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Date: Fri, 11 Jun 2021 07:37:30 +0900
From: Kuniyuki Iwashima <kuniyu@...zon.co.jp>
To: <eric.dumazet@...il.com>
CC: <andrii@...nel.org>, <ast@...nel.org>, <benh@...zon.com>,
<bpf@...r.kernel.org>, <daniel@...earbox.net>,
<davem@...emloft.net>, <edumazet@...gle.com>, <kafai@...com>,
<kuba@...nel.org>, <kuni1840@...il.com>, <kuniyu@...zon.co.jp>,
<linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: Re: [PATCH v7 bpf-next 03/11] tcp: Keep TCP_CLOSE sockets in the reuseport group.
From: Eric Dumazet <eric.dumazet@...il.com>
Date: Thu, 10 Jun 2021 19:59:15 +0200
> On 5/21/21 8:20 PM, Kuniyuki Iwashima wrote:
> > When we close a listening socket, to migrate its connections to another
> > listener in the same reuseport group, we have to handle two kinds of child
> > sockets. One is that a listening socket has a reference to, and the other
> > is not.
> >
> > The former is the TCP_ESTABLISHED/TCP_SYN_RECV sockets, and they are in the
> > accept queue of their listening socket. So we can pop them out and push
> > them into another listener's queue at close() or shutdown() syscalls. On
> > the other hand, the latter, the TCP_NEW_SYN_RECV socket is during the
> > three-way handshake and not in the accept queue. Thus, we cannot access
> > such sockets at close() or shutdown() syscalls. Accordingly, we have to
> > migrate immature sockets after their listening socket has been closed.
> >
> > Currently, if their listening socket has been closed, TCP_NEW_SYN_RECV
> > sockets are freed at receiving the final ACK or retransmitting SYN+ACKs. At
> > that time, if we could select a new listener from the same reuseport group,
> > no connection would be aborted. However, we cannot do that because
> > reuseport_detach_sock() sets NULL to sk_reuseport_cb and forbids access to
> > the reuseport group from closed sockets.
> >
> > This patch allows TCP_CLOSE sockets to remain in the reuseport group and
> > access it while any child socket references them. The point is that
> > reuseport_detach_sock() was called twice from inet_unhash() and
> > sk_destruct(). This patch replaces the first reuseport_detach_sock() with
> > reuseport_stop_listen_sock(), which checks if the reuseport group is
> > capable of migration. If capable, it decrements num_socks, moves the socket
> > backwards in socks[] and increments num_closed_socks. When all connections
> > are migrated, sk_destruct() calls reuseport_detach_sock() to remove the
> > socket from socks[], decrement num_closed_socks, and set NULL to
> > sk_reuseport_cb.
> >
> > By this change, closed or shutdowned sockets can keep sk_reuseport_cb.
> > Consequently, calling listen() after shutdown() can cause EADDRINUSE or
> > EBUSY in inet_csk_bind_conflict() or reuseport_add_sock() which expects
> > such sockets not to have the reuseport group. Therefore, this patch also
> > loosens such validation rules so that a socket can listen again if it has a
> > reuseport group with num_closed_socks more than 0.
> >
> > When such sockets listen again, we handle them in reuseport_resurrect(). If
> > there is an existing reuseport group (reuseport_add_sock() path), we move
> > the socket from the old group to the new one and free the old one if
> > necessary. If there is no existing group (reuseport_alloc() path), we
> > allocate a new reuseport group, detach sk from the old one, and free it if
> > necessary, not to break the current shutdown behaviour:
> >
> > - we cannot carry over the eBPF prog of shutdowned sockets
> > - we cannot attach/detach an eBPF prog to/from listening sockets via
> > shutdowned sockets
> >
> > Note that when the number of sockets gets over U16_MAX, we try to detach a
> > closed socket randomly to make room for the new listening socket in
> > reuseport_grow().
> >
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.co.jp>
> > Signed-off-by: Martin KaFai Lau <kafai@...com>
> > ---
> > include/net/sock_reuseport.h | 1 +
> > net/core/sock_reuseport.c | 184 ++++++++++++++++++++++++++++++--
> > net/ipv4/inet_connection_sock.c | 12 ++-
> > net/ipv4/inet_hashtables.c | 2 +-
> > 4 files changed, 188 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/net/sock_reuseport.h b/include/net/sock_reuseport.h
> > index 0e558ca7afbf..1333d0cddfbc 100644
> > --- a/include/net/sock_reuseport.h
> > +++ b/include/net/sock_reuseport.h
> > @@ -32,6 +32,7 @@ extern int reuseport_alloc(struct sock *sk, bool bind_inany);
> > extern int reuseport_add_sock(struct sock *sk, struct sock *sk2,
> > bool bind_inany);
> > extern void reuseport_detach_sock(struct sock *sk);
> > +void reuseport_stop_listen_sock(struct sock *sk);
> > extern struct sock *reuseport_select_sock(struct sock *sk,
> > u32 hash,
> > struct sk_buff *skb,
> > diff --git a/net/core/sock_reuseport.c b/net/core/sock_reuseport.c
> > index 079bd1aca0e7..ea0e900d3e97 100644
> > --- a/net/core/sock_reuseport.c
> > +++ b/net/core/sock_reuseport.c
> > @@ -17,6 +17,8 @@
> > DEFINE_SPINLOCK(reuseport_lock);
> >
> > static DEFINE_IDA(reuseport_ida);
> > +static int reuseport_resurrect(struct sock *sk, struct sock_reuseport *old_reuse,
> > + struct sock_reuseport *reuse, bool bind_inany);
> >
> > static int reuseport_sock_index(struct sock *sk,
> > struct sock_reuseport *reuse,
> > @@ -61,6 +63,29 @@ static bool __reuseport_detach_sock(struct sock *sk,
> > return true;
> > }
> >
> > +static void __reuseport_add_closed_sock(struct sock *sk,
> > + struct sock_reuseport *reuse)
> > +{
> > + reuse->socks[reuse->max_socks - reuse->num_closed_socks - 1] = sk;
> > + /* paired with READ_ONCE() in inet_csk_bind_conflict() */
> > + WRITE_ONCE(reuse->num_closed_socks, reuse->num_closed_socks + 1);
> > +}
> > +
> > +static bool __reuseport_detach_closed_sock(struct sock *sk,
> > + struct sock_reuseport *reuse)
> > +{
> > + int i = reuseport_sock_index(sk, reuse, true);
> > +
> > + if (i == -1)
> > + return false;
> > +
> > + reuse->socks[i] = reuse->socks[reuse->max_socks - reuse->num_closed_socks];
> > + /* paired with READ_ONCE() in inet_csk_bind_conflict() */
> > + WRITE_ONCE(reuse->num_closed_socks, reuse->num_closed_socks - 1);
> > +
> > + return true;
> > +}
> > +
> > static struct sock_reuseport *__reuseport_alloc(unsigned int max_socks)
> > {
> > unsigned int size = sizeof(struct sock_reuseport) +
> > @@ -92,6 +117,14 @@ int reuseport_alloc(struct sock *sk, bool bind_inany)
> > reuse = rcu_dereference_protected(sk->sk_reuseport_cb,
> > lockdep_is_held(&reuseport_lock));
> > if (reuse) {
> > + if (reuse->num_closed_socks) {
> > + /* sk was shutdown()ed before */
> > + int err = reuseport_resurrect(sk, reuse, NULL, bind_inany);
> > +
> > + spin_unlock_bh(&reuseport_lock);
> > + return err;
>
> It seems coding style in this function would rather do
> ret = reuseport_resurrect(sk, reuse, NULL, bind_inany);
> goto out;
>
> Overall, changes in this commit are a bit scarry.
I will change the style with ret and goto.
Thank you.
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