Linux-kernel mailing list archive 2000-02,: Re: [2.2.13aa6 (bugfix release II) ]

# Re: [2.2.13aa6 (bugfix release II) ]

Andrea Arcangeli (andrea@suse.de)
Mon, 10 Jan 2000 16:11:28 +0100 (CET)

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On Sun, 9 Jan 2000, David S. Miller wrote:

> Date: Sun, 9 Jan 2000 21:43:01 +0100 (CET)
> From: Andrea Arcangeli <andrea@suse.de>
>
> I think I spotted and fixed the bug that is soft-deadlocking your
> 2.2.x compaq cluster (all seems to make sense :). Could you try the
> below patch against 2.2.14 (or 2.2.14aa1 or 2.2.13 or 2.2.13aa6)?
>
>Wrong, all callers of tcp_send_delayed_ack _guarentee_ that the
>quickack bit is clear. Your patch does nothing, put an assert

tcp_delack_timer doesn't really guarantee that, see:

void tcp_delack_timer(unsigned long data)
{
struct sock *sk = (struct sock*)data;

if(!sk->zapped &&
sk->tp_pinfo.af_tcp.delayed_acks &&
sk->state != TCP_CLOSE) {
/* If socket is currently locked, defer the ACK. */
if (!atomic_read(&sk->sock_readers))
tcp_send_ack(sk);
else
tcp_send_delayed_ack(&(sk->tp_pinfo.af_tcp), HZ/10);
}
}

You should as well guarantee by design that none timer is pending after
you turn on the quickack bit and before dropping the bh or sock lock. It
seems to me you are guaranteeing that by always sending an ack on the wire
after you set up the quickack bit but it's not trivial to prove and right
now the only explanation for the deadlock reported by urban and that other
people is experiencing is that a delack timer triggers while delayed_acks
is > 0 and the quickack bit is set.

If the quickack bit is set while calling tcp_send_delayed_ack the kernel
will lockup immediatly in a way that matches the reports from ursus.

The reason for the deadlock is that the expired field of the timer will be
set in the past and so the timer will reinsert inself in the first heap
slot and so it will continue to reinsert and rexecute it in an infinite
loop -> soft deadlock. I'd like to also make the timer code robust against
these kind of subsystem bugs later but actually I am only focused to fix
the offending code in TCP.

I have to admit that I can't yet see exactly the path that sets the
quickack bit without sending data on the wire but you agree with me that
the tcp_send_delayed_ack function is not interested about the quickack bit
and it's interested only about the real "ato" information, so my patch is
obviously correct and in the worst case it won't change anything. I
believe it will as well fix the lockup fine, and that it's the right
approch to avoid these kind of subtle mistakes. It makes more sense than
destroying the quickack information before reinserting the the delack
timer from the delack timer, no?

BTW, while reading the code I found a lockup-unrelated bug in the delack
handling:

--- 2.2.14/net/ipv4/tcp_input.c Fri Jan 7 18:19:25 2000
+++ /tmp/tcp_input.c Mon Jan 10 03:41:10 2000
@@ -1428,6 +1428,7 @@
if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
/* A retransmit, 2nd most common case. Force an imediate ack. */
SOCK_DEBUG(sk, "retransmit received: seq %X\n", TCP_SKB_CB(skb)->seq);
+ tp->delayed_acks++;
tcp_enter_quickack_mode(tp);
kfree_skb(skb);
return;

Anyway the above fix is not interesting for real world because it seems
impossible to me to reach such path in the TCP code (so basically such
check is useless but after all I like it for completeness of the
function). This because we call tcp_data only when we know the packet is
in our receive window (otherwise we force an ack by hand prior calling
tcp_queue).

And really tp->delayed_acks is meaningless as far I can tell and the right
thing to do is to remove the delayed_acks field completly (this must be
done at least in 2.3.x). Removing it will avoid wasting time in the TCP
code and will decrease half of the the delacks code braindamage :).

Ursus, please apply also this patch on the top of my fix in the previous
email as for David correct suggestion of putting an assert there. If
you'll see a printk with the below patch applyed we'll have the proof my
theory about the source of your deadlocks is correct and that my fix
made the difference for you. Without the below patch applyed you could
think you are not deadlocking anymore because of luck :).

--- 2.2.14/net/ipv4/tcp_timer.c Fri Jan 7 18:19:25 2000
+++ /tmp/tcp_timer.c Mon Jan 10 16:02:14 2000
@@ -173,7 +173,12 @@
if (!atomic_read(&sk->sock_readers))
tcp_send_ack(sk);
else
+ {
+ struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
+ if (tcp_in_quickack_mode(tp))
+ printk(KERN_ERR "quickack bit set!!!!\n");
tcp_send_delayed_ack(&(sk->tp_pinfo.af_tcp), HZ/10);
+ }
}
}

If my fix doesn't fix the deadlock completly I have really no other
rasonable ideas on what can be going wrong right now. Thinking thinking...

Comments?

Andrea

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