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Message-ID: <CANn89i+kZv9H2fKgSVLh4iDMT0kEu286YySt-m3RnXJxBLmmWw@mail.gmail.com>
Date: Tue, 16 Mar 2021 10:52:36 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Pavel Machek <pavel@...x.de>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org,
"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 4.19 012/120] tcp: annotate tp->write_seq lockless reads
On Tue, Mar 16, 2021 at 10:50 AM Pavel Machek <pavel@...x.de> wrote:
>
> Hi!
>
> > From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> >
> > From: Eric Dumazet <edumazet@...gle.com>
>
> Dup.
>
>
> > We need to add READ_ONCE() annotations, and also make
> > sure write sides use corresponding WRITE_ONCE() to avoid
> > store-tearing.
>
> > @@ -1037,7 +1037,7 @@ new_segment:
> > sk->sk_wmem_queued += copy;
> > sk_mem_charge(sk, copy);
> > skb->ip_summed = CHECKSUM_PARTIAL;
> > - tp->write_seq += copy;
> > + WRITE_ONCE(tp->write_seq, tp->write_seq + copy);
> > TCP_SKB_CB(skb)->end_seq += copy;
> > tcp_skb_pcount_set(skb, 0);
> >
>
> I wonder if this needs to do READ_ONCE, too?
No, because we hold the socket lock.
This is a backport to ease another backport, please try to review
patches when they hit mainline, if you have any concerns.
>
> > @@ -1391,7 +1391,7 @@ new_segment:
> > if (!copied)
> > TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_PSH;
> >
> > - tp->write_seq += copy;
> > + WRITE_ONCE(tp->write_seq, tp->write_seq + copy);
> > TCP_SKB_CB(skb)->end_seq += copy;
> > tcp_skb_pcount_set(skb, 0);
> >
>
> And here.
>
> > @@ -2593,9 +2594,12 @@ int tcp_disconnect(struct sock *sk, int
> > sock_reset_flag(sk, SOCK_DONE);
> > tp->srtt_us = 0;
> > tp->rcv_rtt_last_tsecr = 0;
> > - tp->write_seq += tp->max_window + 2;
> > - if (tp->write_seq == 0)
> > - tp->write_seq = 1;
> > +
> > + seq = tp->write_seq + tp->max_window + 2;
> > + if (!seq)
> > + seq = 1;
> > + WRITE_ONCE(tp->write_seq, seq);
>
> And here.
>
> > --- a/net/ipv4/tcp_minisocks.c
> > +++ b/net/ipv4/tcp_minisocks.c
> > @@ -510,7 +510,7 @@ struct sock *tcp_create_openreq_child(co
> > newtp->app_limited = ~0U;
> >
> > tcp_init_xmit_timers(newsk);
> > - newtp->write_seq = newtp->pushed_seq = treq->snt_isn + 1;
> > + WRITE_ONCE(newtp->write_seq, newtp->pushed_seq = treq->snt_isn + 1);
>
> Would it be better to do assignment to pushed_seq outside of
> WRITE_ONCE macro? This is ... "interesting".
>
> Best regards,
> Pavel
> --
> DENX Software Engineering GmbH, Managing Director: Wolfgang Denk
> HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
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