Messages in this thread Patch in this message |  | | From | Eric Dumazet <> | Date | Sat, 14 Jan 2023 13:30:51 +0100 | Subject | Re: [PATCH net] tcp: avoid the lookup process failing to get sk in ehash table |
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On Sat, Jan 14, 2023 at 1:06 PM Jason Xing <kerneljasonxing@gmail.com> wrote: > > On Sat, Jan 14, 2023 at 5:45 PM Eric Dumazet <edumazet@google.com> wrote: > > > > On Thu, Jan 12, 2023 at 7:54 AM Jason Xing <kerneljasonxing@gmail.com> wrote: > > > > > > From: Jason Xing <kernelxing@tencent.com> > > > > > > While one cpu is working on looking up the right socket from ehash > > > table, another cpu is done deleting the request socket and is about > > > to add (or is adding) the big socket from the table. It means that > > > we could miss both of them, even though it has little chance. > > > > > > Let me draw a call trace map of the server side. > > > CPU 0 CPU 1 > > > ----- ----- > > > tcp_v4_rcv() syn_recv_sock() > > > inet_ehash_insert() > > > -> sk_nulls_del_node_init_rcu(osk) > > > __inet_lookup_established() > > > -> __sk_nulls_add_node_rcu(sk, list) > > > > > > Notice that the CPU 0 is receiving the data after the final ack > > > during 3-way shakehands and CPU 1 is still handling the final ack. > > > > > > Why could this be a real problem? > > > This case is happening only when the final ack and the first data > > > receiving by different CPUs. Then the server receiving data with > > > ACK flag tries to search one proper established socket from ehash > > > table, but apparently it fails as my map shows above. After that, > > > the server fetches a listener socket and then sends a RST because > > > it finds a ACK flag in the skb (data), which obeys RST definition > > > in RFC 793. > > > > > > Many thanks to Eric for great help from beginning to end. > > > > > > Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions") > > > Signed-off-by: Jason Xing <kernelxing@tencent.com> > > > --- > > > net/ipv4/inet_hashtables.c | 10 ++++++++++ > > > 1 file changed, 10 insertions(+) > > > > > > diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c > > > index 24a38b56fab9..18f88cb4efcb 100644 > > > --- a/net/ipv4/inet_hashtables.c > > > +++ b/net/ipv4/inet_hashtables.c > > > @@ -650,7 +650,16 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk) > > > spin_lock(lock); > > > if (osk) { > > > WARN_ON_ONCE(sk->sk_hash != osk->sk_hash); > > > + if (sk_hashed(osk)) > > > + /* Before deleting the node, we insert a new one to make > > > + * sure that the look-up=sk process would not miss either > > > + * of them and that at least one node would exist in ehash > > > + * table all the time. Otherwise there's a tiny chance > > > + * that lookup process could find nothing in ehash table. > > > + */ > > > + __sk_nulls_add_node_rcu(sk, list); > > > > In our private email exchange, I suggested to insert sk at the _tail_ > > of the hash bucket. > > > > Yes, I noticed that. At that time I kept considering the race > condition of the RCU itself, not the scene you mentioned as below. > > > Inserting it at the _head_ would still leave a race condition, because > > a concurrent reader might > > have already started the bucket traversal, and would not see 'sk'. > > Thanks for the detailed explanation. Now I see why. I'll replace it > with __sk_nulls_add_node_tail_rcu() function and send the v2 patch. > > By the way, I checked the removal of TIMEWAIT socket which is included > in this patch. > I write down the call-trace: > inet_hash_connect() > -> __inet_hash_connect() > -> if (sk_unhashed(sk)) { > inet_ehash_nolisten(sk, (struct sock *)tw, NULL); > -> inet_ehash_insert(sk, osk, found_dup_sk); > Therefore, this patch covers the timewait case.
This is the path handling the TIME_WAIT ---> ESTABLISH case.
I was referring to the more common opposite case which is the case where a race could possibly happen.
This is inet_twsk_hashdance, and I suspect we want something like:
diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c index 1d77d992e6e77f7d96bd061be6dbb802c2566b3f..6d681ef52bb24b984a9dbda25b19291fc4393914 100644 --- a/net/ipv4/inet_timewait_sock.c +++ b/net/ipv4/inet_timewait_sock.c @@ -91,10 +91,10 @@ void inet_twsk_put(struct inet_timewait_sock *tw) } EXPORT_SYMBOL_GPL(inet_twsk_put);
-static void inet_twsk_add_node_rcu(struct inet_timewait_sock *tw, +static void inet_twsk_add_node_tail_rcu(struct inet_timewait_sock *tw, struct hlist_nulls_head *list) { - hlist_nulls_add_head_rcu(&tw->tw_node, list); + hlist_nulls_add_tail_rcu(&tw->tw_node, list); }
static void inet_twsk_add_bind_node(struct inet_timewait_sock *tw, @@ -147,7 +147,7 @@ void inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
spin_lock(lock);
- inet_twsk_add_node_rcu(tw, &ehead->chain); + inet_twsk_add_node_tail_rcu(tw, &ehead->chain);
/* Step 3: Remove SK from hash chain */ if (__sk_nulls_del_node_init_rcu(sk))
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