind hash. Original socket stays there too. + /* Put TW into bind hash. Original socket stays there too. Note, that any socket with inet->num != 0 MUST be bound in binding cache, even if it is closed. */ @@ -140,14 +140,6 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw, spin_lock(lock); - /* Step 2: Hash TW into tcp ehash chain */ - inet_twsk_add_node_rcu(tw, &ehead->chain); - - /* Step 3: Remove SK from hash chain */ - if (__sk_nulls_del_node_init_rcu(sk)) - sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1); - - /* Ensure above writes are committed into memory before updating the * refcount. * Provides ordering vs later refcount_inc(). @@ -162,6 +154,9 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw, */ refcount_set(&tw->tw_refcnt, 3); + hlist_nulls_replace_init_rcu(&sk->sk_nulls_node, &tw->tw_node); + sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1); + inet_twsk_schedule(tw, timeo); spin_unlock(lock); -- 2.25.1