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Date: Mon, 27 May 2024 20:50:05 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Neal Cardwell <ncardwell@...gle.com>, "David S . Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
eric.dumazet@...il.com
Subject: Re: [PATCH net 1/4] tcp: add tcp_done_with_error() helper
On Mon, May 27, 2024 at 4:56 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Mon, May 27, 2024 at 10:07 AM Jason Xing <kerneljasonxing@...il.com> wrote:
> >
> > Hi Neal,
> >
> > On Sat, May 25, 2024 at 10:14 PM Neal Cardwell <ncardwell@...gle.com> wrote:
> > >
> > > On Fri, May 24, 2024 at 3:36 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > > >
> > > > tcp_reset() ends with a sequence that is carefuly ordered.
> > > >
> > > > We need to fix [e]poll bugs in the following patches,
> > > > it makes sense to use a common helper.
> > > >
> > > > Suggested-by: Neal Cardwell <ncardwell@...gle.com>
> > > > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > > > ---
> > > > include/net/tcp.h | 1 +
> > > > net/ipv4/tcp.c | 2 +-
> > > > net/ipv4/tcp_input.c | 25 +++++++++++++++++--------
> > > > 3 files changed, 19 insertions(+), 9 deletions(-)
> > > >
> > > > diff --git a/include/net/tcp.h b/include/net/tcp.h
> > > > index 060e95b331a286ad7c355be11dc03250d2944920..2e7150f6755a5f5bf7b45454da0b33c5fac78183 100644
> > > > --- a/include/net/tcp.h
> > > > +++ b/include/net/tcp.h
> > > > @@ -677,6 +677,7 @@ void tcp_skb_collapse_tstamp(struct sk_buff *skb,
> > > > /* tcp_input.c */
> > > > void tcp_rearm_rto(struct sock *sk);
> > > > void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req);
> > > > +void tcp_done_with_error(struct sock *sk);
> > > > void tcp_reset(struct sock *sk, struct sk_buff *skb);
> > > > void tcp_fin(struct sock *sk);
> > > > void tcp_check_space(struct sock *sk);
> > > > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > > > index 681b54e1f3a64387787738ab6495531b8abe1771..2a8f8d8676ff1d30ea9f8cd47ccf9236940eb299 100644
> > > > --- a/net/ipv4/tcp.c
> > > > +++ b/net/ipv4/tcp.c
> > > > @@ -598,7 +598,7 @@ __poll_t tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
> > > > */
> > > > mask |= EPOLLOUT | EPOLLWRNORM;
> > > > }
> > > > - /* This barrier is coupled with smp_wmb() in tcp_reset() */
> > > > + /* This barrier is coupled with smp_wmb() in tcp_done_with_error() */
> > > > smp_rmb();
> > > > if (READ_ONCE(sk->sk_err) ||
> > > > !skb_queue_empty_lockless(&sk->sk_error_queue))
> > > > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > > > index 9c04a9c8be9dfaa0ec2437b3748284e57588b216..5af716f1bc74e095d22f64d605624decfe27cefe 100644
> > > > --- a/net/ipv4/tcp_input.c
> > > > +++ b/net/ipv4/tcp_input.c
> > > > @@ -4436,6 +4436,22 @@ static enum skb_drop_reason tcp_sequence(const struct tcp_sock *tp,
> > > > return SKB_NOT_DROPPED_YET;
> > > > }
> > > >
> > > > +
> > > > +void tcp_done_with_error(struct sock *sk)
> > > > +{
> > > > + /* Our caller wrote a value into sk->sk_err.
> > > > + * This barrier is coupled with smp_rmb() in tcp_poll()
> > > > + */
> > > > + smp_wmb();
> > > > +
> > > > + tcp_write_queue_purge(sk);
> > > > + tcp_done(sk);
> > > > +
> > > > + if (!sock_flag(sk, SOCK_DEAD))
> > > > + sk_error_report(sk);
> > > > +}
> > > > +EXPORT_SYMBOL(tcp_done_with_error);
> > > > +
> > > > /* When we get a reset we do this. */
> > > > void tcp_reset(struct sock *sk, struct sk_buff *skb)
> > > > {
> > > > @@ -4460,14 +4476,7 @@ void tcp_reset(struct sock *sk, struct sk_buff *skb)
> > > > default:
> > > > WRITE_ONCE(sk->sk_err, ECONNRESET);
> > > > }
> > > > - /* This barrier is coupled with smp_rmb() in tcp_poll() */
> > > > - smp_wmb();
> > > > -
> > > > - tcp_write_queue_purge(sk);
> > > > - tcp_done(sk);
> > > > -
> > > > - if (!sock_flag(sk, SOCK_DEAD))
> > > > - sk_error_report(sk);
> > > > + tcp_done_with_error(sk);
> > > > }
> > > >
> > > > /*
> > > > --
> > >
> > > Thanks, Eric!
> > >
> > > Thinking about this more, I wonder if there is another aspect to this issue.
> > >
> > > I am thinking about this part of tcp_done():
> > >
> > > void tcp_done(struct sock *sk)
> > > {
> > > ...
> > > sk->sk_shutdown = SHUTDOWN_MASK;
> > >
> > > if (!sock_flag(sk, SOCK_DEAD))
> > > sk->sk_state_change(sk);
> > >
> > > The tcp_poll() code reads sk->sk_shutdown to decide whether to set
> > > EPOLLHUP and other bits. However, sk->sk_shutdown is not set until
> > > here in tcp_done(). And in the tcp_done() code there is no smp_wmb()
> > > to ensure that the sk->sk_shutdown is visible to other CPUs before
> > > tcp_done() calls sk->sk_state_change() to wake up threads sleeping on
> > > sk->sk_wq.
> > >
> > > So AFAICT we could have cases where this sk->sk_state_change() (or the
> > > later sk_error_report()?) wakes a thread doing a tcp_poll() on another
> > > CPU, and the tcp_poll() code may correctly see the sk->sk_err because
> > > it was updated before the smp_wmb() in tcp_done_with_error(), but may
> > > fail to see the "sk->sk_shutdown = SHUTDOWN_MASK" write because that
> > > happened after the smp_wmb() in tcp_done_with_error().
> >
> > I agree. Accessing sk_shutdown with a pair of smp operations makes
> > sure that another cpu can see the consistency of both sk_shutdown and
> > sk_err in tcp_poll().
> >
> > >
> > > So AFAICT maybe we need two changes?
> > >
> > > (1) AFAICT the call to smp_wmb() should actually instead be inside
> > > tcp_done(), after we set sk->sk_shutdown?
> > >
> > > void tcp_done(struct sock *sk)
> > > {
> > > ...
> > > sk->sk_shutdown = SHUTDOWN_MASK;
> > >
> > > /* Ensure previous writes to sk->sk_err, sk->sk_state,
> > > * sk->sk_shutdown are visible to others.
> > > * This barrier is coupled with smp_rmb() in tcp_poll()
> > > */
> > > smp_wmb();
> >
> > I wonder if it would affect those callers who have no interest in
> > pairing smp operations, like tcp_v4_syn_recv_sock()? For those
> > callers, WRITE_ONCE/READ_ONCE() is enough to protect itself only.
>
> WRITE_ONCE()/READ_ONCE() and smp_rmb()/smp_wmb() have different purposes.
>
> smp_rmb()/smp_wmb() are order of magnitude more expensive than
> WRITE_ONCE()/READ_ONCE()
>
> You should use them only when absolutely needed.
Sure, I know them. What I was trying to say is putting a smp_wmb()
into tcp_done() is not appropriate because other callers don't want
this expansive protection for sk_shutdown which can be protected with
WRITE/READ_ONCE.
Thanks,
Jason
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