Hi,
I've been using the kernel socket API and noticed that every once in a
while the data received at the remote end of a TCP connection doesn't match
the data sent locally using sosend(). I tracked it down to the piece of
code in tcp_do_segment() starting at line 2665: (svn rev 245286)
if (acked > so->so_snd.sb_cc) {
tp->snd_wnd -= so->so_snd.sb_cc;
sbdrop_locked(&so->so_snd, (int)so->so_snd.sb_cc);
ourfinisacked = 1;
} else {
sbdrop_locked(&so->so_snd, acked);
tp->snd_wnd -= acked;
ourfinisacked = 0;
}
sowwakeup_locked(so);
/* Detect una wraparound. */
if (!IN_RECOVERY(tp->t_flags) &&
SEQ_GT(tp->snd_una, tp->snd_recover) &&
SEQ_LEQ(th->th_ack, tp->snd_recover))
tp->snd_recover = th->th_ack - 1;
/* XXXLAS: Can this be moved up into cc_post_recovery? */
if (IN_RECOVERY(tp->t_flags) &&
SEQ_GEQ(th->th_ack, tp->snd_recover)) {
EXIT_RECOVERY(tp->t_flags);
}
tp->snd_una = th->th_ack;
if (tp->t_flags & TF_SACK_PERMIT) {
if (SEQ_GT(tp->snd_una, tp->snd_recover))
tp->snd_recover = tp->snd_una;
}
if (SEQ_LT(tp->snd_nxt, tp->snd_una))
tp->snd_nxt = tp->snd_una;
The issue is that sowwakeup_locked() is called before all the tcpcb fields
are updated to account for the current ACK processing as can be seen from
the tp->snd_una = th->th_ack in line 2686. My code that uses the kernel
socket API calls sosend() in the upcall path resulting from
sowwakeup_locked() which in turn can lead to tcp_output() being called with
inconsistent TCP state. If I move the call to sowwakeup_locked() to after
the 'if (SEQ_LT(tp->snd_nxt, tp->snd_una))' line in the code snippet above
the data corruption issue seems to be fixed.
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