On Tue, Sep 16, 2025 at 8:27 PM luoxuanqiang [off-list ref] wrote:

在 2025/9/17 03:48, Kuniyuki Iwashima 写道:

quoted

On Tue, Sep 16, 2025 at 3:31 AM [off-list ref] wrote:

quoted

From: Xuanqiang Luo <redacted>

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.

Fixes: 3ab5aee7fe84 ("net: Convert TCP & DCCP hash tables to use RCU / hlist_nulls")
Signed-off-by: Xuanqiang Luo <redacted>
---
  net/ipv4/inet_timewait_sock.c | 15 ++++++---------
  1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
index 5b5426b8ee92..1ba20c4cb73b 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 +154,11 @@ void inet_twsk_hashdance_schedule(struct inet_timewait_sock *tw,
          */
         refcount_set(&tw->tw_refcnt, 3);

+       if (hlist_nulls_replace_init_rcu(&sk->sk_nulls_node, &tw->tw_node))
+               sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
+       else
+               inet_twsk_add_node_rcu(tw, &ehead->chain);

When hlist_nulls_replace_init_rcu() returns false ?

When hlist_nulls_replace_init_rcu() returns false, it means
sk is unhashed,

and how does this happen ?

Here is under lock_sock() I think, for example, you can
find a lockdep annotation in the path:

tcp_time_wait_init
  tp->af_specific->md5_lookup / tcp_v4_md5_lookup
    tcp_md5_do_lookup
      __tcp_md5_do_lookup
        rcu_dereference_check(tp->md5sig_info, lockdep_sock_is_held(sk));

So, is there a path that unhashes socket without holding
lock_sock() ?

the replacement operation failed, we need
to insert tw, and this doesn't change the original logic.

quoted

quoted

+
         inet_twsk_schedule(tw, timeo);

         spin_unlock(lock);
--
2.25.1