| Message ID | 1493677788.31837.25.camel@edumazet-glaptop3.roam.corp.google.com |
|---|---|
| State | Accepted, archived |
| Delegated to: | David Miller |
| Headers | show |
On Mon, 01 May 2017 15:29:48 -0700 Eric Dumazet <eric.dumazet@gmail.com> wrote: > From: Eric Dumazet <edumazet@google.com> > > Be careful when comparing tcp_time_stamp to some u32 quantity, > otherwise result can be surprising. > > Fixes: 7c106d7e782b ("[TCP]: TCP Low Priority congestion control") > Signed-off-by: Eric Dumazet <edumazet@google.com> > --- > net/ipv4/tcp_lp.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/net/ipv4/tcp_lp.c b/net/ipv4/tcp_lp.c > index 046fd3910873306d74207615d6997e1c847ea361..d6fb6c067af4641f232b94e7c590c212648e8173 100644 > --- a/net/ipv4/tcp_lp.c > +++ b/net/ipv4/tcp_lp.c > @@ -264,13 +264,15 @@ static void tcp_lp_pkts_acked(struct sock *sk, const struct ack_sample *sample) > { > struct tcp_sock *tp = tcp_sk(sk); > struct lp *lp = inet_csk_ca(sk); > + u32 delta; > > if (sample->rtt_us > 0) > tcp_lp_rtt_sample(sk, sample->rtt_us); > > /* calc inference */ > - if (tcp_time_stamp > tp->rx_opt.rcv_tsecr) > - lp->inference = 3 * (tcp_time_stamp - tp->rx_opt.rcv_tsecr); > + delta = tcp_time_stamp - tp->rx_opt.rcv_tsecr; > + if ((s32)delta > 0) > + lp->inference = 3 * delta; Agreed time wraparound would cause problems. But why not use existing time_after() macro here?
On Mon, May 1, 2017 at 7:56 PM, Stephen Hemminger <stephen@networkplumber.org> wrote: > On Mon, 01 May 2017 15:29:48 -0700 > Agreed time wraparound would cause problems. > But why not use existing time_after() macro here? > I suspect this is because time_after() asserts that it is being used on unsigned long (64 bits), and tcp_time_stamp is 32 bits. I suppose for tcp_time_stamp comparisons we could re-use the u32 TCP sequence macros for before() and after()? Even the comment for before()/after() is already generic enough to apply to tcp_time_stamp: "The next routines deal with comparing 32 bit unsigned ints and worry about wraparound (automatic with unsigned arithmetic)." That might be nice. neal
On Mon, 2017-05-01 at 16:56 -0700, Stephen Hemminger wrote: > On Mon, 01 May 2017 15:29:48 -0700 > Eric Dumazet <eric.dumazet@gmail.com> wrote: > > > From: Eric Dumazet <edumazet@google.com> > > > > Be careful when comparing tcp_time_stamp to some u32 quantity, > > otherwise result can be surprising. > > > > Fixes: 7c106d7e782b ("[TCP]: TCP Low Priority congestion control") > > Signed-off-by: Eric Dumazet <edumazet@google.com> > > --- > > net/ipv4/tcp_lp.c | 6 ++++-- > > 1 file changed, 4 insertions(+), 2 deletions(-) > > > > diff --git a/net/ipv4/tcp_lp.c b/net/ipv4/tcp_lp.c > > index 046fd3910873306d74207615d6997e1c847ea361..d6fb6c067af4641f232b94e7c590c212648e8173 100644 > > --- a/net/ipv4/tcp_lp.c > > +++ b/net/ipv4/tcp_lp.c > > @@ -264,13 +264,15 @@ static void tcp_lp_pkts_acked(struct sock *sk, const struct ack_sample *sample) > > { > > struct tcp_sock *tp = tcp_sk(sk); > > struct lp *lp = inet_csk_ca(sk); > > + u32 delta; > > > > if (sample->rtt_us > 0) > > tcp_lp_rtt_sample(sk, sample->rtt_us); > > > > /* calc inference */ > > - if (tcp_time_stamp > tp->rx_opt.rcv_tsecr) > > - lp->inference = 3 * (tcp_time_stamp - tp->rx_opt.rcv_tsecr); > > + delta = tcp_time_stamp - tp->rx_opt.rcv_tsecr; > > + if ((s32)delta > 0) > > + lp->inference = 3 * delta; > > Agreed time wraparound would cause problems. > But why not use existing time_after() macro here? > Simply to not perform (tcp_time_stamp - tp->rx_opt.rcv_tsecr) twice. jiffies being volatile, this can not be optimized by the compiler. I have a patch series (for linux-4.13) that will switch TCP stack to 1ms TS options, regardless of CONFIG_HZ value, and when cooking it I found this bug.
On Mon, 2017-05-01 at 18:04 -0700, Eric Dumazet wrote: > > Simply to not perform (tcp_time_stamp - tp->rx_opt.rcv_tsecr) twice. > > jiffies being volatile, this can not be optimized by the compiler. > > I have a patch series (for linux-4.13) that will switch TCP stack to 1ms > TS options, regardless of CONFIG_HZ value, and when cooking it I found > this bug. I forgot to say that after this upcoming patch series, tcp_time_stamp will become a more expensive function, no longer a plain (u32)jiffies.
From: Eric Dumazet <eric.dumazet@gmail.com> Date: Mon, 01 May 2017 15:29:48 -0700 > From: Eric Dumazet <edumazet@google.com> > > Be careful when comparing tcp_time_stamp to some u32 quantity, > otherwise result can be surprising. > > Fixes: 7c106d7e782b ("[TCP]: TCP Low Priority congestion control") > Signed-off-by: Eric Dumazet <edumazet@google.com> Applied and queued up for -stable, thanks.
diff --git a/net/ipv4/tcp_lp.c b/net/ipv4/tcp_lp.c index 046fd3910873306d74207615d6997e1c847ea361..d6fb6c067af4641f232b94e7c590c212648e8173 100644 --- a/net/ipv4/tcp_lp.c +++ b/net/ipv4/tcp_lp.c @@ -264,13 +264,15 @@ static void tcp_lp_pkts_acked(struct sock *sk, const struct ack_sample *sample) { struct tcp_sock *tp = tcp_sk(sk); struct lp *lp = inet_csk_ca(sk); + u32 delta; if (sample->rtt_us > 0) tcp_lp_rtt_sample(sk, sample->rtt_us); /* calc inference */ - if (tcp_time_stamp > tp->rx_opt.rcv_tsecr) - lp->inference = 3 * (tcp_time_stamp - tp->rx_opt.rcv_tsecr); + delta = tcp_time_stamp - tp->rx_opt.rcv_tsecr; + if ((s32)delta > 0) + lp->inference = 3 * delta; /* test if within inference */ if (lp->last_drop && (tcp_time_stamp - lp->last_drop < lp->inference))